diff --git a/CHANGELOG.md b/CHANGELOG.md
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--- /dev/null
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+<!---
+SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+SPDX-License-Identifier: CC0-1.0
+-->
+
+# v0.1.0 (2020-09-01)
+
+Initial release providing the `Merge` trait and some merge strategies in the
+`bool`, `num`, `ord` and `vec` modules.
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+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies
+#
+# If you believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+edition = "2018"
+name = "merge"
+version = "0.1.0"
+authors = ["Robin Krahl <robin.krahl@ireas.org>"]
+exclude = [".builds/*"]
+description = "Merge multiple values into one"
+keywords = ["merge", "macros", "derive"]
+categories = ["rust-patterns"]
+license = "Apache-2.0 OR MIT"
+repository = "https://git.sr.ht/~ireas/merge-rs"
+[dependencies.merge_derive]
+version = "0.1.0"
+optional = true
+
+[dependencies.num-traits]
+version = "0.2.12"
+optional = true
+[dev-dependencies.envy]
+version = "0.4"
+
+[dev-dependencies.serde]
+version = "1.0"
+features = ["derive"]
+
+[dev-dependencies.structopt]
+version = "0.3"
+
+[dev-dependencies.toml]
+version = "0.5"
+
+[dev-dependencies.trybuild]
+version = "1.0"
+
+[features]
+default = ["derive", "num", "std"]
+derive = ["merge_derive"]
+num = ["num-traits"]
+std = []
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
new file mode 100644
index 0000000..aac6575
--- /dev/null
+++ b/Cargo.toml.orig
@@ -0,0 +1,34 @@
+# SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+# SPDX-License-Identifier: CC0-1.0
+
+[package]
+name = "merge"
+version = "0.1.0"
+authors = ["Robin Krahl <robin.krahl@ireas.org>"]
+edition = "2018"
+description = "Merge multiple values into one"
+repository = "https://git.sr.ht/~ireas/merge-rs"
+keywords = ["merge", "macros", "derive"]
+categories = ["rust-patterns"]
+license = "Apache-2.0 OR MIT"
+exclude = [".builds/*"]
+
+[dependencies]
+merge_derive = { path = "merge_derive", version = "0.1.0", optional = true }
+num-traits = { version = "0.2.12", optional = true }
+
+[dev-dependencies]
+envy = "0.4"
+toml = "0.5"
+trybuild = "1.0"
+structopt = "0.3"
+
+[dev-dependencies.serde]
+version = "1.0"
+features = ["derive"]
+
+[features]
+default = ["derive", "num", "std"]
+derive = ["merge_derive"]
+num = ["num-traits"]
+std = []
diff --git a/LICENSE b/LICENSE
new file mode 120000
index 0000000..5431dc1
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1 @@
+LICENSES/Apache-2.0.txt
\ No newline at end of file
diff --git a/LICENSES/Apache-2.0.txt b/LICENSES/Apache-2.0.txt
new file mode 100644
index 0000000..d645695
--- /dev/null
+++ b/LICENSES/Apache-2.0.txt
@@ -0,0 +1,202 @@
+
+                                 Apache License
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+                        http://www.apache.org/licenses/
+
+   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
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+      whether in tort (including negligence), contract, or otherwise,
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+   APPENDIX: How to apply the Apache License to your work.
+
+      To apply the Apache License to your work, attach the following
+      boilerplate notice, with the fields enclosed by brackets "[]"
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+   you may not use this file except in compliance with the License.
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+
+   Unless required by applicable law or agreed to in writing, software
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diff --git a/METADATA b/METADATA
new file mode 100644
index 0000000..3204e4f
--- /dev/null
+++ b/METADATA
@@ -0,0 +1,19 @@
+name: "merge"
+description: "Merge multiple values into one"
+third_party {
+  url {
+    type: HOMEPAGE
+    value: "https://crates.io/crates/merge"
+  }
+  url {
+    type: ARCHIVE
+    value: "https://static.crates.io/crates/merge/merge-0.1.0.crate"
+  }
+  version: "0.1.0"
+  license_type: NOTICE
+  last_upgrade_date {
+    year: 2022
+    month: 11
+    day: 28
+  }
+}
diff --git a/MODULE_LICENSE_APACHE2 b/MODULE_LICENSE_APACHE2
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/MODULE_LICENSE_APACHE2
diff --git a/OWNERS b/OWNERS
new file mode 100644
index 0000000..4e1e118
--- /dev/null
+++ b/OWNERS
@@ -0,0 +1,2 @@
+include platform/prebuilts/rust:master:/OWNERS
+fmayle@google.com
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..1bdb16a
--- /dev/null
+++ b/README.md
@@ -0,0 +1,98 @@
+<!---
+Copyright (C) 2020 Robin Krahl <robin.krahl@ireas.org>
+SPDX-License-Identifier: CC0-1.0
+-->
+
+# merge-rs
+
+The `merge` crate provides the `Merge` trait that can be used to merge multiple
+values into one:
+
+```rust
+trait Merge {
+    fn merge(&mut self, other: Self);
+}
+```
+
+`Merge` is implemented for `Option` and can be derived for structs:
+
+<!-- should be kept in sync with examples/user.rs -->
+```rust
+use merge::Merge;
+
+#[derive(Merge)]
+struct User {
+    // Fields with the skip attribute are skipped by Merge
+    #[merge(skip)]
+    pub name: &'static str,
+
+    // The Merge implementation for Option replaces its value if it is None
+    pub location: Option<&'static str>,
+
+    // The strategy attribute is used to customize the merge behavior
+    #[merge(strategy = merge::vec::append)]
+    pub groups: Vec<&'static str>,
+}
+
+let defaults = User {
+    name: "",
+    location: Some("Internet"),
+    groups: vec!["rust"],
+};
+let mut ferris = User {
+    name: "Ferris",
+    location: None,
+    groups: vec!["mascot"],
+};
+ferris.merge(defaults);
+
+assert_eq!("Ferris", ferris.name);
+assert_eq!(Some("Internet"), ferris.location);
+assert_eq!(vec!["mascot", "rust"], ferris.groups);
+```
+
+A merge strategy is a function with the signature `fn merge<T>(left: &mut T,
+right: T)` that merges `right` into `left`.  The `merge` crate provides
+strategies for the most common types, but you can also define your own
+strategies.
