[clang] [C++20] [Modules] Warn if we found #include <filename> in module purview (PR #69555)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 created https://github.com/llvm/llvm-project/pull/69555

Close https://github.com/llvm/llvm-project/issues/68615.

It is generally wrong to include <filename> in the module purview. Although there are cases to include files in the module purview, generally these use cases should include files by quotes instead of by angles. Here we think the files got included by angles are the system headers.

This is consistency with MSVC too: https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warnings-by-compiler-version?view=msvc-170#warnings-introduced-in-visual-studio-2022-version-170-compiler-version-1930

>From 95427d21575ab912b63292b7811bbedd83ccf8b8 Mon Sep 17 00:00:00 2001
From: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
Date: Thu, 19 Oct 2023 11:28:01 +0800
Subject: [PATCH] [C++20] [Modules] Warn if we found #include <filename> in
 module purview

Address https://github.com/llvm/llvm-project/issues/68615.

It is generally wrong to include <filename> in the module purview.
Although there are cases to include files in the module purview,
generally these use cases should include files by quotes instead of by
angles. Here we think the files got included by angles are the system
headers.
---
 .../include/clang/Basic/DiagnosticLexKinds.td |  4 ++
 clang/lib/Lex/PPDirectives.cpp                |  3 +
 .../include-in-module-purview.cppm            | 60 +++++++++++++++++++
 3 files changed, 67 insertions(+)
 create mode 100644 clang/test/Preprocessor/include-in-module-purview.cppm

diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
index 940cca67368492f..5d96bcb9359951f 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -922,6 +922,10 @@ 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>' in the module purview appears to be erroneous; "
+  "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">,
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 2892d4b777846ff..243f7a729681ce9 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -2530,6 +2530,9 @@ Preprocessor::ImportAction Preprocessor::HandleHeaderIncludeOrImport(
     return {ImportAction::None};
   }
 
+  if (isAngled && isInNamedModule())
+    Diag(FilenameTok, diag::warn_pp_include_angled_in_module_purview);
+
   // 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
diff --git a/clang/test/Preprocessor/include-in-module-purview.cppm b/clang/test/Preprocessor/include-in-module-purview.cppm
new file mode 100644
index 000000000000000..a06088b3b408fbc
--- /dev/null
+++ b/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>' in the module purview is probably incorrect
+// CHECK: a.cppm:10:10: warning: '#include <filename>' in the module purview is probably incorrect
+// CHECK: a.cppm:11:10: warning: '#include <filename>' in the module purview is probably incorrect
+// CHECK: In file included from {{.*}}/a.cppm:11
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' in the module purview is probably incorrect; 
+// CHECK: In file included from {{.*}}/a.cppm:13
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' in the module purview is probably incorrect; 
+
+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>' in the module purview is probably incorrect;
+// CHECK: a.cppm:25:10: warning: '#include <filename>' in the module purview is probably incorrect
+// CHECK: a.cppm:26:10: warning: '#include <filename>' in the module purview is probably incorrect
+// CHECK: In file included from {{.*}}/a.cppm:26
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' in the module purview is probably incorrect; 
+// CHECK: In file included from {{.*}}/a.cppm:28
+// CHECK-NEXT: b.h:1:10: warning: '#include <filename>' in the module purview is probably incorrect; 
+
+// We should have catched all warnings.
+// CHECK: 10 warnings generated.
+
+// CHECK-NO-WARN-NOT: warning