[clang] [Clang] Fix a crash on invalid concept (PR #150186)

[Younan Zhang via cfe-commits]

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@@ -588,6 +588,9 @@ static bool CheckConstraintSatisfaction(
     return true;
 
   for (const AssociatedConstraint &AC : AssociatedConstraints) {
+    if (AC.isNull())
+      return true;
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zyn0217 wrote:

Can we continue?

https://github.com/llvm/llvm-project/pull/150186