+
+The trait can be used to merge configuration from different sources, for
+example environment variables, multiple configuration files and command-line
+arguments, see the `args.rs` example.
+
+## Features
+
+This crate has the following features:
+
+- `derive` (default):  Enables the derive macro for the `Merge` trait using the
+  `merge_derive` crate.
+- `num` (default): Enables the merge strategies in the `num` module that
+  require the `num_traits` crate.
+- `std` (default): Enables the merge strategies in the `vec` module that
+  require the standard library.  If this feature is not set, `merge` is a
+  `no_std` library.
+
+## Minimum Supported Rust Version
+
+This crate supports Rust 1.36.0 or later.
+
+## Contact
+
+For bug reports, patches, feature requests and other messages, please send a
+mail to [~ireas/public-inbox@lists.sr.ht][] using the `[merge-rs]` prefix in
+the subject.
+
+## License
+
+This project is dual-licensed under the [Apache-2.0][] and [MIT][] licenses.
+The documentation and configuration files contained in this repository are
+licensed under the [Creative Commons Zero][CC0] license.  You can find a copy
+of the license texts in the `LICENSES` directory.
+
+`merge-rs` complies with [version 3.0 of the REUSE specification][reuse].
+
+[~ireas/public-inbox@lists.sr.ht]: mailto:~ireas/public-inbox@lists.sr.ht
+[Apache-2.0]: https://opensource.org/licenses/Apache-2.0
+[MIT]: https://opensource.org/licenses/MIT
+[CC0]: https://creativecommons.org/publicdomain/zero/1.0/
+[reuse]: https://reuse.software/practices/3.0/
diff --git a/examples/args.rs b/examples/args.rs
new file mode 100644
index 0000000..616a1d4
--- /dev/null
+++ b/examples/args.rs
@@ -0,0 +1,43 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: CC0-1.0
+
+use merge::Merge;
+use serde::Deserialize;
+use structopt::StructOpt;
+
+#[derive(Debug, Default, Deserialize, Merge, StructOpt)]
+#[serde(default)]
+struct Args {
+    #[structopt(short, long)]
+    #[merge(strategy = merge::bool::overwrite_false)]
+    debug: bool,
+
+    input: Option<String>,
+
+    output: Option<String>,
+}
+
+fn get_config() -> Option<Args> {
+    let path: &std::path::Path = "args.toml".as_ref();
+    if path.is_file() {
+        let s = std::fs::read_to_string(path).expect("Could not read configuration file");
+        Some(toml::from_str(&s).expect("Could not parse configuration"))
+    } else {
+        None
+    }
+}
+
+fn get_env() -> Args {
+    envy::prefixed("ARGS_")
+        .from_env()
+        .expect("Could not read environment variables")
+}
+
+fn main() {
+    let mut args = Args::from_args();
+    args.merge(get_env());
+    if let Some(config) = get_config() {
+        args.merge(config);
+    }
+    println!("{:?}", args);
+}
diff --git a/examples/user.rs b/examples/user.rs
new file mode 100644
index 0000000..60161d7
--- /dev/null
+++ b/examples/user.rs
@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: CC0-1.0
+
+use merge::Merge;
+
+#[derive(Merge)]
+struct User {
+    #[merge(skip)]
+    pub name: &'static str,
+    pub location: Option<&'static str>,
+    #[merge(strategy = merge::vec::append)]
+    pub groups: Vec<&'static str>,
+}
+
+fn main() {
+    let defaults = User {
+        name: "",
+        location: Some("Internet"),
+        groups: vec!["rust"],
+    };
+    let mut ferris = User {
+        name: "Ferris",
+        location: None,
+        groups: vec!["mascot"],
+    };
+    ferris.merge(defaults);
+
+    assert_eq!("Ferris", ferris.name);
+    assert_eq!(Some("Internet"), ferris.location);
+    assert_eq!(vec!["mascot", "rust"], ferris.groups);
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..2deebca
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,223 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+//! Provides [`Merge`][], a trait for objects that can be merged.
