[clang] [Clang] Handle consteval expression in array bounds expressions (PR #66222)

[Erich Keane via cfe-commits]

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@@ -221,6 +221,15 @@ ExprResult Parser::ParseConstantExpression() {
   return ParseConstantExpressionInExprEvalContext(NotTypeCast);
 }
 
+ExprResult Parser::ParseArrayBoundExpression() {
+  EnterExpressionEvaluationContext ConstantEvaluated(
+      Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+  // If we parse the bound of a VLA... we parse a non-constant
+  // constant-expression!
+  Actions.ExprEvalContexts.back().InConditionallyConstantEvaluateContext = true;
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erichkeane wrote:

In reality, I'd probably prefer something like; 

``` auto Expr = ExpressionEvaluationContext::CreateForVLA```

kind of thing (or perhaps a builder-pattern like below), but that is probably a sizable refactor for everything.  I think I can hold my nose here and be OK for now.

``` ExprEvalContext Eva(ThingsEverythingNeeds).ForVLA()```



https://github.com/llvm/llvm-project/pull/66222