feat(3p/nix/libutil): Add util::overloaded class for std::visitor r/1277

Implements the fairly common lambda overload class used for std::visit
over variants and other things that require groups of callables.

Change-Id: Ia7448b7e1bd349b4909974758e6e6303a80d86d7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1137
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: edef <edef@edef.eu>

2 files changed, 20 insertions, 0 deletions

diff --git a/third_party/nix/src/libutil/CMakeLists.txt b/third_party/nix/src/libutil/CMakeLists.txt
index 43b9e8525f78..a9736b37bc05 100644
--- a/third_party/nix/src/libutil/CMakeLists.txt
+++ b/third_party/nix/src/libutil/CMakeLists.txt
@@ -25,6 +25,7 @@ set(HEADER_FILES
     thread-pool.hh
     types.hh
     util.hh
+    visitor.hh
     xml-writer.hh
 )
 
diff --git a/third_party/nix/src/libutil/visitor.hh b/third_party/nix/src/libutil/visitor.hh
new file mode 100644
index 000000000000..bf1d665af77f
--- /dev/null
+++ b/third_party/nix/src/libutil/visitor.hh
@@ -0,0 +1,19 @@
+#pragma once
+
+namespace nix::util {
+
+// Helper class used for visiting std::variants by creating a variadic
+// list of lambda expressions that delegates calls to each of the
+// callables.
+//
+// See e.g.
+// https://dev.to/tmr232/that-overloaded-trick-overloading-lambdas-in-c17
+template <class... Ts>
+struct overloaded : Ts... {
+  using Ts::operator()...;
+};
+
+template <class... Ts>
+overloaded(Ts...) -> overloaded<Ts...>;
+
+}  // namespace nix::util