+//!
+//! # Usage
+//!
+//! ```
+//! trait Merge {
+//!     fn merge(&mut self, other: Self);
+//! }
+//! ```
+//!
+//! The [`Merge`][] trait can be used to merge two objects of the same type into one.  The intended
+//! use case is merging configuration from different sources, for example environment variables,
+//! multiple configuration files and command-line arguments, see the [`args.rs`][] example.
+//!
+//! `Merge` is implemented for `Option` and can be derived for structs.  When deriving the `Merge`
+//! trait for a struct, you can provide custom merge strategies for the fields that don’t implement
+//! `Merge`.  A merge strategy is a function with the signature `fn merge<T>(left: &mut T, right:
+//! T)` that merges `right` into `left`.  The submodules of this crate provide strategies for the
+//! most common types, but you can also define your own strategies.
+//!
+//! ## Features
+//!
+//! This crate has the following features:
+//!
+//! - `derive` (default):  Enables the derive macro for the `Merge` trait using the `merge_derive`
+//!   crate.
+//! - `num` (default): Enables the merge strategies in the `num` module that require the
+//!   `num_traits` crate.
+//! - `std` (default): Enables the merge strategies in the `vec` module that require the standard
+//!   library.  If this feature is not set, `merge` is a `no_std` library.
+//!
+//! # Example
+//!
+//! ```
+//! use merge::Merge;
+//!
+//! #[derive(Merge)]
+//! struct User {
+//!     // Fields with the skip attribute are skipped by Merge
+//!     #[merge(skip)]
+//!     pub name: &'static str,
+//!
+//!     // The Merge implementation for Option replaces its value if it is None
+//!     pub location: Option<&'static str>,
+//!
+//!     // The strategy attribute is used to customize the merge behavior
+//!     #[merge(strategy = merge::vec::append)]
+//!     pub groups: Vec<&'static str>,
+//! }
+//!
+//! let defaults = User {
+//!     name: "",
+//!     location: Some("Internet"),
+//!     groups: vec!["rust"],
+//! };
+//! let mut ferris = User {
+//!     name: "Ferris",
+//!     location: None,
+//!     groups: vec!["mascot"],
+//! };
+//! ferris.merge(defaults);
+//!
+//! assert_eq!("Ferris", ferris.name);
+//! assert_eq!(Some("Internet"), ferris.location);
+//! assert_eq!(vec!["mascot", "rust"], ferris.groups);
+//! ```
+//!
+//! [`Merge`]: trait.Merge.html
+//! [`args.rs`]: https://git.sr.ht/~ireas/merge-rs/tree/master/examples/args.rs
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(feature = "derive")]
+pub use merge_derive::*;
+
+/// A trait for objects that can be merged.
+///
+/// # Deriving
+///
+/// `Merge` can be derived for structs if the `derive` feature is enabled.  The generated
+/// implementation calls the `merge` method for all fields, or the merge strategy function if set.
+/// You can use these field attributes to configure the generated implementation:
+/// - `skip`: Skip this field in the `merge` method.
+/// - `strategy = f`: Call `f(self.field, other.field)` instead of calling the `merge` function for
+///    this field.
+///
+/// # Examples
+///
+/// Using the `Merge` implementation for `Option`:
+///
+/// ```
+/// use merge::Merge as _;
+///
+/// let mut val = None;
+/// val.merge(Some(42));
+/// assert_eq!(Some(42), val);
+/// ```
+///
+/// Deriving `Merge` for a struct:
+///
+/// ```
+/// use merge::Merge;
+///
+/// #[derive(Debug, PartialEq, Merge)]
+/// struct S {
+///     option: Option<usize>,
+///
+///     #[merge(skip)]
+///     s: String,
+///
+///     #[merge(strategy = merge::bool::overwrite_false)]
+///     flag: bool,
+/// }
+///
+/// let mut val = S {
+///     option: None,
+///     s: "some ignored value".to_owned(),
+///     flag: false,
+/// };
+/// val.merge(S {
+///     option: Some(42),
+///     s: "some other ignored value".to_owned(),
+///     flag: true,
+/// });
+/// assert_eq!(S {
+///     option: Some(42),
+///     s: "some ignored value".to_owned(),
+///     flag: true,
+/// }, val);
+/// ```
+pub trait Merge {
+    /// Merge another object into this object.
+    fn merge(&mut self, other: Self);
+}
+
+impl<T> Merge for Option<T> {
+    fn merge(&mut self, mut other: Self) {
+        if !self.is_some() {
+            *self = other.take();
+        }
+    }
+}
+
+/// Merge strategies for boolean types.
+pub mod bool {
+    /// Overwrite left with right if the value of left is false.
+    pub fn overwrite_false(left: &mut bool, right: bool) {
+        if !*left {
+            *left = right;
+        }
+    }
+
+    /// Overwrite left with right if the value of left is true.
