[clang] fix explicit incomplete enum (PR #184210)

[Erich Keane via cfe-commits]

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@@ -6464,6 +6464,9 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
   if (checkPlaceholderForOverload(S, From))
     return ExprError();
 
+  if (From->containsErrors())
+    return S.CreateRecoveryExpr(From->getBeginLoc(), From->getEndLoc(), {From}, T);
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erichkeane wrote:

@mizvekov is at least a little right in that having a `RecoveryExpr` inside a `RecoveryExpr` is a little goofy.  BUT IMO, we're better off trying to maintain AST fidelity here to let other cascading issues appear, over returning `ExprError`.

My thoughts are: We have two options for that; 1- Emit a RecoveryExpr that wraps a RecoveryExpr.  We do that rarely.

2- Figure out how to 'continue' this function with the RecoveryExpr we already have.  So rather than bailing out here, we 'teach' the PerformImplicitConversion logic to handle a RecoveryExpr in some way, by figuring out how we want this to 'cast' right.  IMO, this is quite a bit more work, and seemed like a greater risk.

IF you were to look into 2 for Mattheus (to at least evaluate the difficulty here), it might make everyone more comfortable with this.

https://github.com/llvm/llvm-project/pull/184210