[C++20] Implement P2113R0: Changes to the Partial Ordering of Constrained Functions

This implementation matches GCC behavior in that [[ https://eel.is/c++draft/temp.func.order#6.2.1 | temp.func.order p6.2.1 ]] is not implemented [1]. I reached out to the GCC author to confirm that some changes elsewhere to overload resolution are probably needed, but no solution has been developed sufficiently [3].

Most of the wordings are implemented straightforwardly. However,
for [[ https://eel.is/c++draft/temp.func.order#6.2.2 | temp.func.order p6.2.2 ]] "... or if the function parameters that positionally correspond between the two templates are not of the same type", the "same type" is not very clear ([2] is a bug related to this). Here is a quick example
```
template <C T, C U>        int f(T, U);
template <typename T, C U> int f(U, T);

int x = f(0, 0);
```
Is the `U` and `T` from different `f`s the "same type"? The answer is NO even though both `U` and `T` are deduced to be `int` in this case. The reason is that `U` and `T` are dependent types, according to [[ https://eel.is/c++draft/temp.over.link#3 |  temp.over.link p3 ]], they can not be the "same type".

To check if two function parameters are the "same type":
* For //function template//: compare the function parameter canonical types and return type between two function templates.
* For //class template/partial specialization//: by [[ https://eel.is/c++draft/temp.spec.partial.order#1.2 | temp.spec.partial.order p1.2 ]], compare the injected template arguments between two templates using hashing(TemplateArgument::Profile) is enough.

[1] https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=57b4daf8dc4ed7b669cc70638866ddb00f5b7746
[2] #49308
[3] https://lists.isocpp.org/core/2020/06/index.php#msg9392

Fixes #54039
Fixes #49308 (PR49964)

Reviewed By: royjacobson, #clang-language-wg, mizvekov

Differential Revision: https://reviews.llvm.org/D128750

Author: Yuanfang Chen

clang/docs/ReleaseNotes.rst

@@ -534,6 +534,9 @@ C++20 Feature Support
   which removes the requirement for the ``typename`` keyword in certain contexts.
 - Implemented The Equality Operator You Are Looking For (`P2468 <http://wg21.link/p2468r2>`_).
 
+- Implemented `P2113R0: Proposed resolution for 2019 comment CA 112 <https://wg21.link/P2113R0>`_
+  ([temp.func.order]p6.2.1 is not implemented, matching GCC).
+
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/AST/DeclTemplate.h

@@ -847,6 +847,15 @@ class RedeclarableTemplateDecl : public TemplateDecl,
     /// The first value in the array is the number of specializations/partial
     /// specializations that follow.
     uint32_t *LazySpecializations = nullptr;
+
+    /// The set of "injected" template arguments used within this
+    /// template.
+    ///
+    /// This pointer refers to the template arguments (there are as
+    /// many template arguments as template parameaters) for the
+    /// template, and is allocated lazily, since most templates do not
+    /// require the use of this information.
+    TemplateArgument *InjectedArgs = nullptr;
   };
 
   /// Pointer to the common data shared by all declarations of this
@@ -954,6 +963,14 @@ class RedeclarableTemplateDecl : public TemplateDecl,
     getCommonPtr()->InstantiatedFromMember.setPointer(TD);
   }
 
+  /// Retrieve the "injected" template arguments that correspond to the
+  /// template parameters of this template.
+  ///
+  /// Although the C++ standard has no notion of the "injected" template
+  /// arguments for a template, the notion is convenient when
+  /// we need to perform substitutions inside the definition of a template.
+  ArrayRef<TemplateArgument> getInjectedTemplateArgs();
+
   using redecl_range = redeclarable_base::redecl_range;
   using redecl_iterator = redeclarable_base::redecl_iterator;
 
@@ -998,15 +1015,6 @@ class FunctionTemplateDecl : public RedeclarableTemplateDecl {
     /// template, including explicit specializations and instantiations.
     llvm::FoldingSetVector<FunctionTemplateSpecializationInfo> Specializations;
 
-    /// The set of "injected" template arguments used within this
-    /// function template.
-    ///
-    /// This pointer refers to the template arguments (there are as
-    /// many template arguments as template parameaters) for the function
-    /// template, and is allocated lazily, since most function templates do not
-    /// require the use of this information.
-    TemplateArgument *InjectedArgs = nullptr;
-
     Common() = default;
   };
 