+    pub fn overwrite_true(left: &mut bool, right: bool) {
+        if *left {
+            *left = right;
+        }
+    }
+}
+
+/// Merge strategies for numeric types.
+///
+/// These strategies are only available if the `num` feature is enabled.
+#[cfg(feature = "num")]
+pub mod num {
+    /// Set left to the saturated some of left and right.
+    pub fn saturating_add<T: num_traits::SaturatingAdd>(left: &mut T, right: T) {
+        *left = left.saturating_add(&right);
+    }
+
+    /// Overwrite left with right if the value of left is zero.
+    pub fn overwrite_zero<T: num_traits::Zero>(left: &mut T, right: T) {
+        if left.is_zero() {
+            *left = right;
+        }
+    }
+}
+
+/// Merge strategies for types that form a total order.
+pub mod ord {
+    use core::cmp;
+
+    /// Set left to the maximum of left and right.
+    pub fn max<T: cmp::Ord>(left: &mut T, right: T) {
+        if cmp::Ord::cmp(left, &right) == cmp::Ordering::Less {
+            *left = right;
+        }
+    }
+
+    /// Set left to the minimum of left and right.
+    pub fn min<T: cmp::Ord>(left: &mut T, right: T) {
+        if cmp::Ord::cmp(left, &right) == cmp::Ordering::Greater {
+            *left = right;
+        }
+    }
+}
+
+/// Merge strategies for vectors.
+///
+/// These strategies are only available if the `std` feature is enabled.
+#[cfg(feature = "std")]
+pub mod vec {
+    /// Overwrite left with right if left is empty.
+    pub fn overwrite_empty<T>(left: &mut Vec<T>, mut right: Vec<T>) {
+        if left.is_empty() {
+            left.append(&mut right);
+        }
+    }
+
+    /// Append the contents of right to left.
+    pub fn append<T>(left: &mut Vec<T>, mut right: Vec<T>) {
+        left.append(&mut right);
+    }
+
+    /// Prepend the contents of right to left.
+    pub fn prepend<T>(left: &mut Vec<T>, mut right: Vec<T>) {
+        right.append(left);
+        *left = right;
+    }
+}
diff --git a/tests/compile.rs b/tests/compile.rs
new file mode 100644
index 0000000..aa1bda7
--- /dev/null
+++ b/tests/compile.rs
@@ -0,0 +1,9 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+#[test]
+#[ignore]
+fn test_compile_fail() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/compile/*.rs");
+}
diff --git a/tests/compile/derive-enum.rs b/tests/compile/derive-enum.rs
new file mode 100644
index 0000000..6c4f9b2
--- /dev/null
+++ b/tests/compile/derive-enum.rs
@@ -0,0 +1,13 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+use merge::Merge;
+
+#[derive(Merge)]
+enum E {
+    V1,
+    V2,
+    V3,
+}
+
+fn main() {}
diff --git a/tests/compile/derive-enum.stderr b/tests/compile/derive-enum.stderr
new file mode 100644
index 0000000..4783549
--- /dev/null
+++ b/tests/compile/derive-enum.stderr
@@ -0,0 +1,7 @@
+error: merge::Merge can only be derived for structs
+ --> $DIR/derive-enum.rs:6:10
+  |
+6 | #[derive(Merge)]
+  |          ^^^^^
+  |
+  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/tests/compile/derive-invalid-attribute.rs b/tests/compile/derive-invalid-attribute.rs
new file mode 100644
index 0000000..cbc3e54
--- /dev/null
+++ b/tests/compile/derive-invalid-attribute.rs
@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+use merge::Merge;
+
+#[derive(Merge)]
+struct S {
+    #[merge(ignore)]
+    field1: Option<u8>,
+}
+
+fn main() {}
diff --git a/tests/compile/derive-invalid-attribute.stderr b/tests/compile/derive-invalid-attribute.stderr
new file mode 100644
index 0000000..ee1b2c6
--- /dev/null
+++ b/tests/compile/derive-invalid-attribute.stderr
@@ -0,0 +1,5 @@
+error: Unexpected attribute: ignore
+ --> $DIR/derive-invalid-attribute.rs:8:13
+  |
+8 |     #[merge(ignore)]
+  |             ^^^^^^
diff --git a/tests/compile/derive-invalid-skip.rs b/tests/compile/derive-invalid-skip.rs
new file mode 100644
index 0000000..78ed93c
--- /dev/null
+++ b/tests/compile/derive-invalid-skip.rs
@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+use merge::Merge;
+
+#[derive(Merge)]
+struct S {
+    #[merge(skip = true)]
+    field1: u8,
+}
+
+fn main() {}
diff --git a/tests/compile/derive-invalid-skip.stderr b/tests/compile/derive-invalid-skip.stderr
new file mode 100644
index 0000000..65679f9
--- /dev/null
+++ b/tests/compile/derive-invalid-skip.stderr
@@ -0,0 +1,5 @@
+error: expected `,`
+ --> $DIR/derive-invalid-skip.rs:8:18
+  |
+8 |     #[merge(skip = true)]
+  |                  ^
diff --git a/tests/compile/derive-invalid-strategy.rs b/tests/compile/derive-invalid-strategy.rs
new file mode 100644
index 0000000..46dc259
