[clang] [Clang][Sema] Refactor collection of multi-level template argument lists (PR #106585)

[Krystian Stasiowski via cfe-commits]

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@@ -2112,6 +2078,33 @@ DeclResult Sema::CheckClassTemplate(
   NewClass->setLexicalDeclContext(CurContext);
   NewTemplate->setLexicalDeclContext(CurContext);
 
+  // Ensure that the template parameter lists are compatible. Skip this check
+  // for a friend in a dependent context: the template parameter list itself
+  // could be dependent.
+  if (ShouldAddRedecl && PrevClassTemplate &&
+      !TemplateParameterListsAreEqual(
+          NewTemplate, TemplateParams, PrevClassTemplate,
+          PrevClassTemplate->getTemplateParameters(),
+          /*Complain=*/true, TPL_TemplateMatch))
+    return true;
+
+  // Check the template parameter list of this declaration, possibly
+  // merging in the template parameter list from the previous class
+  // template declaration. Skip this check for a friend in a dependent
+  // context, because the template parameter list might be dependent.
+  if (ShouldAddRedecl &&
+      CheckTemplateParameterList(
+          TemplateParams,
+          PrevClassTemplate ? PrevClassTemplate->getTemplateParameters()
+                            : nullptr,
+          (SS.isSet() && SemanticContext && SemanticContext->isRecord() &&
+           SemanticContext->isDependentContext())
+              ? TPC_ClassTemplateMember
+          : TUK == TagUseKind::Friend ? TPC_FriendClassTemplate
+                                      : TPC_ClassTemplate,
+          SkipBody))
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sdkrystian wrote:

@zyn0217 Since this is existing code which I moved, I would prefer to keep it as-is and perhaps address this in some future NFC patch.

https://github.com/llvm/llvm-project/pull/106585