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

[Matheus Izvekov 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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mizvekov wrote:

We are finding one declaration within all the redeclarations of the primary template.
The list of redeclarations can't have a null element in it.

Otherwise, if you meant a null iterator, we shouldn't get here without a definition, as we should have diagnosed that before.
We could perhaps add a null check for that. Is that what you meant?

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