[C++20] [Modules] Warn if we found #include <filename> in module purview

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -922,6 +922,11 @@ def err_header_import_semi_in_macro : Error<
 def err_header_import_not_header_unit : Error<
   "header file %0 (aka '%1') cannot be imported because "
   "it is not known to be a header unit">;
+def warn_pp_include_angled_in_module_purview : Warning<
+  "'#include <filename>' attaches the declarations to the named module '%0'"
+  ", which is not usually intended; consider moving that directive before "
+  "the module declaration">,
+  InGroup<DiagGroup<"include-angled-in-module-purview">>;
 
 def warn_header_guard : Warning<
   "%0 is used as a header guard here, followed by #define of a different macro">,

clang/lib/Lex/PPDirectives.cpp

@@ -2530,6 +2530,10 @@ Preprocessor::ImportAction Preprocessor::HandleHeaderIncludeOrImport(
     return {ImportAction::None};
   }
 
+  if (isAngled && isInNamedModule())
+    Diag(FilenameTok, diag::warn_pp_include_angled_in_module_purview)
+        << getNamedModuleName();
+
   // Look up the file, create a File ID for it.
   SourceLocation IncludePos = FilenameTok.getLocation();
   // If the filename string was the result of macro expansions, set the include

clang/test/Preprocessor/include-in-module-purview.cppm

@@ -0,0 +1,60 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -E -P -I%t -o %t/tmp 2>&1 | FileCheck %t/a.cppm
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -E -P -I%t -o - 2>&1 \
+// RUN:     -Wno-include-angled-in-module-purview | FileCheck %t/a.cppm --check-prefix=CHECK-NO-WARN
+
+//--- a.h
+// left empty
+
+//--- b.h
+#include <stddef.h>
+// The headers not get included shouldn't be affected.
+#ifdef WHATEVER
+#include <stdint.h>
+#endif
+
+//--- a.cppm
+module;
+#include <stddef.h>
+#include <a.h>
+#include <b.h>
+#include "a.h"
+#include "b.h"
+export module a;
+
+#include <stddef.h>
+#include <a.h>
+#include <b.h>
+#include "a.h"
+#include "b.h"
+
+// CHECK: a.cppm:9:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: a.cppm:10:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: a.cppm:11:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: In file included from {{.*}}/a.cppm:11
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: In file included from {{.*}}/a.cppm:13
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+
+module :private;
+#include <stddef.h>
+#include <a.h>
+#include <b.h>
+#include "a.h"
+#include "b.h"
+
+// CHECK: a.cppm:24:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: a.cppm:25:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: a.cppm:26:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: In file included from {{.*}}/a.cppm:26
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+// CHECK: In file included from {{.*}}/a.cppm:28
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' attaches the declarations to the named module 'a'
+
+// We should have catched all warnings.
+// CHECK: 10 warnings generated.
+
+// CHECK-NO-WARN-NOT: warning