[clang] check deduction consistency when partial ordering function templates (#100692)

This makes partial ordering of function templates consistent with other
entities, by implementing [temp.deduct.type]p1 in that case.

Fixes #18291

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -317,6 +317,8 @@ Bug Fixes to C++ Support
 - Clang now properly handles the order of attributes in `extern` blocks. (#GH101990).
 - Fixed an assertion failure by preventing null explicit object arguments from being deduced. (#GH102025).
 - Correctly check constraints of explicit instantiations of member functions. (#GH46029)
+- When performing partial ordering of function templates, clang now checks that
+  the deduction was consistent. Fixes (#GH18291).
 - Fixed an assertion failure about a constraint of a friend function template references to a value with greater
   template depth than the friend function template. (#GH98258)
 - Clang now rebuilds the template parameters of out-of-line declarations and specializations in the context

clang/include/clang/Sema/Sema.h

@@ -13283,6 +13283,10 @@ class Sema final : public SemaBase {
   /// \param AllowDeducedTST Whether a DeducedTemplateSpecializationType is
   /// acceptable as the top level type of the result.
   ///
+  /// \param IsIncompleteSubstitution If provided, the pointee will be set
+  /// whenever substitution would perform a replacement with a null or
+  /// non-existent template argument.
+  ///
   /// \returns If the instantiation succeeds, the instantiated
   /// type. Otherwise, produces diagnostics and returns a NULL type.
   TypeSourceInfo *SubstType(TypeSourceInfo *T,
@@ -13292,7 +13296,8 @@ class Sema final : public SemaBase {
 
   QualType SubstType(QualType T,
                      const MultiLevelTemplateArgumentList &TemplateArgs,
-                     SourceLocation Loc, DeclarationName Entity);
+                     SourceLocation Loc, DeclarationName Entity,
+                     bool *IsIncompleteSubstitution = nullptr);
 
   TypeSourceInfo *SubstType(TypeLoc TL,
                             const MultiLevelTemplateArgumentList &TemplateArgs,

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -1344,15 +1344,21 @@ namespace {
     DeclarationName Entity;
     // Whether to evaluate the C++20 constraints or simply substitute into them.
     bool EvaluateConstraints = true;
+    // Whether Substitution was Incomplete, that is, we tried to substitute in
+    // any user provided template arguments which were null.
+    bool IsIncomplete = false;
+    // Whether an incomplete substituion should be treated as an error.
+    bool BailOutOnIncomplete;
 
   public:
     typedef TreeTransform<TemplateInstantiator> inherited;
 
     TemplateInstantiator(Sema &SemaRef,
                          const MultiLevelTemplateArgumentList &TemplateArgs,
-                         SourceLocation Loc, DeclarationName Entity)
+                         SourceLocation Loc, DeclarationName Entity,
+                         bool BailOutOnIncomplete = false)
         : inherited(SemaRef), TemplateArgs(TemplateArgs), Loc(Loc),
-          Entity(Entity) {}
+          Entity(Entity), BailOutOnIncomplete(BailOutOnIncomplete) {}
 
     void setEvaluateConstraints(bool B) {
       EvaluateConstraints = B;
@@ -1374,6 +1380,9 @@ namespace {
     /// Returns the name of the entity being instantiated, if any.
     DeclarationName getBaseEntity() { return Entity; }
 
+    /// Returns whether any substitution so far was incomplete.
+    bool getIsIncomplete() const { return IsIncomplete; }
+
     /// Sets the "base" location and entity when that
     /// information is known based on another transformation.
     void setBase(SourceLocation Loc, DeclarationName Entity) {
@@ -1420,6 +1429,10 @@ namespace {
         if (TemplateArgs.hasTemplateArgument(Depth, Index)) {
           Result = TemplateArgs(Depth, Index);
           TemplateArgs.setArgument(Depth, Index, TemplateArgument());
+        } else {
+          IsIncomplete = true;
+          if (BailOutOnIncomplete)
+            return TemplateArgument();
         }
       }
 
@@ -1820,8 +1833,10 @@ Decl *TemplateInstantiator::TransformDecl(SourceLocation Loc, Decl *D) {
       // template arguments in a function template, but there were some
       // arguments left unspecified.
       if (!TemplateArgs.hasTemplateArgument(TTP->getDepth(),
-                                            TTP->getPosition()))
-        return D;
+                                            TTP->getPosition())) {
+        IsIncomplete = true;
+        return BailOutOnIncomplete ? nullptr : D;
+      }
 
       TemplateArgument Arg = TemplateArgs(TTP->getDepth(), TTP->getPosition());
 
@@ -1967,8 +1982,10 @@ TemplateName TemplateInstantiator::TransformTemplateName(
       // template arguments in a function template, but there were some
       // arguments left unspecified.
       if (!TemplateArgs.hasTemplateArgument(TTP->getDepth(),
-                                            TTP->getPosition()))
-        return Name;
+                                            TTP->getPosition())) {
+        IsIncomplete = true;
+        return BailOutOnIncomplete ? TemplateName() : Name;
+      }
 
