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

[Younan Zhang via cfe-commits]

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@@ -1994,8 +1995,10 @@ TemplateDeclInstantiator::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
   // Link the instantiation back to the pattern *unless* this is a
   // non-definition friend declaration.
   if (!InstTemplate->getInstantiatedFromMemberTemplate() &&
-      !(isFriend && !D->getTemplatedDecl()->isThisDeclarationADefinition()))
+      !(isFriend && !D->getTemplatedDecl()->isThisDeclarationADefinition())) {
+    InstTemplate->setInstantiatedFromDefinition();
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zyn0217 wrote:

Is it possible that `isFriend = false` and `D->getTemplatedDecl()->isThisDeclarationADefinition() = false`?

In that case, `InstTemplate` actually might not be instantiated from a definition, so the flag is then misleading from what I see?

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