[clang] [Clang] [Sema] Fix crash instantiating a member variable template partial specialization when the enclosing class template is deserialized (PR #202958)

[Matheus Izvekov via cfe-commits]

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@@ -2697,6 +2697,14 @@ void ASTDeclReader::VisitVarTemplatePartialSpecializationDecl(
 
   RedeclarableResult Redecl = VisitVarTemplateSpecializationDeclImpl(D);
 
+  // A partial specialization whose primary template is invalid cannot be
+  // instantiated. Mark it invalid too so InstantiateClass skips it, the same
+  // way it already skips the invalid primary, instead of later looking for an
+  // instantiated primary that was never created.
+  if (VarTemplateDecl *Primary = D->getSpecializedTemplate();
+      Primary && Primary->isInvalidDecl())
+    D->setInvalidDecl();
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mizvekov wrote:

Can we address the root cause instead? Why does this not crash without serialization, but does with it?

https://github.com/llvm/llvm-project/pull/202958