4 files changed, 34 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.
diff --git a/clang/test/Modules/Inputs/elaborated-type-structs.h b/clang/test/Modules/Inputs/elaborated-type-structs.h
new file mode 100644
index 00000000000..da3940998fd
--- /dev/null
+++ b/clang/test/Modules/Inputs/elaborated-type-structs.h
@@ -0,0 +1,3 @@
+struct S1;
+struct S2 { int x; };
+struct S3 { int x; };
diff --git a/clang/test/Modules/Inputs/module.map b/clang/test/Modules/Inputs/module.map
index 226d45fdf44..632517dd363 100644
--- a/clang/test/Modules/Inputs/module.map
+++ b/clang/test/Modules/Inputs/module.map
@@ -386,3 +386,10 @@ module TypedefTag {
     header "typedef-tag-hidden.h"
   }
 }
+
+module ElaboratedTypeStructs {
+  module Empty {}
+  module Structs {
+    header "elaborated-type-structs.h"
+  }
+}
diff --git a/clang/test/Modules/elaborated-type-specifier-from-hidden-module.m b/clang/test/Modules/elaborated-type-specifier-from-hidden-module.m
new file mode 100644
index 00000000000..0ca1c24bba0
--- /dev/null
+++ b/clang/test/Modules/elaborated-type-specifier-from-hidden-module.m
@@ -0,0 +1,18 @@
+// 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);