Everywhere: Make Lagom build with GCC 13

GCC 13 was released on 2023-04-26. This commit fixes Lagom build errors
when using an updated host toolchain:
- Adds a workaround for a bug in constraint handling, which made LibJS
  fail to compile: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109683
- Silences the new `-Wdangling-reference` diagnostic globally. It
  produces multiple false positives with no clear way to silence them
  without `#pragmas`.
- Silences `-Wself-move` in `RefPtr` tests as GCC 13 adds this
  previously Clang-exclusive warning.

Author: BertalanD

AK/Variant.h

@@ -130,15 +130,16 @@ struct VariantConstructTag {
 
 template<typename T, typename Base>
 struct VariantConstructors {
+    // The pointless `typename Base` constraints are a workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109683
     ALWAYS_INLINE VariantConstructors(T&& t)
-    requires(requires { T(move(t)); })
+    requires(requires { T(move(t)); typename Base; })
     {
         internal_cast().clear_without_destruction();
         internal_cast().set(move(t), VariantNoClearTag {});
     }
 
     ALWAYS_INLINE VariantConstructors(T const& t)
-    requires(requires { T(t); })
+    requires(requires { T(t); typename Base; })
     {
         internal_cast().clear_without_destruction();
         internal_cast().set(t, VariantNoClearTag {});

Meta/CMake/common_compile_options.cmake

@@ -28,4 +28,7 @@ elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     # Only ignore expansion-to-defined for g++, clang's implementation doesn't complain about function-like macros
     add_compile_options(-Wno-expansion-to-defined)
     add_compile_options(-Wno-literal-suffix)
+
+    # FIXME: This warning seems useful but has too many false positives with GCC 13.
+    add_compile_options(-Wno-dangling-reference)
 endif()

Tests/AK/TestRefPtr.cpp

@@ -97,14 +97,11 @@ TEST_CASE(assign_moved_self)
 {
     RefPtr<Object> object = adopt_ref(*new Object);
     EXPECT_EQ(object->ref_count(), 1u);
-#if defined(AK_COMPILER_CLANG)
-#    pragma clang diagnostic push
-#    pragma clang diagnostic ignored "-Wself-move"
-#endif
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpragmas"
+#pragma GCC diagnostic ignored "-Wself-move"
     object = move(object);
-#if defined(AK_COMPILER_CLANG)
-#    pragma clang diagnostic pop
-#endif
+#pragma GCC diagnostic pop
     EXPECT_EQ(object->ref_count(), 1u);
 }