Downgrade reserved module identifier error into a warning

Any project that wants to import std; potentially needs to be able to
build a module that does export std;. We silenced the error diagnostic
if the module identified itself as a system header, but this isn't
quite good enough, what we really need is a way to identify a system
module. It would be nice for that feature to be shared among the major
implementations, so this downgrades the diagnostic from an error to a
warning temporarily to give implementers time to determine what that
mechanism will look like. We may convert this warning back into an
error in a future release of Clang, but it's not guaranteed we will do
so.

Fixes https://github.com/llvm/llvm-project/issues/61446
Differential Revision: https://reviews.llvm.org/D146986

1 files changed, 9 insertions, 6 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index 8c120d2..40ebc9c 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -156,11 +156,15 @@ static bool DiagReservedModuleName(Sema &S, const IdentifierInfo *II,
   if (Reason == Reserved && S.getSourceManager().isInSystemHeader(Loc))
     Reason = Valid;
 
-  if (Reason != Valid) {
-    S.Diag(Loc, diag::err_invalid_module_name) << II << (int)Reason;
-    return true;
+  switch (Reason) {
+  case Valid:
+    return false;
+  case Invalid:
+    return S.Diag(Loc, diag::err_invalid_module_name) << II;
+  case Reserved:
+    return S.Diag(Loc, diag::warn_reserved_module_name) << II;
   }
-  return false;
+  llvm_unreachable("fell off a fully covered switch");
 }
 
 Sema::DeclGroupPtrTy
@@ -264,8 +268,7 @@ Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
       (FirstComponentName == "std" ||
        (FirstComponentName.startswith("std") &&
         llvm::all_of(FirstComponentName.drop_front(3), &llvm::isDigit)))) {
-    Diag(Path[0].second, diag::err_invalid_module_name)
-        << Path[0].first << /*reserved*/ 1;
+    Diag(Path[0].second, diag::warn_reserved_module_name) << Path[0].first;
     return nullptr;
   }