Constexpr matching support in IsEmpty and SizeIs matchers

docs/matchers.md

@@ -491,6 +491,15 @@ TEST_CASE("Constexpr support for matchers", "[constexpr][matchers]") {
 }
 ```
 
+### First party constexpr matchers
+
+Some (but not all) of Catch2's generic matchers support `constexpr`
+matching. Currently, this includes:
+
+* `IsEmpty()`
+* `SizeIs(size_t target_size)`
+* `SizeIs(Matcher size_matcher)`
+
 
 ---
 

src/catch2/matchers/catch_matchers_container_properties.cpp

@@ -22,13 +22,5 @@ namespace Matchers {
         return sstr.str();
     }
 
-    IsEmptyMatcher IsEmpty() {
-        return {};
-    }
-
-    HasSizeMatcher SizeIs(std::size_t sz) {
-        return HasSizeMatcher{ sz };
-    }
-
 } // end namespace Matchers
 } // end namespace Catch

src/catch2/matchers/catch_matchers_container_properties.hpp

@@ -18,7 +18,7 @@ namespace Catch {
         class IsEmptyMatcher final : public MatcherGenericBase {
         public:
             template <typename RangeLike>
-            bool match(RangeLike&& rng) const {
+            constexpr bool match(RangeLike&& rng) const {
 #if defined(CATCH_CONFIG_POLYFILL_NONMEMBER_CONTAINER_ACCESS)
                 using Catch::Detail::empty;
 #else
@@ -33,12 +33,12 @@ namespace Catch {
         class HasSizeMatcher final : public MatcherGenericBase {
             std::size_t m_target_size;
         public:
-            explicit HasSizeMatcher(std::size_t target_size):
+            constexpr explicit HasSizeMatcher(std::size_t target_size):
                 m_target_size(target_size)
             {}
 
             template <typename RangeLike>
-            bool match(RangeLike&& rng) const {
+            constexpr bool match(RangeLike&& rng) const {
 #if defined(CATCH_CONFIG_POLYFILL_NONMEMBER_CONTAINER_ACCESS)
                 using Catch::Detail::size;
 #else
@@ -54,12 +54,12 @@ namespace Catch {
         class SizeMatchesMatcher final : public MatcherGenericBase {
             Matcher m_matcher;
         public:
-            explicit SizeMatchesMatcher(Matcher m):
+            constexpr explicit SizeMatchesMatcher(Matcher m):
                 m_matcher(CATCH_MOVE(m))
             {}
 
             template <typename RangeLike>
-            bool match(RangeLike&& rng) const {
+            constexpr bool match(RangeLike&& rng) const {
 #if defined(CATCH_CONFIG_POLYFILL_NONMEMBER_CONTAINER_ACCESS)
                 using Catch::Detail::size;
 #else
@@ -75,11 +75,13 @@ namespace Catch {
 
 
         //! Creates a matcher that accepts empty ranges/containers
-        IsEmptyMatcher IsEmpty();
+        inline CATCH_DESTRUCTOR_CONSTEXPR IsEmptyMatcher IsEmpty() { return {}; }
         //! Creates a matcher that accepts ranges/containers with specific size
-        HasSizeMatcher SizeIs(std::size_t sz);
+        inline CATCH_DESTRUCTOR_CONSTEXPR HasSizeMatcher SizeIs( std::size_t sz ) {
+            return HasSizeMatcher{ sz };
+        }
         template <typename Matcher>
-        std::enable_if_t<Detail::is_matcher_v<Matcher>,
+        constexpr std::enable_if_t<Detail::is_matcher_v<Matcher>,
         SizeMatchesMatcher<Matcher>> SizeIs(Matcher&& m) {
             return SizeMatchesMatcher<Matcher>{CATCH_FORWARD(m)};
         }

tests/SelfTest/UsageTests/MatchersConstexpr.tests.cpp

@@ -8,6 +8,9 @@
 
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/matchers/catch_matchers_templated.hpp>
+#include <catch2/matchers/catch_matchers_container_properties.hpp>
+
+#include <array>
 
 #if defined( CATCH_INTERNAL_CONSTEXPR_MATCHERS_ENABLED )
 
@@ -34,6 +37,15 @@ TEST_CASE( "Constexpr support for matchers", "[constexpr][matchers][approvals]"
     STATIC_REQUIRE_THAT( 1, MatchAll() );
 }
 
+TEST_CASE( "IsEmpty and HasSize matchers can be used in constexpr contexts",
+           "[constexpr][matchers][approvals]" ){
+    static constexpr std::array<int, 0> empty{};
+    STATIC_REQUIRE_THAT( empty, Catch::Matchers::IsEmpty() );
+    static constexpr int arr[1] = { 2 };
+    STATIC_REQUIRE_THAT( arr, Catch::Matchers::SizeIs( 1 ) );
+    STATIC_REQUIRE_THAT( arr, Catch::Matchers::SizeIs( MatchAll() ) );
+}
+
 // Combining matchers needs C++26 and P2738, so they are in separate preprocessor block
 #    if __cpp_constexpr >= 202306L