--- /dev/null
+++ b/tests/compile/derive-invalid-strategy.rs
@@ -0,0 +1,16 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+use merge::Merge;
+
+#[derive(Merge)]
+struct S {
+    #[merge(strategy = my_custom_merge_strategy)]
+    field1: u8,
+}
+
+fn my_custom_merge_strategy(left: u8, right: u8) -> u8 {
+    left + right
+}
+
+fn main() {}
diff --git a/tests/compile/derive-invalid-strategy.stderr b/tests/compile/derive-invalid-strategy.stderr
new file mode 100644
index 0000000..ecdd085
--- /dev/null
+++ b/tests/compile/derive-invalid-strategy.stderr
@@ -0,0 +1,13 @@
+error[E0308]: mismatched types
+ --> $DIR/derive-invalid-strategy.rs:8:24
+  |
+8 |       #[merge(strategy = my_custom_merge_strategy)]
+  |  ________________________^
+9 | |     field1: u8,
+  | |__________^ expected `u8`, found `&mut u8`
+  |
+help: consider removing the borrow
+  |
+8 |     #[merge(strategy = my_custom_merge_strategy)]
+9 |     field1: u8,
+  |
diff --git a/tests/compile/derive-missing-strategy.rs b/tests/compile/derive-missing-strategy.rs
new file mode 100644
index 0000000..c9c1073
--- /dev/null
+++ b/tests/compile/derive-missing-strategy.rs
@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+use merge::Merge;
+
+#[derive(Merge)]
+struct S {
+    #[merge(strategy = my_custom_merge_strategy)]
+    field1: u8,
+}
+
+fn main() {}
diff --git a/tests/compile/derive-missing-strategy.stderr b/tests/compile/derive-missing-strategy.stderr
new file mode 100644
index 0000000..9fb1a03
--- /dev/null
+++ b/tests/compile/derive-missing-strategy.stderr
@@ -0,0 +1,5 @@
+error[E0425]: cannot find function `my_custom_merge_strategy` in this scope
+ --> $DIR/derive-missing-strategy.rs:8:24
+  |
+8 |     #[merge(strategy = my_custom_merge_strategy)]
+  |                        ^^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
diff --git a/tests/compile/derive-no-strategy.rs b/tests/compile/derive-no-strategy.rs
new file mode 100644
index 0000000..10b95fa
--- /dev/null
+++ b/tests/compile/derive-no-strategy.rs
@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+use merge::Merge;
+
+#[derive(Merge)]
+struct S {
+    #[merge(strategy)]
+    field1: u8,
+}
+
+fn main() {}
diff --git a/tests/compile/derive-no-strategy.stderr b/tests/compile/derive-no-strategy.stderr
new file mode 100644
index 0000000..9da8914
--- /dev/null
+++ b/tests/compile/derive-no-strategy.stderr
@@ -0,0 +1,5 @@
+error: expected `=`
+ --> $DIR/derive-no-strategy.rs:8:12
+  |
+8 |     #[merge(strategy)]
+  |            ^^^^^^^^^^
diff --git a/tests/compile/derive-u8.rs b/tests/compile/derive-u8.rs
new file mode 100644
index 0000000..25bfd9b
--- /dev/null
+++ b/tests/compile/derive-u8.rs
@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+use merge::Merge;
+
+#[derive(Merge)]
+struct S {
+    field1: Option<u8>,
+    field2: u8,
+}
+
+fn main() {}
diff --git a/tests/compile/derive-u8.stderr b/tests/compile/derive-u8.stderr
new file mode 100644
index 0000000..de3b5c2
--- /dev/null
+++ b/tests/compile/derive-u8.stderr
@@ -0,0 +1,7 @@
+error[E0277]: the trait bound `u8: merge::Merge` is not satisfied
+ --> $DIR/derive-u8.rs:9:5
+  |
+9 |     field2: u8,
+  |     ^^^^^^ the trait `merge::Merge` is not implemented for `u8`
+  |
+  = note: required by `merge::Merge::merge`
diff --git a/tests/derive.rs b/tests/derive.rs
new file mode 100644
index 0000000..0ac4163
--- /dev/null
+++ b/tests/derive.rs
@@ -0,0 +1,540 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+#![cfg(feature = "derive")]
+
+use merge::Merge;
+
+fn test<T: std::fmt::Debug + Merge + PartialEq>(expected: T, mut left: T, right: T) {
+    left.merge(right);
+    assert_eq!(expected, left);
+}
+
+#[test]
+fn test_one_option_field() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S {
+        field1: Option<usize>,
+    }
+
+    impl S {
+        pub fn new(field1: Option<usize>) -> S {
+            S { field1 }
+        }
+    }
+
+    test(S::new(Some(1)), S::new(Some(1)), S::new(Some(2)));
+    test(S::new(Some(1)), S::new(Some(1)), S::new(None));
+    test(S::new(Some(2)), S::new(None), S::new(Some(2)));
+    test(S::new(None), S::new(None), S::new(None));
+}
+
+#[test]
+fn test_two_option_fields() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S {
+        field1: Option<usize>,
+        field2: Option<usize>,
+    }
+
+    impl S {
+        pub fn new(field1: Option<usize>, field2: Option<usize>) -> S {
+            S { field1, field2 }
+        }
+    }
+
+    // left.field1 == Some(1)
+    // right.field1 == Some(2)
