Fix -Wmismatched-tags to not warn on redeclarations of structs in system

headers.

Previously, we would only check whether the new declaration is in a
system header, but that requires the user to be able to correctly guess
whether a declaration in a system header is declared as a struct or a
class when specializing standard library traits templates.

We now entirely ignore declarations for which the warning was disabled
when determining whether to warn on a tag mismatch.

Also extend the diagnostic message to clarify that
 a) code containing such a tag mismatch is in fact valid and correct,
    and
 b) the (non-coding-style) reason to emit such a warning is that the
    Microsoft C++ ABI is broken and includes the tag kind in decorated
    names,
as it seems a lot of users are confused by our diagnostic here (either
not understanding why we produce it, or believing that it represents an
actual language rule).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@348233 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 8 deletions

diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index c725da9752..1a6d0d035b 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -4806,16 +4806,18 @@ def err_nested_redefinition : Error<"nested redefinition of %0">;
 def err_use_with_wrong_tag : Error<
   "use of %0 with tag type that does not match previous declaration">;
 def warn_struct_class_tag_mismatch : Warning<
-    "%select{struct|interface|class}0%select{| template}1 %2 was previously "
-    "declared as a %select{struct|interface|class}3%select{| template}1">,
-    InGroup<MismatchedTags>, DefaultIgnore;
+  "%select{struct|interface|class}0%select{| template}1 %2 was previously "
+  "declared as a %select{struct|interface|class}3%select{| template}1; "
+  "this is valid, but may result in linker errors under the Microsoft C++ ABI">,
+  InGroup<MismatchedTags>, DefaultIgnore;
 def warn_struct_class_previous_tag_mismatch : Warning<
-    "%2 defined as %select{a struct|an interface|a class}0%select{| template}1 "
-    "here but previously declared as "
-    "%select{a struct|an interface|a class}3%select{| template}1">,
-     InGroup<MismatchedTags>, DefaultIgnore;
+  "%2 defined as %select{a struct|an interface|a class}0%select{| template}1 "
+  "here but previously declared as "
+  "%select{a struct|an interface|a class}3%select{| template}1; "
+  "this is valid, but may result in linker errors under the Microsoft C++ ABI">,
+  InGroup<MismatchedTags>, DefaultIgnore;
 def note_struct_class_suggestion : Note<
-    "did you mean %select{struct|interface|class}0 here?">;
+  "did you mean %select{struct|interface|class}0 here?">;
 def ext_forward_ref_enum : Extension<
   "ISO C forbids forward references to 'enum' types">;
 def err_forward_ref_enum : Error<