Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -5291,18 +5291,29 @@
   // __attribute__((aligned)) struct A;
   // Attributes should be placed after tag to apply to type declaration.
   if (!DS.getAttributes().empty() || !DeclAttrs.empty()) {
-    DeclSpec::TST TypeSpecType = DS.getTypeSpecType();
+    auto WarnAttributeIgnored = [this, TypeSpecType](const ParsedAttr &AL) {
+      if (AL.isAlignasAttribute()) {
+        // Don't use the message with placement with _Alignas.
+        // This is because C doesnt let you use _Alignas on type declarations.
+        Diag(AL.getLoc(), diag::warn_attribute_ignored)
+            << GetDiagnosticTypeSpecifierID(TypeSpecType);
+      } else {
+        Diag(AL.getLoc(), diag::warn_declspec_attribute_ignored)
+            << AL << GetDiagnosticTypeSpecifierID(TypeSpecType);
+      }
+    };
+
     if (TypeSpecType == DeclSpec::TST_class ||
         TypeSpecType == DeclSpec::TST_struct ||
         TypeSpecType == DeclSpec::TST_interface ||
         TypeSpecType == DeclSpec::TST_union ||
         TypeSpecType == DeclSpec::TST_enum) {
-      for (const ParsedAttr &AL : DS.getAttributes())
-        Diag(AL.getLoc(), diag::warn_declspec_attribute_ignored)
-            << AL << GetDiagnosticTypeSpecifierID(TypeSpecType);
-      for (const ParsedAttr &AL : DeclAttrs)
-        Diag(AL.getLoc(), diag::warn_declspec_attribute_ignored)
-            << AL << GetDiagnosticTypeSpecifierID(TypeSpecType);
+      for (const ParsedAttr &AL : DS.getAttributes()) {
+        WarnAttributeIgnored(AL);
+      }
+      for (const ParsedAttr &AL : DeclAttrs) {
+        WarnAttributeIgnored(AL);
+      }
     }
   }
 
Index: clang/test/Parser/c1x-alignas.c
===================================================================
--- clang/test/Parser/c1x-alignas.c
+++ clang/test/Parser/c1x-alignas.c
@@ -9,5 +9,7 @@
 
 char _Alignas(_Alignof(int)) c5;
 
+_Alignas(int) struct c6; // expected-warning {{1 attribute ignored}}
+
 // CHECK-EXT: '_Alignas' is a C11 extension
 // CHECK-EXT: '_Alignof' is a C11 extension