a.cpp

    {
      EnterExpressionEvaluationContext Eval(
        Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated,
        /*LambdaContextDecl=*/
        nullptr,
        /*ExprContext=*/
        Sema::ExpressionEvaluationContextRecord::EK_Other,
        /*ShouldEnter=*/
        CK == Sema::ConditionKind::ConstexprIf);

      // Parse the expression.
      Expr = ParseExpression(); // expression
    }

    if (Expr.isInvalid())
      return Sema::ConditionError();

    if (InitStmt && Tok.is(tok::semi)) {
      WarnOnInit();
      * InitStmt = Actions.ActOnExprStmt(Expr.get());
      ConsumeToken();
      return ParseCXXCondition(nullptr, Loc, CK, MissingOK);
    }

    return Actions.ActOnCondition(getCurScope(), Loc, Expr.get(), CK,
      MissingOK);