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

This is causing assertion failures; reverting until I can fix.

This reverts commit r255267

llvm-svn: 255324

clang/lib/Sema/SemaDecl.cpp

@@ -12121,12 +12121,10 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
     // In C++, we need to do a redeclaration lookup to properly
     // diagnose some problems.
-    // 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 (getLangOpts().CPlusPlus) {
+      Previous.setRedeclarationKind(ForRedeclaration);
+      LookupQualifiedName(Previous, SearchDC);
+    }
   }
   // If we have a known previous declaration to use, then use it.

clang/test/Modules/Inputs/elaborated-type-structs.h

-struct S1;
-struct S2 { int x; };
-struct S3 { int x; };

clang/test/Modules/Inputs/module.map

@@ -386,10 +386,3 @@ module TypedefTag {
     header "typedef-tag-hidden.h"
   }
 }
-
-module ElaboratedTypeStructs {
-  module Empty {}
-  module Structs {
-    header "elaborated-type-structs.h"
-  }
-}

clang/test/Modules/elaborated-type-specifier-from-hidden-module.m

-// RUN: rm -rf %t
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs %s -verify
-
-@import ElaboratedTypeStructs.Empty; // The structs are now hidden.
-struct S1 *x;
-struct S2 *y;
-// FIXME: compatible definition should not be an error.
-struct S2 { int x; }; // expected-error {{redefinition}}
-struct S3 *z;
-// Incompatible definition.
-struct S3 { float y; }; // expected-error {{redefinition}}
-// expected-note@elaborated-type-structs.h:* 2 {{previous definition is here}}
-
-@import ElaboratedTypeStructs.Structs;
-
-void useS1(struct S1 *x);
-void useS2(struct S2 *x);
-void useS2(struct S2 *x);