Support decomposing types that only compare with literal 0

This is primarily done to support new `std::*_ordering` types,
but the refactoring also supports any other type with this
property.

The compilation overhead is surprisingly low. Testing it with
clang on a Linux machine, compiling our SelfTest project takes
only 2-3% longer with these changes than it takes otherwise.

Closes #2555

4 files changed, 65 insertions, 30 deletions

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index c6f93d96..b64ecd08 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -133,6 +133,7 @@ set(TEST_SOURCES
 
 set(TEST_HEADERS
   ${SELF_TEST_DIR}/helpers/parse_test_spec.hpp
+  ${SELF_TEST_DIR}/helpers/type_with_lit_0_comparisons.hpp
 )
 
 
diff --git a/tests/SelfTest/IntrospectiveTests/Traits.tests.cpp b/tests/SelfTest/IntrospectiveTests/Traits.tests.cpp
index 35d30c76..459e0d48 100644
--- a/tests/SelfTest/IntrospectiveTests/Traits.tests.cpp
+++ b/tests/SelfTest/IntrospectiveTests/Traits.tests.cpp
@@ -8,36 +8,7 @@
 
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/internal/catch_compare_traits.hpp>
-
-// Should only be constructible from literal 0.
-// Used by `TypeWithLit0Comparisons` for testing comparison
-// ops that only work with literal zero, the way std::*orderings do
-struct ZeroLiteralDetector {
-    constexpr ZeroLiteralDetector( ZeroLiteralDetector* ) noexcept {}
-
-    template <typename T,
-              typename = std::enable_if_t<!std::is_same<T, int>::value>>
-    constexpr ZeroLiteralDetector( T ) = delete;
-};
-
-struct TypeWithLit0Comparisons {
-#define DEFINE_COMP_OP( op )                                                  \
-    friend bool operator op( TypeWithLit0Comparisons, ZeroLiteralDetector ) { \
-        return true;                                                          \
-    }                                                                         \
-    friend bool operator op( ZeroLiteralDetector, TypeWithLit0Comparisons ) { \
-        return false;                                                          \
-    }
-
-    DEFINE_COMP_OP( < )
-    DEFINE_COMP_OP( <= )
-    DEFINE_COMP_OP( > )
-    DEFINE_COMP_OP( >= )
-    DEFINE_COMP_OP( == )
-    DEFINE_COMP_OP( != )
-
-#undef DEFINE_COMP_OP
-};
+#include <helpers/type_with_lit_0_comparisons.hpp>
 
 
 #define ADD_TRAIT_TEST_CASE( op )                                         \
diff --git a/tests/SelfTest/UsageTests/Compilation.tests.cpp b/tests/SelfTest/UsageTests/Compilation.tests.cpp
index 5998ce62..a61a9674 100644
--- a/tests/SelfTest/UsageTests/Compilation.tests.cpp
+++ b/tests/SelfTest/UsageTests/Compilation.tests.cpp
@@ -6,6 +6,8 @@
 
 // SPDX-License-Identifier: BSL-1.0
 
+#include <helpers/type_with_lit_0_comparisons.hpp>
+
 #include <type_traits>
 
 // Setup for #1403 -- look for global overloads of operator << for classes
@@ -310,3 +312,20 @@ TEST_CASE("ADL universal operators don't hijack expression deconstruction", "[co
     REQUIRE(0 & adl::always_true{});
     REQUIRE(0 ^ adl::always_true{});
 }
+
+TEST_CASE( "#2555 - types that can only be compared with 0 literal (not int/long) are supported", "[compilation][approvals]" ) {
+    REQUIRE( TypeWithLit0Comparisons{} < 0 );
+    REQUIRE_FALSE( 0 < TypeWithLit0Comparisons{} );
+    REQUIRE( TypeWithLit0Comparisons{} <= 0 );
+    REQUIRE_FALSE( 0 > TypeWithLit0Comparisons{} );
+
+    REQUIRE( TypeWithLit0Comparisons{} > 0 );
+    REQUIRE_FALSE( 0 > TypeWithLit0Comparisons{} );
+    REQUIRE( TypeWithLit0Comparisons{} >= 0 );
+    REQUIRE_FALSE( 0 >= TypeWithLit0Comparisons{} );
+
+    REQUIRE( TypeWithLit0Comparisons{} == 0 );
+    REQUIRE_FALSE( 0 == TypeWithLit0Comparisons{} );
+    REQUIRE( TypeWithLit0Comparisons{} != 0 );
+    REQUIRE_FALSE( 0 != TypeWithLit0Comparisons{} );
+}
diff --git a/tests/SelfTest/helpers/type_with_lit_0_comparisons.hpp b/tests/SelfTest/helpers/type_with_lit_0_comparisons.hpp
new file mode 100644
index 00000000..202c3af4
--- /dev/null
+++ b/tests/SelfTest/helpers/type_with_lit_0_comparisons.hpp
@@ -0,0 +1,44 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+#ifndef CATCH_TEST_HELPERS_TYPE_WITH_LIT_0_COMPARISONS_HPP_INCLUDED
+#define CATCH_TEST_HELPERS_TYPE_WITH_LIT_0_COMPARISONS_HPP_INCLUDED
+
+#include <type_traits>
+
+// Should only be constructible from literal 0.
+// Used by `TypeWithLit0Comparisons` for testing comparison
+// ops that only work with literal zero, the way std::*orderings do
+struct ZeroLiteralDetector {
+    constexpr ZeroLiteralDetector( ZeroLiteralDetector* ) noexcept {}
+
+    template <typename T,
+              typename = std::enable_if_t<!std::is_same<T, int>::value>>
+    constexpr ZeroLiteralDetector( T ) = delete;
+};
+
+struct TypeWithLit0Comparisons {
+#define DEFINE_COMP_OP( op )                                                  \
+    friend bool operator op( TypeWithLit0Comparisons, ZeroLiteralDetector ) { \
+        return true;                                                          \
+    }                                                                         \
+    friend bool operator op( ZeroLiteralDetector, TypeWithLit0Comparisons ) { \
+        return false;                                                         \
+    }
+
+    DEFINE_COMP_OP( < )
+    DEFINE_COMP_OP( <= )
+    DEFINE_COMP_OP( > )
+    DEFINE_COMP_OP( >= )
+    DEFINE_COMP_OP( == )
+    DEFINE_COMP_OP( != )
+
+#undef DEFINE_COMP_OP
+};
+
+#endif // CATCH_TEST_HELPERS_TYPE_WITH_LIT_0_COMPARISONS_HPP_INCLUDED