[Modules] Fix regression when an elaborated-type-specifier mentions a hidden tag

This makes non-C++ languages find the same decl as C++ does to
workaround a regression introduced in r252960.

rdar://problem/23784203

llvm-svn: 255267

1 files changed, 6 insertions, 4 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index af5fdc95c90..1d8f9b7d203 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -12121,10 +12121,12 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
 
     // In C++, we need to do a redeclaration lookup to properly
     // diagnose some problems.
-    if (getLangOpts().CPlusPlus) {
-      Previous.setRedeclarationKind(ForRedeclaration);
-      LookupQualifiedName(Previous, SearchDC);
-    }
+    // FIXME: this lookup is also used (with and without C++) to find a hidden
+    // declaration so that we don't get ambiguity errors when using a type
+    // declared by an elaborated-type-specifier.  In C that is not correct and
+    // we should instead merge compatible types found by lookup.
+    Previous.setRedeclarationKind(ForRedeclaration);
+    LookupQualifiedName(Previous, SearchDC);
   }
 
   // If we have a known previous declaration to use, then use it.