From ed3c184edaa94f898d25eb4d69e635cbd6995369 Mon Sep 17 00:00:00 2001
From: Asa Hammond <asa@automaton.is>
Date: Mon, 19 Feb 2024 09:52:38 -0800
Subject: [PATCH] make match constexpr

---
 BUILD          |  1 +
 lager/util.hpp | 15 +++++++++------
 test/util.cpp  |  8 ++++++++
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/BUILD b/BUILD
index 90307081..ade01d7e 100644
--- a/BUILD
+++ b/BUILD
@@ -11,6 +11,7 @@ cc_library(
         "@boost//:intrusive_ptr",
         "@zug//:zug",
         "@cereal//:cereal",
+        "@immer",
     ],
     includes = [".", "lager/"],
     visibility = ["//visibility:public"],
diff --git a/lager/util.hpp b/lager/util.hpp
index d09339b8..7fe9f92c 100644
--- a/lager/util.hpp
+++ b/lager/util.hpp
@@ -55,7 +55,7 @@ template <typename TupleT>
 struct matcher : TupleT
 {
     template <typename... Visitors>
-    decltype(auto) operator()(Visitors&&... vs) &&
+    constexpr decltype(auto) operator()(Visitors&&... vs) &&
     {
         return std::apply(
             [&](auto&&... xs) {
@@ -70,7 +70,7 @@ template <typename T>
 matcher(T) -> matcher<T>;
 
 template <typename... Ts>
-decltype(auto) as_variant(std::variant<Ts...> const& x)
+constexpr decltype(auto) as_variant(std::variant<Ts...> const& x)
 {
     return x;
 }
@@ -109,7 +109,8 @@ constexpr bool is_variant_v = is_variant<T>::value;
  * value is forwarded as a variant.
  */
 template <typename T>
-auto forward_variant(typename std::remove_reference<T>::type& v) noexcept
+constexpr auto
+forward_variant(typename std::remove_reference<T>::type& v) noexcept
     -> std::enable_if_t<is_variant_v<T>,
                         zug::detail::copy_decay_t<T, get_variant_t<T>>&&>
 {
@@ -117,7 +118,8 @@ auto forward_variant(typename std::remove_reference<T>::type& v) noexcept
 }
 
 template <typename T>
-auto forward_variant(typename std::remove_reference<T>::type& v) noexcept
+constexpr auto
+forward_variant(typename std::remove_reference<T>::type& v) noexcept
     -> std::enable_if_t<!is_variant_v<T>, T&&>
 {
     static_assert(is_variant<T>::value,
@@ -147,7 +149,7 @@ auto forward_variant(typename std::remove_reference<T>::type& v) noexcept
  * @endcode
  */
 template <typename... Variants>
-auto match(Variants&&... vs)
+constexpr auto match(Variants&&... vs)
 {
     return detail::matcher{
         std::forward_as_tuple(forward_variant<Variants>(vs)...)};
@@ -158,7 +160,8 @@ ZUG_INLINE_CONSTEXPR struct noop_t
 {
     template <typename... T>
     void operator()(T&&...) const
-    {}
+    {
+    }
 } noop{};
 
 //! Function that returns its first arguemnt
diff --git a/test/util.cpp b/test/util.cpp
index 839e58e7..baa68bb6 100644
--- a/test/util.cpp
+++ b/test/util.cpp
@@ -36,3 +36,11 @@ TEST_CASE("match deriv")
     auto y = lager::match(v)([](int x) { return x; }, [](auto) { return 0; });
     CHECK(y == 42);
 }
+
+TEST_CASE("match deriv constexpr ")
+{
+    constexpr auto v = deriv_t{42};
+    constexpr auto y =
+        lager::match(v)([](int x) { return x; }, [](auto) { return 0; });
+    CHECK(y == 42);
+}