       TemplateArgument Arg = TemplateArgs(TTP->getDepth(), TTP->getPosition());
 
@@ -2050,8 +2067,10 @@ TemplateInstantiator::TransformTemplateParmRefExpr(DeclRefExpr *E,
   // template arguments in a function template, but there were some
   // arguments left unspecified.
   if (!TemplateArgs.hasTemplateArgument(NTTP->getDepth(),
-                                        NTTP->getPosition()))
-    return E;
+                                        NTTP->getPosition())) {
+    IsIncomplete = true;
+    return BailOutOnIncomplete ? ExprError() : E;
+  }
 
   TemplateArgument Arg = TemplateArgs(NTTP->getDepth(), NTTP->getPosition());
 
@@ -2470,6 +2489,10 @@ TemplateInstantiator::TransformTemplateTypeParmType(TypeLocBuilder &TLB,
     // template arguments in a function template class, but there were some
     // arguments left unspecified.
     if (!TemplateArgs.hasTemplateArgument(T->getDepth(), T->getIndex())) {
+      IsIncomplete = true;
+      if (BailOutOnIncomplete)
+        return QualType();
+
       TemplateTypeParmTypeLoc NewTL
         = TLB.push<TemplateTypeParmTypeLoc>(TL.getType());
       NewTL.setNameLoc(TL.getNameLoc());
@@ -2841,7 +2864,8 @@ TypeSourceInfo *Sema::SubstType(TypeLoc TL,
 /// Deprecated form of the above.
 QualType Sema::SubstType(QualType T,
                          const MultiLevelTemplateArgumentList &TemplateArgs,
-                         SourceLocation Loc, DeclarationName Entity) {
+                         SourceLocation Loc, DeclarationName Entity,
+                         bool *IsIncompleteSubstitution) {
   assert(!CodeSynthesisContexts.empty() &&
          "Cannot perform an instantiation without some context on the "
          "instantiation stack");
@@ -2851,8 +2875,13 @@ QualType Sema::SubstType(QualType T,
   if (!T->isInstantiationDependentType() && !T->isVariablyModifiedType())
     return T;
 
-  TemplateInstantiator Instantiator(*this, TemplateArgs, Loc, Entity);
-  return Instantiator.TransformType(T);
+  TemplateInstantiator Instantiator(
+      *this, TemplateArgs, Loc, Entity,
+      /*BailOutOnIncomplete=*/IsIncompleteSubstitution != nullptr);
+  QualType QT = Instantiator.TransformType(T);
+  if (IsIncompleteSubstitution && Instantiator.getIsIncomplete())
+    *IsIncompleteSubstitution = true;
+  return QT;
 }
 
 static bool NeedsInstantiationAsFunctionType(TypeSourceInfo *T) {

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -8,7 +8,7 @@ void f() {
   // The important thing is that we provide OVERLOAD signature in all those cases.
   //
   // RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:%(line-5):7 %s -o - | FileCheck --check-prefix=CHECK-1 %s
-  // CHECK-1: OVERLOAD: [#void#]fun(<#T x#>, Args args...)
+  // CHECK-1: OVERLOAD: [#void#]fun(<#T x#>)
   // RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:%(line-7):10 %s -o - | FileCheck --check-prefix=CHECK-2 %s
   // CHECK-2: OVERLOAD: [#void#]fun(int x)
   // RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:%(line-9):13 %s -o - | FileCheck --check-prefix=CHECK-3 %s

clang/test/CodeCompletion/variadic-template.cpp

@@ -409,7 +409,8 @@ struct Bar2Template : public BarTemplates {
 // CHECK-CC22-NEXT: Objective-C interface
 
 // RUN: c-index-test -code-completion-at=%s:64:10 %s | FileCheck -check-prefix=CHECK-CC23 %s
-// CHECK-CC23: OverloadCandidate:{ResultType void}{Text foo_12}{LeftParen (}{CurrentParameter int}{Comma , }{Placeholder int}{RightParen )} (1)
+
+// CHECK-CC23: OverloadCandidate:{ResultType void}{Text foo_12}{LeftParen (}{CurrentParameter T}{Comma , }{Placeholder T}{RightParen )} (1)
 // CHECK-CC23: Completion contexts:
 // CHECK-CC23-NEXT: Any type
 // CHECK-CC23-NEXT: Any value
@@ -702,7 +703,7 @@ struct Bar2Template : public BarTemplates {
 // CHECK-CC46-NEXT: Objective-C interface
 
