[CodeGen] Fix a crash on valid when constant folding 'default:' statement in switch

[erik.pilkington]

Summary

The following valid code crashes Clang in CodeGen:

void other_test() {
  switch(0) {
  case 0:
    do {
    default:;
    } while(0);
  }
}

The problem is that when we constant fold the switch into the case 0: case, the default: attempts to find the enclosing switch it is attached to, which does not exist. The solution is to ignore the default and emit it's child statement. (Which is the exact same solution as in the case statement, in EmitCaseStmt just above this)

Fixes PR28609.

[manmanren]

LGTM.

Manman

cfe/trunk/lib/CodeGen/CGStmt.cpp

while (NextCase && NextCase->getRHS() == nullptr) {
     NextCase = dyn_cast<CaseStmt>(CurCase->getSubStmt());
   }
 
   // Normal default recursion for non-cases.
   EmitStmt(CurCase->getSubStmt());
 }
 
 void CodeGenFunction::EmitDefaultStmt(const DefaultStmt &S) {
+  // If there is no enclosing switch instance that we're aware of, then this
+  // default statement can be elided. This situation only happens when we've
+  // constant-folded the switch.
+  if (!SwitchInsn) {
+    EmitStmt(S.getSubStmt());
+    return;
+  }
+
   llvm::BasicBlock *DefaultBlock = SwitchInsn->getDefaultDest();
   assert(DefaultBlock->empty() &&
          "EmitDefaultStmt: Default block already defined?");
 
   EmitBlockWithFallThrough(DefaultBlock, &S);
 
   EmitStmt(S.getSubStmt());
 }

cfe/trunk/test/CodeGenCXX/switch-case-folding-2.cpp

  }
  return 0;
 }
 
 int main(void) {
  return test(5);
 }
 
+void other_test() {
+  switch(0) {
+  case 0:
+    do {
+    default:;
+    } while(0);
+  }
+}
+
 // CHECK: call i32 (i8*, ...) @_Z6printfPKcz