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

[mizvekov]

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
Fixes #91787

[llvmbot]

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/91833.diff

5 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/include/clang/Sema/Sema.h (+7-1)
• (modified) clang/lib/Sema/SemaTemplate.cpp (+9-3)
• (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+3-1)
• (renamed) clang/test/SemaTemplate/temp_arg_template_p0522.cpp (+10-2)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 7c5dcc59c7016..ddc5d5ff16a34 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -707,6 +707,7 @@ Bug Fixes to C++ Support
   initialized, rather than evaluating them as a part of the larger manifestly constant evaluated
   expression.
 - Fix a bug in access control checking due to dealyed checking of friend declaration. Fixes (#GH12361).
+- 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 4efd3878e861b..3fc59488243e5 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -9216,6 +9216,12 @@ 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 PartialOrderTTP If true, assume these template arguments are
+  /// the injected template arguments for a template template parameter.
+  /// This will relax the requirement that all it's 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,
@@ -9223,7 +9229,7 @@ class Sema final : public SemaBase {
       SmallVectorImpl<TemplateArgument> &SugaredConverted,
       SmallVectorImpl<TemplateArgument> &CanonicalConverted,
       bool UpdateArgsWithConversions = true,
-      bool *ConstraintsNotSatisfied = nullptr);
+      bool *ConstraintsNotSatisfied = nullptr, bool PartialOrderTTP = false);
 
   bool CheckTemplateTypeArgument(
       TemplateTypeParmDecl *Param, TemplateArgumentLoc &Arg,
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 480c0103ae335..88d0af191e6c8 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -6436,7 +6436,8 @@ bool Sema::CheckTemplateArgumentList(
     TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
     SmallVectorImpl<TemplateArgument> &SugaredConverted,
     SmallVectorImpl<TemplateArgument> &CanonicalConverted,
-    bool UpdateArgsWithConversions, bool *ConstraintsNotSatisfied) {
+    bool UpdateArgsWithConversions, bool *ConstraintsNotSatisfied,
+    bool PartialOrderTTP) {
 
   if (ConstraintsNotSatisfied)
     *ConstraintsNotSatisfied = false;
@@ -6507,8 +6508,13 @@ bool Sema::CheckTemplateArgumentList(
       bool PackExpansionIntoNonPack =
           NewArgs[ArgIdx].getArgument().isPackExpansion() &&
           (!(*Param)->isTemplateParameterPack() || getExpandedPackSize(*Param));
-      if (PackExpansionIntoNonPack && (isa<TypeAliasTemplateDecl>(Template) ||
-                                       isa<ConceptDecl>(Template))) {
+      // Core issue 1430: 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 && !PartialOrderTTP &&
+          (isa<TypeAliasTemplateDecl>(Template) ||
+           isa<ConceptDecl>(Template))) {
         // Core issue 1430: 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.
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index fe7e35d841510..853c0e1b50619 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/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;
   }
diff --git a/clang/test/SemaTemplate/temp_arg_template_cxx1z.cpp b/clang/test/SemaTemplate/temp_arg_template_p0522.cpp
similarity index 91%
rename from clang/test/SemaTemplate/temp_arg_template_cxx1z.cpp
rename to clang/test/SemaTemplate/temp_arg_template_p0522.cpp
index 372a00efc601e..251b6fc7d2be1 100644
--- a/clang/test/SemaTemplate/temp_arg_template_cxx1z.cpp
+++ b/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

[cor3ntin]

Looks sensible to me but let's wait a bit (I'll try to run tests on stdexec with that change)

[cor3ntin]

I have confirmed that with this change, stdexec compiles successfully, addressing both #89807 and #91787

[cor3ntin]

LGTM, thanks!