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(Some(2), None),
+    );
+    test(
+        S::new(Some(1), Some(2)),
+        S::new(Some(1), None),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(Some(2), None),
+    );
+
+    // left.field1 == Some(1)
+    // right.field1 == None
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(None, None),
+    );
+    test(
+        S::new(Some(1), Some(2)),
+        S::new(Some(1), None),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(None, None),
+    );
+
+    // left.field1 == None
+    // right.field1 == Some(2)
+    test(
+        S::new(Some(2), Some(1)),
+        S::new(None, Some(1)),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(2), Some(1)),
+        S::new(None, Some(1)),
+        S::new(Some(2), None),
+    );
+    test(
+        S::new(Some(2), Some(2)),
+        S::new(None, None),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(2), None),
+        S::new(None, None),
+        S::new(Some(2), None),
+    );
+
+    // left.field1 == None
+    // right.field1 == None
+    test(
+        S::new(None, Some(1)),
+        S::new(None, Some(1)),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(None, Some(1)),
+        S::new(None, Some(1)),
+        S::new(None, None),
+    );
+    test(
+        S::new(None, Some(2)),
+        S::new(None, None),
+        S::new(None, Some(2)),
+    );
+    test(S::new(None, None), S::new(None, None), S::new(None, None));
+}
+
+#[test]
+fn test_skip_valid() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S {
+        field1: Option<usize>,
+        #[merge(skip)]
+        field2: Option<usize>,
+    }
+
+    impl S {
+        pub fn new(field1: Option<usize>, field2: Option<usize>) -> S {
+            S { field1, field2 }
+        }
+    }
+
+    // left.field1 == Some(1)
+    // right.field1 == Some(2)
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(Some(2), None),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(Some(2), None),
+    );
+
+    // left.field1 == Some(1)
+    // right.field1 == None
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(None, None),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(None, None),
+    );
+
+    // left.field1 == None
+    // right.field1 == Some(2)
+    test(
+        S::new(Some(2), Some(1)),
+        S::new(None, Some(1)),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(2), Some(1)),
+        S::new(None, Some(1)),
+        S::new(Some(2), None),
+    );
+    test(
+        S::new(Some(2), None),
+        S::new(None, None),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(2), None),
+        S::new(None, None),
+        S::new(Some(2), None),
+    );
+
+    // left.field1 == None
+    // right.field1 == None
+    test(
+        S::new(None, Some(1)),
+        S::new(None, Some(1)),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(None, Some(1)),
+        S::new(None, Some(1)),
+        S::new(None, None),
+    );
+    test(
+        S::new(None, None),
+        S::new(None, None),
+        S::new(None, Some(2)),
+    );
+    test(S::new(None, None), S::new(None, None), S::new(None, None));
+}
+
+#[test]
+fn test_skip_invalid() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S {
+        field1: Option<usize>,
+        #[merge(skip)]
+        field2: usize,
+    }
+
+    impl S {
+        pub fn new(field1: Option<usize>, field2: usize) -> S {
+            S { field1, field2 }
+        }
+    }
+
+    // left.field1 == Some(1)
+    // right.field1 == Some(2)
+    test(S::new(Some(1), 1), S::new(Some(1), 1), S::new(Some(2), 2));
+    test(S::new(Some(1), 1), S::new(Some(1), 1), S::new(Some(2), 0));
+    test(S::new(Some(1), 0), S::new(Some(1), 0), S::new(Some(2), 2));
+    test(S::new(Some(1), 0), S::new(Some(1), 0), S::new(Some(2), 0));
+
+    // left.field1 == Some(1)
+    // right.field1 == None
+    test(S::new(Some(1), 1), S::new(Some(1), 1), S::new(None, 2));
+    test(S::new(Some(1), 1), S::new(Some(1), 1), S::new(None, 0));
+    test(S::new(Some(1), 0), S::new(Some(1), 0), S::new(None, 2));
+    test(S::new(Some(1), 0), S::new(Some(1), 0), S::new(None, 0));
+
+    // left.field1 == None
+    // right.field1 == Some(2)
+    test(S::new(Some(2), 1), S::new(None, 1), S::new(Some(2), 2));
+    test(S::new(Some(2), 1), S::new(None, 1), S::new(Some(2), 0));
+    test(S::new(Some(2), 0), S::new(None, 0), S::new(Some(2), 2));
+    test(S::new(Some(2), 0), S::new(None, 0), S::new(Some(2), 0));
+
+    // left.field1 == None
+    // right.field1 == None
