[C++20] [Modules] Set the feature testing macro to 1

[ChuanqiXu9]

See #71364 for details.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Chuanqi Xu (ChuanqiXu9)

Changes

See #71364 for details.

---

Full diff: https://github.com/llvm/llvm-project/pull/161034.diff

2 Files Affected:

• (modified) clang/lib/Frontend/InitPreprocessor.cpp (+4-1)
• (modified) clang/test/Lexer/cxx-features.cpp (+1-1)

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index edf0a091e087c..877ab02850667 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -742,7 +742,10 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_impl_coroutine", "201902L");
     Builder.defineMacro("__cpp_designated_initializers", "201707L");
     Builder.defineMacro("__cpp_impl_three_way_comparison", "201907L");
-    //Builder.defineMacro("__cpp_modules", "201907L");
+    // Intentionally to set __cpp_modules to 1.
+    // See https://github.com/llvm/llvm-project/issues/71364 for details.
+    // Builder.defineMacro("__cpp_modules", "201907L");
+    Builder.defineMacro("__cpp_modules", "1");
     Builder.defineMacro("__cpp_using_enum", "201907L");
   }
   // C++23 features.
diff --git a/clang/test/Lexer/cxx-features.cpp b/clang/test/Lexer/cxx-features.cpp
index ced5bcaf0db16..8eb9ea032879c 100644
--- a/clang/test/Lexer/cxx-features.cpp
+++ b/clang/test/Lexer/cxx-features.cpp
@@ -148,7 +148,7 @@
 
 // init_captures checked below
 
-#if check(modules, 0, 0, 0, 0, 0, 0, 0)
+#if check(modules, 0, 0, 0, 0, 1, 1, 1)
 // FIXME: 201907 in C++20
 #error "wrong value for __cpp_modules"
 #endif

[zygoloid]

This seems like a reasonable compromise to me.

[shafik]

I am a bit uncomfortable just putting a link w/o at least a TL;DR; giving a high level explanation here.