[clang] [NFC][Clang] Fix potential dereferencing of nullptr (PR #86759)

[Tom Honermann via cfe-commits]

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@@ -2974,7 +2974,7 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
     if (auto *FPrime = SemaRef.InstantiateFunctionDeclaration(
             F, TemplateArgListForBuildingFPrime, AliasTemplate->getLocation(),
             Sema::CodeSynthesisContext::BuildingDeductionGuides)) {
-      auto *GG = dyn_cast<CXXDeductionGuideDecl>(FPrime);
+      auto *GG = cast<CXXDeductionGuideDecl>(FPrime);
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tahonermann wrote:

I think this looks fine too. It is not trivial to ensure, but seems reasonable.

https://github.com/llvm/llvm-project/pull/86759