Diagnose labels in expansion statements.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3250,8 +3250,8 @@ def err_injected_decl_outside_cone : Error <
 
 def err_compute_expansion_size_index : Error<
   "could not compute %select{size|index}0 of expansion">;
-def err_cannot_expand_over_type : Error<
-  "cannot expand over an expression of type %0">;
+def err_expanded_identifier_label : Error<
+  "identifier labels are not allowed in expansion statements">;
 
 // C++11 char16_t/char32_t
 def warn_cxx98_compat_unicode_type : Warning<

clang/lib/Sema/SemaExpand.cpp

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/Decl.h"
+#include "clang/AST/DynamicRecursiveASTVisitor.h"
 #include "clang/Basic/DiagnosticSema.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/EnterExpressionEvaluationContext.h"
@@ -518,6 +519,18 @@ StmtResult Sema::FinishCXXExpansionStmt(Stmt *Heading, Stmt *Body) {
   if (!Heading || !Body)
     return StmtError();
 
+  // Diagnose identifier labels.
+  struct DiagnoseLabels : DynamicRecursiveASTVisitor {
+    Sema &SemaRef;
+    DiagnoseLabels(Sema &S) : SemaRef(S) {}
+    bool VisitLabelStmt(LabelStmt *S) override {
+      SemaRef.Diag(S->getIdentLoc(), diag::err_expanded_identifier_label);
+      return false;
+    }
+  } Visitor(*this);
+  if (!Visitor.TraverseStmt(Body))
+    return StmtError();
+
   CXXExpansionStmt *Expansion = cast<CXXExpansionStmt>(Heading);
   Expansion->setBody(Body);