From 698885113770ae8b407170a211719dc6638c6c1b Mon Sep 17 00:00:00 2001
From: Ben Langmuir <blangmuir@apple.com>
Date: Thu, 10 Dec 2015 17:28:51 +0000
Subject: [PATCH] [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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@255267 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Sema/SemaDecl.cpp                          | 10 ++++++----
 test/Modules/Inputs/elaborated-type-structs.h  |  3 +++
 test/Modules/Inputs/module.map                 |  7 +++++++
 ...borated-type-specifier-from-hidden-module.m | 18 ++++++++++++++++++
 4 files changed, 34 insertions(+), 4 deletions(-)
 create mode 100644 test/Modules/Inputs/elaborated-type-structs.h
 create mode 100644 test/Modules/elaborated-type-specifier-from-hidden-module.m

diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index af5fdc95c9..1d8f9b7d20 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/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/test/Modules/Inputs/elaborated-type-structs.h b/test/Modules/Inputs/elaborated-type-structs.h
new file mode 100644
index 0000000000..da3940998f
--- /dev/null
+++ b/test/Modules/Inputs/elaborated-type-structs.h
@@ -0,0 +1,3 @@
+struct S1;
+struct S2 { int x; };
+struct S3 { int x; };
diff --git a/test/Modules/Inputs/module.map b/test/Modules/Inputs/module.map
index 226d45fdf4..632517dd36 100644
--- a/test/Modules/Inputs/module.map
+++ b/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/test/Modules/elaborated-type-specifier-from-hidden-module.m b/test/Modules/elaborated-type-specifier-from-hidden-module.m
new file mode 100644
index 0000000000..0ca1c24bba
--- /dev/null
+++ b/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);
-- 
2.40.0