Index: cfe/trunk/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDeclAttr.cpp
+++ cfe/trunk/lib/Sema/SemaDeclAttr.cpp
@@ -3044,10 +3044,14 @@
     return;
   }
 
+  if (FirstType->isIncompleteType())
+    return;
   uint64_t FirstSize = S.Context.getTypeSize(FirstType);
   uint64_t FirstAlign = S.Context.getTypeAlign(FirstType);
   for (; Field != FieldEnd; ++Field) {
     QualType FieldType = Field->getType();
+    if (FieldType->isIncompleteType())
+      return;
     // FIXME: this isn't fully correct; we also need to test whether the
     // members of the union would all have the same calling convention as the
     // first member of the union. Checking just the size and alignment isn't
Index: cfe/trunk/test/Sema/transparent-union.c
===================================================================
--- cfe/trunk/test/Sema/transparent-union.c
+++ cfe/trunk/test/Sema/transparent-union.c
@@ -89,3 +89,12 @@
     unsigned int u3;
   } __attribute__((aligned(8)));
 } __attribute__((transparent_union));
+
+union pr30520v { void b; } __attribute__((transparent_union)); // expected-error {{field has incomplete type 'void'}}
+
+union pr30520a { int b[]; } __attribute__((transparent_union)); // expected-error {{field has incomplete type 'int []'}}
+
+// expected-note@+1 2 {{forward declaration of 'struct stb'}}
+union pr30520s { struct stb b; } __attribute__((transparent_union)); // expected-error {{field has incomplete type 'struct stb'}}
+
+union pr30520s2 { int *v; struct stb b; } __attribute__((transparent_union)); // expected-error {{field has incomplete type 'struct stb'}}