@@ -1106,15 +1114,6 @@ class FunctionTemplateDecl : public RedeclarableTemplateDecl {
     return makeSpecIterator(getSpecializations(), true);
   }
 
-  /// Retrieve the "injected" template arguments that correspond to the
-  /// template parameters of this function template.
-  ///
-  /// Although the C++ standard has no notion of the "injected" template
-  /// arguments for a function template, the notion is convenient when
-  /// we need to perform substitutions inside the definition of a function
-  /// template.
-  ArrayRef<TemplateArgument> getInjectedTemplateArgs();
-
   /// Return whether this function template is an abbreviated function template,
   /// e.g. `void foo(auto x)` or `template<typename T> void foo(auto x)`
   bool isAbbreviated() const {

clang/include/clang/Sema/Sema.h

@@ -8329,14 +8329,17 @@ class Sema final {
       const NamedDecl *NewInstFrom, TemplateParameterList *New,
       const NamedDecl *OldInstFrom, TemplateParameterList *Old, bool Complain,
       TemplateParameterListEqualKind Kind,
-      SourceLocation TemplateArgLoc = SourceLocation());
+      SourceLocation TemplateArgLoc = SourceLocation(),
+      bool PartialOrdering = false);
 
   bool TemplateParameterListsAreEqual(
       TemplateParameterList *New, TemplateParameterList *Old, bool Complain,
       TemplateParameterListEqualKind Kind,
-      SourceLocation TemplateArgLoc = SourceLocation()) {
+      SourceLocation TemplateArgLoc = SourceLocation(),
+      bool PartialOrdering = false) {
     return TemplateParameterListsAreEqual(nullptr, New, nullptr, Old, Complain,
-                                          Kind, TemplateArgLoc);
+                                          Kind, TemplateArgLoc,
+                                          PartialOrdering);
   }
 
   bool CheckTemplateDeclScope(Scope *S, TemplateParameterList *TemplateParams);
@@ -9021,8 +9024,7 @@ class Sema final {
   FunctionTemplateDecl *getMoreSpecializedTemplate(
       FunctionTemplateDecl *FT1, FunctionTemplateDecl *FT2, SourceLocation Loc,
       TemplatePartialOrderingContext TPOC, unsigned NumCallArguments1,
-      unsigned NumCallArguments2, bool Reversed = false,
-      bool AllowOrderingByConstraints = true);
+      unsigned NumCallArguments2, bool Reversed = false);
   UnresolvedSetIterator
   getMostSpecialized(UnresolvedSetIterator SBegin, UnresolvedSetIterator SEnd,
                      TemplateSpecCandidateSet &FailedCandidates,

clang/lib/AST/DeclTemplate.cpp

@@ -353,6 +353,22 @@ void RedeclarableTemplateDecl::addSpecializationImpl(
                                       SETraits::getDecl(Entry));
 }
 
+ArrayRef<TemplateArgument> RedeclarableTemplateDecl::getInjectedTemplateArgs() {
+  TemplateParameterList *Params = getTemplateParameters();
+  auto *CommonPtr = getCommonPtr();
+  if (!CommonPtr->InjectedArgs) {
+    auto &Context = getASTContext();
+    SmallVector<TemplateArgument, 16> TemplateArgs;
+    Context.getInjectedTemplateArgs(Params, TemplateArgs);
+    CommonPtr->InjectedArgs =
+        new (Context) TemplateArgument[TemplateArgs.size()];
+    std::copy(TemplateArgs.begin(), TemplateArgs.end(),
+              CommonPtr->InjectedArgs);
+  }
+
+  return llvm::makeArrayRef(CommonPtr->InjectedArgs, Params->size());
+}
+
 //===----------------------------------------------------------------------===//
 // FunctionTemplateDecl Implementation
 //===----------------------------------------------------------------------===//
@@ -403,22 +419,6 @@ void FunctionTemplateDecl::addSpecialization(
                                               InsertPos);
 }
 
