[clang] Allow pack expansions when partial ordering against template template parameters

When partial ordering alias templates against template template parameters,
allow pack expansions when the alias has a fixed-size parameter list.

These expansions were generally disallowed by proposed resolution for CWG1430.

By previously diagnosing these when checking template template parameters, we
would be too strict in trying to prevent any potential invalid use.

This flows against the more general idea that template template parameters are
weakly typed, that we would rather allow an argument that might be possibly
misused, and only diagnose the actual misuses during instantiation.

Since this interaction between P0522R0 and CWG1430 is also a backwards-compat
breaking change, we implement provisional wording to allow these.

Fixes #62529

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -708,6 +708,8 @@ Bug Fixes to C++ Support
   expression.
 - Fix a bug in access control checking due to dealyed checking of friend declaration. Fixes (#GH12361).
 - Correctly treat the compound statement of an ``if consteval`` as an immediate context. Fixes (#GH91509).
+- When partial ordering alias templates against template template parameters,
+  allow pack expansions when the alias has a fixed-size parameter list. Fixes (#GH62529).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -9216,14 +9216,20 @@ class Sema final : public SemaBase {
   /// receive true if the cause for the error is the associated constraints of
   /// the template not being satisfied by the template arguments.
   ///
+  /// \param PartialOrderingTTP If true, assume these template arguments are
+  /// the injected template arguments for a template template parameter.
+  /// This will relax the requirement that all its possible uses are valid:
+  /// TTP checking is loose, and assumes that invalid uses will be diagnosed
+  /// during instantiation.
+  ///
   /// \returns true if an error occurred, false otherwise.
   bool CheckTemplateArgumentList(
       TemplateDecl *Template, SourceLocation TemplateLoc,
       TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
       SmallVectorImpl<TemplateArgument> &SugaredConverted,
       SmallVectorImpl<TemplateArgument> &CanonicalConverted,
       bool UpdateArgsWithConversions = true,
-      bool *ConstraintsNotSatisfied = nullptr);
+      bool *ConstraintsNotSatisfied = nullptr, bool PartialOrderingTTP = false);
 
   bool CheckTemplateTypeArgument(
       TemplateTypeParmDecl *Param, TemplateArgumentLoc &Arg,

clang/lib/Sema/SemaTemplate.cpp

@@ -6556,7 +6556,8 @@ bool Sema::CheckTemplateArgumentList(
     TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
     SmallVectorImpl<TemplateArgument> &SugaredConverted,
     SmallVectorImpl<TemplateArgument> &CanonicalConverted,
-    bool UpdateArgsWithConversions, bool *ConstraintsNotSatisfied) {
+    bool UpdateArgsWithConversions, bool *ConstraintsNotSatisfied,
+    bool PartialOrderingTTP) {
 
   if (ConstraintsNotSatisfied)
     *ConstraintsNotSatisfied = false;
@@ -6627,9 +6628,14 @@ bool Sema::CheckTemplateArgumentList(
       bool PackExpansionIntoNonPack =
           NewArgs[ArgIdx].getArgument().isPackExpansion() &&
           (!(*Param)->isTemplateParameterPack() || getExpandedPackSize(*Param));
-      if (PackExpansionIntoNonPack && (isa<TypeAliasTemplateDecl>(Template) ||
-                                       isa<ConceptDecl>(Template))) {
-        // Core issue 1430: we have a pack expansion as an argument to an
+      // CWG1430: Don't diagnose this pack expansion when partial
+      // ordering template template parameters. Some uses of the template could
+      // be valid, and invalid uses will be diagnosed later during
+      // instantiation.
+      if (PackExpansionIntoNonPack && !PartialOrderingTTP &&
+          (isa<TypeAliasTemplateDecl>(Template) ||
+           isa<ConceptDecl>(Template))) {
+        // CWG1430: we have a pack expansion as an argument to an
         // alias template, and it's not part of a parameter pack. This
         // can't be canonicalized, so reject it now.
         // As for concepts - we cannot normalize constraints where this

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -6243,7 +6243,9 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
     //   specialized as A.
     SmallVector<TemplateArgument, 4> SugaredPArgs;
     if (CheckTemplateArgumentList(AArg, Loc, PArgList, false, SugaredPArgs,
-                                  PArgs) ||
+                                  PArgs, /*UpdateArgsWithConversions=*/true,
+                                  /*ConstraintsNotSatisfied=*/nullptr,
+                                  /*PartialOrderTTP=*/true) ||
         Trap.hasErrorOccurred())
       return false;
   }

clang/test/SemaTemplate/temp_arg_template_cxx1z.cpp renamed to clang/test/SemaTemplate/temp_arg_template_p0522.cpp

@@ -1,6 +1,6 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 
-// expected-note@temp_arg_template_cxx1z.cpp:* 1+{{}}
+// expected-note@temp_arg_template_p0522.cpp:* 1+{{}}
 
 template<template<int> typename> struct Ti;
 template<template<int...> typename> struct TPi;
@@ -118,3 +118,11 @@ namespace Auto {
   TInt<SubstFailure> isf; // FIXME: this should be ill-formed
   TIntPtr<SubstFailure> ipsf;
 }
+
+namespace GH62529 {
+  // Note: the constraint here is just for bypassing a fast-path.
+  template<class T1> requires(true) using A = int;
+  template<template<class ...T2s> class TT1, class T3> struct B {};
+  template<class T4> B<A, T4> f();
+  auto t = f<int>();
+} // namespace GH62529