[clang] [Concepts] Avoid substituting into constraints for invalid TemplateDecls (PR #75697)

[via cfe-commits]

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@@ -353,6 +353,10 @@ static ExprResult calculateConstraintSatisfaction(
           if (Inst.isInvalid())
             return ExprError();
 
+          // An empty expression for substitution failure messages.
+          if (Template && Template->isInvalidDecl())
+            return ExprEmpty();
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cor3ntin wrote:

Can you explain why not return ExprError here? Does it just produce more diagnostics?

https://github.com/llvm/llvm-project/pull/75697