-ArrayRef<TemplateArgument> FunctionTemplateDecl::getInjectedTemplateArgs() {
-  TemplateParameterList *Params = getTemplateParameters();
-  Common *CommonPtr = getCommonPtr();
-  if (!CommonPtr->InjectedArgs) {
-    auto &Context = getASTContext();
-    SmallVector<TemplateArgument, 16> TemplateArgs;
-    Context.getInjectedTemplateArgs(Params, TemplateArgs);
-    CommonPtr->InjectedArgs =
-        new (Context) TemplateArgument[TemplateArgs.size()];
-    std::copy(TemplateArgs.begin(), TemplateArgs.end(),
-              CommonPtr->InjectedArgs);
-  }
-
-  return llvm::makeArrayRef(CommonPtr->InjectedArgs, Params->size());
-}
-
 void FunctionTemplateDecl::mergePrevDecl(FunctionTemplateDecl *Prev) {
   using Base = RedeclarableTemplateDecl;
 

clang/lib/Sema/SemaConcept.cpp

@@ -1104,6 +1104,18 @@ NormalizedConstraint::fromConstraintExpr(Sema &S, NamedDecl *D, const Expr *E) {
   // - The normal form of an expression (E) is the normal form of E.
   // [...]
   E = E->IgnoreParenImpCasts();
+
+  // C++2a [temp.param]p4:
+  //     [...] If T is not a pack, then E is E', otherwise E is (E' && ...).
+  //
+  // Using the pattern suffices because the partial ordering rules guarantee
+  // the template paramaters are equivalent.
+  if (auto *FoldE = dyn_cast<const CXXFoldExpr>(E)) {
+    assert(FoldE->isRightFold() && FoldE->getOperator() == BO_LAnd);
+    assert(E->IgnoreParenImpCasts() == E);
+    E = FoldE->getPattern();
+  }
+
   if (LogicalBinOp BO = E) {
     auto LHS = fromConstraintExpr(S, D, BO.getLHS());
     if (!LHS)

clang/lib/Sema/SemaOverload.cpp

@@ -9776,19 +9776,13 @@ static bool haveSameParameterTypes(ASTContext &Context, const FunctionDecl *F1,
 
 /// We're allowed to use constraints partial ordering only if the candidates
 /// have the same parameter types:
-/// [temp.func.order]p6.2.2 [...] or if the function parameters that
-/// positionally correspond between the two templates are not of the same type,
-/// neither template is more specialized than the other.
 /// [over.match.best]p2.6
 /// F1 and F2 are non-template functions with the same parameter-type-lists,
 /// and F1 is more constrained than F2 [...]
-static bool canCompareFunctionConstraints(Sema &S,
+static bool sameFunctionParameterTypeLists(Sema &S,
                                           const OverloadCandidate &Cand1,
                                           const OverloadCandidate &Cand2) {
-  // FIXME: Per P2113R0 we also need to compare the template parameter lists
-  // when comparing template functions. 
-  if (Cand1.Function && Cand2.Function && Cand1.Function->hasPrototype() &&
-      Cand2.Function->hasPrototype()) {
+  if (Cand1.Function && Cand2.Function) {
     auto *PT1 = cast<FunctionProtoType>(Cand1.Function->getFunctionType());
     auto *PT2 = cast<FunctionProtoType>(Cand2.Function->getFunctionType());
     if (PT1->getNumParams() == PT2->getNumParams() &&
@@ -10031,15 +10025,14 @@ bool clang::isBetterOverloadCandidate(
             isa<CXXConversionDecl>(Cand1.Function) ? TPOC_Conversion
                                                    : TPOC_Call,
             Cand1.ExplicitCallArguments, Cand2.ExplicitCallArguments,
-            Cand1.isReversed() ^ Cand2.isReversed(),
-            canCompareFunctionConstraints(S, Cand1, Cand2)))
+            Cand1.isReversed() ^ Cand2.isReversed()))
       return BetterTemplate == Cand1.Function->getPrimaryTemplate();
   }
 
