[clang][ExprConst] Avoid initializer diags if there is no init

Author: tbaederr

clang/lib/AST/ExprConstant.cpp

@@ -3596,6 +3596,7 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
   // expressions here; doing so would regress diagnostics for things like
   // reading from a volatile constexpr variable.
   if ((Info.getLangOpts().CPlusPlus && !VD->hasConstantInitialization() &&
+       VD->getAnyInitializer() &&
        VD->mightBeUsableInConstantExpressions(Info.Ctx)) ||
       ((Info.getLangOpts().CPlusPlus || Info.getLangOpts().OpenCL) &&
        !Info.getLangOpts().CPlusPlus11 && !VD->hasICEInitializer(Info.Ctx))) {

clang/test/SemaCXX/constant-expression-p2280r4.cpp

@@ -47,11 +47,10 @@ void splash(Swim& swam) {
 }
 
 extern Swim dc;
-extern Swim& trident; // expected-note {{declared here}}
+extern Swim& trident;
 
 constexpr auto& sandeno   = typeid(dc);         // ok: can only be typeid(Swim)
 constexpr auto& gallagher = typeid(trident);    // expected-error {{constexpr variable 'gallagher' must be initialized by a constant expression}}
-                                                // expected-note@-1 {{initializer of 'trident' is not a constant expression}}
 
 namespace explicitThis {
 struct C {