[clang] [clang] Track function template instantiation from definition (PR #110387)

[Younan Zhang via cfe-commits]

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@@ -5185,9 +5189,24 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
     RebuildTypeSourceInfoForDefaultSpecialMembers();
     SetDeclDefaulted(Function, PatternDecl->getLocation());
   } else {
+    NamedDecl *ND = Function;
+    std::optional<ArrayRef<TemplateArgument>> Innermost;
+    if (auto *Primary = Function->getPrimaryTemplate();
+        Primary &&
+        !isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function)) {
+      ND = *llvm::find_if(Primary->redecls(),
+                          [](const RedeclarableTemplateDecl *RTD) {
+                            auto *FTD = cast<FunctionTemplateDecl>(RTD);
+                            return FTD->isInstantiatedFromDefinition() ||
+                                   FTD->isThisDeclarationADefinition();
+                          });
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zyn0217 wrote:

We'd better have an assertion here, ND shouldn't be a nullptr.

https://github.com/llvm/llvm-project/pull/110387