   //   -— F1 and F2 are non-template functions with the same
   //      parameter-type-lists, and F1 is more constrained than F2 [...],
   if (!Cand1IsSpecialization && !Cand2IsSpecialization &&
-      canCompareFunctionConstraints(S, Cand1, Cand2)) {
+      sameFunctionParameterTypeLists(S, Cand1, Cand2)) {
     Expr *RC1 = Cand1.Function->getTrailingRequiresClause();
     Expr *RC2 = Cand2.Function->getTrailingRequiresClause();
     if (RC1 && RC2) {

clang/lib/Sema/SemaTemplate.cpp

@@ -7652,10 +7652,10 @@ bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
 
     if (isTemplateTemplateParameterAtLeastAsSpecializedAs(Params, Template,
                                                           Arg.getLocation())) {
-      // C++2a[temp.func.order]p2
+      // P2113
+      // C++20[temp.func.order]p2
       //   [...] If both deductions succeed, the partial ordering selects the
-      //   more constrained template as described by the rules in
-      //   [temp.constr.order].
+      // more constrained template (if one exists) as determined below.
       SmallVector<const Expr *, 3> ParamsAC, TemplateAC;
       Params->getAssociatedConstraints(ParamsAC);
       // C++2a[temp.arg.template]p3
@@ -7864,7 +7864,8 @@ Sema::BuildExpressionFromIntegralTemplateArgument(const TemplateArgument &Arg,
 static bool MatchTemplateParameterKind(
     Sema &S, NamedDecl *New, const NamedDecl *NewInstFrom, NamedDecl *Old,
     const NamedDecl *OldInstFrom, bool Complain,
-    Sema::TemplateParameterListEqualKind Kind, SourceLocation TemplateArgLoc) {
+    Sema::TemplateParameterListEqualKind Kind, SourceLocation TemplateArgLoc,
+    bool PartialOrdering) {
   // Check the actual kind (type, non-type, template).
   if (Old->getKind() != New->getKind()) {
     if (Complain) {
@@ -7952,11 +7953,11 @@ static bool MatchTemplateParameterKind(
             (Kind == Sema::TPL_TemplateMatch
                  ? Sema::TPL_TemplateTemplateParmMatch
                  : Kind),
-            TemplateArgLoc))
+            TemplateArgLoc, PartialOrdering))
       return false;
   }
 
-  if (Kind != Sema::TPL_TemplateTemplateArgumentMatch &&
+  if (!PartialOrdering && Kind != Sema::TPL_TemplateTemplateArgumentMatch &&
       !isa<TemplateTemplateParmDecl>(Old)) {
     const Expr *NewC = nullptr, *OldC = nullptr;
 
@@ -8049,7 +8050,8 @@ void DiagnoseTemplateParameterListArityMismatch(Sema &S,
 bool Sema::TemplateParameterListsAreEqual(
     const NamedDecl *NewInstFrom, TemplateParameterList *New,
     const NamedDecl *OldInstFrom, TemplateParameterList *Old, bool Complain,
-    TemplateParameterListEqualKind Kind, SourceLocation TemplateArgLoc) {
+    TemplateParameterListEqualKind Kind, SourceLocation TemplateArgLoc,
+    bool PartialOrdering) {
   if (Old->size() != New->size() && Kind != TPL_TemplateTemplateArgumentMatch) {
     if (Complain)
       DiagnoseTemplateParameterListArityMismatch(*this, New, Old, Kind,
@@ -8081,7 +8083,7 @@ bool Sema::TemplateParameterListsAreEqual(
 
       if (!MatchTemplateParameterKind(*this, *NewParm, NewInstFrom, *OldParm,
                                       OldInstFrom, Complain, Kind,
-                                      TemplateArgLoc))
+                                      TemplateArgLoc, PartialOrdering))
         return false;
 
       ++NewParm;
@@ -8098,7 +8100,7 @@ bool Sema::TemplateParameterListsAreEqual(
     for (; NewParm != NewParmEnd; ++NewParm) {
       if (!MatchTemplateParameterKind(*this, *NewParm, NewInstFrom, *OldParm,
                                       OldInstFrom, Complain, Kind,
-                                      TemplateArgLoc))
+                                      TemplateArgLoc, PartialOrdering))
         return false;
     }
   }
@@ -8112,7 +8114,7 @@ bool Sema::TemplateParameterListsAreEqual(
     return false;
   }
 
-  if (Kind != TPL_TemplateTemplateArgumentMatch) {
+  if (!PartialOrdering && Kind != TPL_TemplateTemplateArgumentMatch) {
     const Expr *NewRC = New->getRequiresClause();
     const Expr *OldRC = Old->getRequiresClause();