+    test(S::new(None, 1), S::new(None, 1), S::new(None, 2));
+    test(S::new(None, 1), S::new(None, 1), S::new(None, 0));
+    test(S::new(None, 0), S::new(None, 0), S::new(None, 2));
+    test(S::new(None, 0), S::new(None, 0), S::new(None, 0));
+}
+
+#[test]
+fn test_strategy_usize_add() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S {
+        #[merge(strategy = add)]
+        field1: usize,
+    }
+
+    impl S {
+        pub fn new(field1: usize) -> S {
+            S { field1 }
+        }
+    }
+
+    fn add(left: &mut usize, right: usize) {
+        *left = *left + right;
+    }
+
+    test(S::new(0), S::new(0), S::new(0));
+    test(S::new(1), S::new(1), S::new(0));
+    test(S::new(1), S::new(0), S::new(1));
+    test(S::new(2), S::new(1), S::new(1));
+}
+
+#[test]
+fn test_strategy_vec_append() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S {
+        #[merge(strategy = append)]
+        field1: Vec<usize>,
+    }
+
+    impl S {
+        pub fn new(field1: Vec<usize>) -> S {
+            S { field1 }
+        }
+    }
+
+    fn append(left: &mut Vec<usize>, mut right: Vec<usize>) {
+        left.append(&mut right);
+    }
+
+    test(
+        S::new(vec![0, 1, 2, 3]),
+        S::new(vec![0, 1]),
+        S::new(vec![2, 3]),
+    );
+    test(
+        S::new(vec![0, 1, 2, 3]),
+        S::new(vec![0, 1, 2, 3]),
+        S::new(vec![]),
+    );
+    test(
+        S::new(vec![0, 1, 2, 3]),
+        S::new(vec![]),
+        S::new(vec![0, 1, 2, 3]),
+    );
+}
+
+#[test]
+fn test_unnamed_fields() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(Option<usize>, Option<usize>);
+
+    impl S {
+        pub fn new(field1: Option<usize>, field2: Option<usize>) -> S {
+            S(field1, field2)
+        }
+    }
+
+    // left.field1 == Some(1)
+    // right.field1 == Some(2)
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(Some(2), None),
+    );
+    test(
+        S::new(Some(1), Some(2)),
+        S::new(Some(1), None),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(Some(2), None),
+    );
+
+    // left.field1 == Some(1)
+    // right.field1 == None
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(None, None),
+    );
+    test(
+        S::new(Some(1), Some(2)),
+        S::new(Some(1), None),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(None, None),
+    );
+
+    // left.field1 == None
+    // right.field1 == Some(2)
+    test(
+        S::new(Some(2), Some(1)),
+        S::new(None, Some(1)),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(2), Some(1)),
+        S::new(None, Some(1)),
+        S::new(Some(2), None),
+    );
+    test(
+        S::new(Some(2), Some(2)),
+        S::new(None, None),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(2), None),
+        S::new(None, None),
+        S::new(Some(2), None),
+    );
+
+    // left.field1 == None
+    // right.field1 == None
+    test(
+        S::new(None, Some(1)),
+        S::new(None, Some(1)),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(None, Some(1)),
+        S::new(None, Some(1)),
+        S::new(None, None),
+    );
+    test(
+        S::new(None, Some(2)),
+        S::new(None, None),
+        S::new(None, Some(2)),
+    );
+    test(S::new(None, None), S::new(None, None), S::new(None, None));
+}
+
+#[test]
+fn test_unnamed_fields_skip() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(Option<usize>, #[merge(skip)] Option<usize>);
+
+    impl S {
+        pub fn new(field1: Option<usize>, field2: Option<usize>) -> S {
+            S(field1, field2)
+        }
+    }
+
+    // left.field1 == Some(1)
+    // right.field1 == Some(2)
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(Some(2), None),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(Some(2), None),
+    );
+
+    // left.field1 == Some(1)
+    // right.field1 == None
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(Some(1), Some(1)),
+        S::new(Some(1), Some(1)),
+        S::new(None, None),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(Some(1), None),
+        S::new(Some(1), None),
+        S::new(None, None),
+    );
+
+    // left.field1 == None
+    // right.field1 == Some(2)
+    test(
+        S::new(Some(2), Some(1)),
+        S::new(None, Some(1)),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(2), Some(1)),
+        S::new(None, Some(1)),
+        S::new(Some(2), None),
+    );
+    test(
+        S::new(Some(2), None),
+        S::new(None, None),
+        S::new(Some(2), Some(2)),
+    );
+    test(
