[clang] Reland: [clang] Track function template instantiation from definition (PR #125266)

[Matheus Izvekov via cfe-commits]

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@@ -5276,9 +5277,31 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
     RebuildTypeSourceInfoForDefaultSpecialMembers();
     SetDeclDefaulted(Function, PatternDecl->getLocation());
   } else {
+    NamedDecl *ND = Function;
+    DeclContext *DC = ND->getLexicalDeclContext();
+    std::optional<ArrayRef<TemplateArgument>> Innermost;
+    if (auto *Primary = Function->getPrimaryTemplate();
+        Primary &&
+        !isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function) &&
+        Function->getTemplateSpecializationKind() !=
+            TSK_ExplicitSpecialization) {
+      auto It = llvm::find_if(Primary->redecls(),
+                              [](const RedeclarableTemplateDecl *RTD) {
+                                return cast<FunctionTemplateDecl>(RTD)
+                                    ->isCompatibleWithDefinition();
+                              });
+      assert(It != Primary->redecls().end() &&
+             "Should't get here without a definition");
+      if (FunctionDecl *Def = cast<FunctionTemplateDecl>(*It)
+                                  ->getTemplatedDecl()
+                                  ->getDefinition())
+        DC = Def->getLexicalDeclContext();
+      else
+        DC = (*It)->getLexicalDeclContext();
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mizvekov wrote:

We need the lexical context because we are interested in the template context of the declaration, ie what set of parameters it sits under, which is a lexical thing.

I don't follow what you mean this situation with friends, can you provide an example?
regression2 is a situation very specific to member function templates, but friends are not members.

https://github.com/llvm/llvm-project/pull/125266