Permit link decls in submodule declarations.

Author: qiongsiwu

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -915,8 +915,10 @@ def err_mmap_expected_attribute : Error<"expected an attribute name">;
 def warn_mmap_link_redeclaration : Warning<"redeclaration of link library '%0'">,
   InGroup<DiagGroup<"module-link-redeclaration">>, DefaultError;
 def note_mmap_prev_link_declaration : Note<"previously declared here">;
-def err_mmap_submodule_link_decl
-    : Error<"link declaration is not allowed in submodules">;
+def warn_mmap_submodule_link_decl
+    : Warning<"link declaration is not allowed in submodules">,
+      InGroup<DiagGroup<"module-submodule-link-decl">>,
+      DefaultError;
 def warn_mmap_unknown_attribute : Warning<"unknown attribute '%0'">,
   InGroup<IgnoredAttributes>;
 def warn_mmap_mismatched_private_submodule : Warning<

clang/lib/Lex/ModuleMapFile.cpp

@@ -856,18 +856,14 @@ std::optional<LinkDecl> ModuleMapFileParser::parseLinkDecl(
   // Make sure we eat all the tokens when we report the errors so parsing
   // can continue.
   if (!Allowed) {
-    Diags.Report(LD.Location, diag::err_mmap_submodule_link_decl);
-    HadError = true;
-    return std::nullopt;
+    Diags.Report(LD.Location, diag::warn_mmap_submodule_link_decl);
   }
 
   auto [It, Inserted] =
       SeenLinkDecl.insert(std::make_pair(Library, LD.Location));
   if (!Inserted) {
     Diags.Report(LD.Location, diag::warn_mmap_link_redeclaration) << Library;
     Diags.Report(It->second, diag::note_mmap_prev_link_declaration);
-    HadError = true;
-    return std::nullopt;
   }
 
   return std::move(LD);

clang/test/ClangScanDeps/link-libraries-diag-dup.c

@@ -51,7 +51,7 @@ module C {
 
 // Note that module D does not report an error because it is explicit.
 // Therefore we can use CHECK-NEXT for the redeclaration error on line 15.
-// CHECK:      module.modulemap:6:5:  error: link declaration is not allowed in submodules
+// CHECK:      module.modulemap:6:5:  error: link declaration is not allowed in submodules [-Wmodule-submodule-link-decl]
 // CHECK-NEXT: module.modulemap:15:3: error: redeclaration of link library 'libraryA' [-Wmodule-link-redeclaration]
 // CHECK-NEXT: module.modulemap:14:3: note: previously declared here
 // CHECK-NOT:  module.modulemap:20:3: error: redeclaration of link library 'libraryA'