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

[Matheus Izvekov 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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mizvekov wrote:

That's true, but the only problematic cases are the friends.

We could make the flag more accurate, as named, for non-friends, but then we don't have a motivation, and we would make searching for a declaration which is compatible with the definition slightly more expensive for those.

Another option is to rename the flag to something like `InstantiatedFromDefinitionForFriend`.

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