[clang] [Clang] Do not mark ambiguous specialization invalid. (PR #147275)

[Corentin Jabot via cfe-commits]

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@@ -4111,7 +4111,10 @@ static ActionResult<CXXRecordDecl *> getPatternForClassTemplateSpecialization(
         if (Ambiguous) {
           // Partial ordering did not produce a clear winner. Complain.
           Inst.Clear();
-          ClassTemplateSpec->setInvalidDecl();
+
+          if (!S.isSFINAEContext())
+            ClassTemplateSpec->setInvalidDecl();
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cor3ntin wrote:

Can we make a decision here?

https://github.com/llvm/llvm-project/pull/147275