Sema: Allow 'constexpr' variables in range loops

This fixes PR22492, which is in response to CWG issue #1204.
Per the CWG issue 'contexpr' variables are now allowed in
for range loops.

llvm-svn: 229716

clang/lib/Sema/SemaDecl.cpp

@@ -9371,8 +9371,6 @@ void Sema::ActOnCXXForRangeDecl(Decl *D) {
   case SC_OpenCLWorkGroupLocal:
     llvm_unreachable("Unexpected storage class");
   }
-  if (VD->isConstexpr())
-    Error = 5;
   if (Error != -1) {
     Diag(VD->getOuterLocStart(), diag::err_for_range_storage_class)
       << VD->getDeclName() << Error;

clang/test/CXX/stmt.stmt/stmt.iter/stmt.ranged/p1.cpp

@@ -50,6 +50,18 @@ namespace X {
   struct NoEndADL {
     null_t alt_begin();
   };
+
+  struct C {
+    C();
+    struct It {
+      int val;
+      operator int &() { return val; }
+    };
+    It begin();
+    It end();
+  };
+
+  constexpr int operator*(const C::It &) { return 0; }
 }
 
 using X::A;
@@ -118,7 +130,7 @@ void g() {
   for (extern int a : A()) {} // expected-error {{loop variable 'a' may not be declared 'extern'}}
   for (static int a : A()) {} // expected-error {{loop variable 'a' may not be declared 'static'}}
   for (register int a : A()) {} // expected-error {{loop variable 'a' may not be declared 'register'}} expected-warning {{deprecated}}
-  for (constexpr int a : A()) {} // expected-error {{loop variable 'a' may not be declared 'constexpr'}}
+  for (constexpr int a : X::C()) {} // OK per CWG issue #1204.
 
   for (auto u : X::NoBeginADL()) { // expected-error {{invalid range expression of type 'X::NoBeginADL'; no viable 'begin' function available}}
   }