Sema: Allow aliases to have incomplete type

gcc supports this behavior and it is pervasively used inside the Linux
kernel.

Note that both gcc and clang will reject code that attempts to do this
in a C++ language mode.

This fixes PR17998.

llvm-svn: 212631

2 files changed, 10 insertions, 5 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 82d28fa0827..13a77f15eb2 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -8897,11 +8897,13 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl,
     if (Var->isInvalidDecl())
       return;
 
-    if (RequireCompleteType(Var->getLocation(), 
-                            Context.getBaseElementType(Type),
-                            diag::err_typecheck_decl_incomplete_type)) {
-      Var->setInvalidDecl();
-      return;
+    if (!Var->hasAttr<AliasAttr>()) {
+      if (RequireCompleteType(Var->getLocation(),
+                              Context.getBaseElementType(Type),
+                              diag::err_typecheck_decl_incomplete_type)) {
+        Var->setInvalidDecl();
+        return;
+      }
     }
 
     // The variable can not have an abstract class type.
diff --git a/clang/test/Sema/attr-alias-elf.c b/clang/test/Sema/attr-alias-elf.c
index 04d13924ac1..f14514dccd2 100644
--- a/clang/test/Sema/attr-alias-elf.c
+++ b/clang/test/Sema/attr-alias-elf.c
@@ -64,3 +64,6 @@ void test3_foo() __attribute__((alias("test3_bar")));
 __attribute__((section("test"))) void test4_bar() { }
 void test4_foo() __attribute__((section("test")));
 void test4_foo() __attribute__((alias("test4_bar")));
+
+int test5_bar = 0;
+extern struct incomplete_type test5_foo __attribute__((alias("test5_bar")));