+        S::new(Some(2), None),
+        S::new(None, None),
+        S::new(Some(2), None),
+    );
+
+    // left.field1 == None
+    // right.field1 == None
+    test(
+        S::new(None, Some(1)),
+        S::new(None, Some(1)),
+        S::new(None, Some(2)),
+    );
+    test(
+        S::new(None, Some(1)),
+        S::new(None, Some(1)),
+        S::new(None, None),
+    );
+    test(
+        S::new(None, None),
+        S::new(None, None),
+        S::new(None, Some(2)),
+    );
+    test(S::new(None, None), S::new(None, None), S::new(None, None));
+}
diff --git a/tests/impls.rs b/tests/impls.rs
new file mode 100644
index 0000000..d8378e5
--- /dev/null
+++ b/tests/impls.rs
@@ -0,0 +1,16 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+use merge::Merge;
+
+fn test<T: std::fmt::Debug + Merge + PartialEq>(expected: T, mut left: T, right: T) {
+    left.merge(right);
+    assert_eq!(expected, left);
+}
+
+#[test]
+fn test_option() {
+    test(Some(1), Some(1), Some(2));
+    test(Some(2), None, Some(2));
+    test(None::<usize>, None, None);
+}
diff --git a/tests/strategies.rs b/tests/strategies.rs
new file mode 100644
index 0000000..3a35ea2
--- /dev/null
+++ b/tests/strategies.rs
@@ -0,0 +1,122 @@
+// SPDX-FileCopyrightText: 2020 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: Apache-2.0 or MIT
+
+#![cfg(feature = "derive")]
+
+use merge::Merge;
+
+fn test<T: std::fmt::Debug + Merge + PartialEq>(expected: T, mut left: T, right: T) {
+    left.merge(right);
+    assert_eq!(expected, left);
+}
+
+#[test]
+fn test_bool_overwrite_false() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::bool::overwrite_false)] bool);
+
+    test(S(false), S(false), S(false));
+    test(S(true), S(false), S(true));
+    test(S(true), S(true), S(false));
+    test(S(true), S(true), S(true));
+}
+
+#[test]
+fn test_bool_overwrite_true() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::bool::overwrite_true)] bool);
+
+    test(S(false), S(false), S(false));
+    test(S(false), S(false), S(true));
+    test(S(false), S(true), S(false));
+    test(S(true), S(true), S(true));
+}
+
+#[cfg(feature = "num")]
+#[test]
+fn test_num_saturating_add() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::num::saturating_add)] u8);
+
+    test(S(0), S(0), S(0));
+    test(S(1), S(0), S(1));
+    test(S(255), S(255), S(10));
+    test(S(40), S(30), S(10));
+}
+
+#[cfg(feature = "num")]
+#[test]
+fn test_num_overwrite_zero() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::num::overwrite_zero)] u8);
+
+    test(S(0), S(0), S(0));
+    test(S(1), S(0), S(1));
+    test(S(255), S(255), S(10));
+}
+
+#[test]
+fn test_ord_max() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::ord::max)] u8);
+
+    test(S(2), S(1), S(2));
+    test(S(2), S(2), S(1));
+    test(S(2), S(2), S(2));
+    test(S(2), S(2), S(0));
+    test(S(2), S(0), S(2));
+    test(S(33), S(33), S(11));
+}
+
+#[test]
+fn test_ord_min() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::ord::min)] u8);
+
+    test(S(1), S(1), S(2));
+    test(S(1), S(2), S(1));
+    test(S(2), S(2), S(2));
+    test(S(0), S(2), S(0));
+    test(S(0), S(0), S(2));
+    test(S(11), S(33), S(11));
+}
+
+#[cfg(feature = "std")]
+#[test]
+fn test_vec_overwrite_empty() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::vec::overwrite_empty)] Vec<u8>);
+
+    test(S(vec![]), S(vec![]), S(vec![]));
+    test(S(vec![1]), S(vec![]), S(vec![1]));
+    test(S(vec![0]), S(vec![0]), S(vec![1]));
+    test(S(vec![255]), S(vec![255]), S(vec![10]));
+}
+
+#[cfg(feature = "std")]
+#[test]
+fn test_vec_append() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::vec::append)] Vec<u8>);
+
+    test(S(vec![]), S(vec![]), S(vec![]));
+    test(S(vec![1]), S(vec![]), S(vec![1]));
+    test(S(vec![0, 1]), S(vec![0]), S(vec![1]));
+    test(S(vec![255, 10]), S(vec![255]), S(vec![10]));
+    test(S(vec![0, 1, 2, 3, 4]), S(vec![0, 1, 2]), S(vec![3, 4]));
+    test(S(vec![3, 4, 0, 1, 2]), S(vec![3, 4]), S(vec![0, 1, 2]));
+}
+
+#[cfg(feature = "std")]
+#[test]
+fn test_vec_prepend() {
+    #[derive(Debug, Merge, PartialEq)]
+    struct S(#[merge(strategy = merge::vec::prepend)] Vec<u8>);
+
+    test(S(vec![]), S(vec![]), S(vec![]));
+    test(S(vec![1]), S(vec![]), S(vec![1]));
+    test(S(vec![1, 0]), S(vec![0]), S(vec![1]));
+    test(S(vec![10, 255]), S(vec![255]), S(vec![10]));
+    test(S(vec![3, 4, 0, 1, 2]), S(vec![0, 1, 2]), S(vec![3, 4]));
+    test(S(vec![0, 1, 2, 3, 4]), S(vec![3, 4]), S(vec![0, 1, 2]));
+}