 // RUN: c-index-test -code-completion-at=%s:84:12 %s | FileCheck -check-prefix=CHECK-CC47 %s
-// CHECK-CC47: OverloadCandidate:{ResultType void}{Text foo_12}{LeftParen (}{CurrentParameter int}{Comma , }{Placeholder int}{RightParen )} (1)
+// CHECK-CC47: OverloadCandidate:{ResultType void}{Text foo_12}{LeftParen (}{CurrentParameter T}{Comma , }{Placeholder T}{RightParen )} (1)
 // CHECK-CC47: Completion contexts:
 // CHECK-CC47-NEXT: Any type
 // CHECK-CC47-NEXT: Any value

clang/test/Index/complete-call.cpp

@@ -1012,6 +1012,49 @@ static_assert(__is_same(X<B{0}>, X<B{0}>));
 static_assert(!__is_same(X<B{0}>, X<B{1}>));
 } // namespace defaulted_compare
 
+namespace static_overloaded_operator {
+struct A {
+  template<auto N>
+  static void operator()(const char (&)[N]);
+  void operator()(this auto &&, auto &&);
+
+  void implicit_this() {
+    operator()("123");
+  }
+};
+
+struct B {
+  template<auto N>
+  void operator()(this auto &&, const char (&)[N]);
+  static void operator()(auto &&);
+
+  void implicit_this() {
+    operator()("123");
+  }
+};
+
+struct C {
+  template<auto N>
+  static void operator[](const char (&)[N]);
+  void operator[](this auto &&, auto &&);
+
+  void implicit_this() {
+    operator[]("123");
+  }
+};
+
+struct D {
+  template<auto N>
+  void operator[](this auto &&, const char (&)[N]);
+  static void operator[](auto &&);
+
+  void implicit_this() {
+    operator[]("123");
+  }
+};
+
+} // namespace static_overloaded_operator
+
 namespace GH102025 {
 struct Foo {
   template <class T>

clang/test/SemaCXX/cxx2b-deducing-this.cpp

@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -std=c++23 -verify %s
+
+namespace t1 {
+template<bool> struct enable_if { typedef void type; };
+template <class T> class Foo {};
+template <class X> constexpr bool check() { return true; }
+template <class X, class Enable = void> struct Bar {};
+
+template<class X> void func(Bar<X, typename enable_if<check<X>()>::type>) {}
+// expected-note@-1 {{candidate function}}
+
+template<class T> void func(Bar<Foo<T>>) {}
+// expected-note@-1 {{candidate function}}
+
+void g() {
+  func(Bar<Foo<int>>()); // expected-error {{call to 'func' is ambiguous}}
+}
+} // namespace t1
+
+namespace t2 {
+template <bool> struct enable_if;
+template <> struct enable_if<true> {
+  typedef int type;
+};
+struct pair {
+  template <int = 0> pair(int);
+  template <class _U2, enable_if<__is_constructible(int &, _U2)>::type = 0>
+  pair(_U2 &&);
+};
+int test_test_i;
+void test() { pair{test_test_i}; }
+} // namespace t2

clang/test/SemaTemplate/GH18291.cpp

@@ -74,6 +74,20 @@ namespace class_template {
   // new-error@-1 {{ambiguous partial specialization}}
 } // namespace class_template
 
+namespace class_template_func {
+  template <class T1, class T2 = float> struct A {};
+
+  template <template <class T4> class TT1, class T5> void f(TT1<T5>);
+  // new-note@-1 {{candidate function}}
+
+  template <class T6, class T7>                      void f(A<T6, T7>) {};
+  // new-note@-1 {{candidate function}}
+
+  void g() {
+    f(A<int>()); // new-error {{call to 'f' is ambiguous}}
+  }
+} // namespace class_template_func
+
 namespace type_pack1 {
   template<class T2> struct A;
   template<template<class ...T3s> class TT1, class T4> struct A<TT1<T4>>   ;

clang/test/SemaTemplate/cwg2398.cpp

@@ -458,17 +458,13 @@ namespace dependent_nested_partial_specialization {
 namespace nondependent_default_arg_ordering {
   int n, m;
   template<typename A, A B = &n> struct X {};
-  template<typename A> void f(X<A>); // expected-note {{candidate}}
-  template<typename A> void f(X<A, &m>); // expected-note {{candidate}}
-  template<typename A, A B> void f(X<A, B>); // expected-note 2{{candidate}}
+  template<typename A> void f(X<A>);
+  template<typename A> void f(X<A, &m>);
+  template<typename A, A B> void f(X<A, B>);
   template<template<typename U, U> class T, typename A, int *B> void f(T<A, B>);
   void g() {
-    // FIXME: The first and second function templates above should be
-    // considered more specialized than the third, but during partial
-    // ordering we fail to check that we actually deduced template arguments
-    // that make the deduced A identical to A.
-    X<int *, &n> x; f(x); // expected-error {{ambiguous}}
-    X<int *, &m> y; f(y); // expected-error {{ambiguous}}
+    X<int *, &n> x; f(x);
+    X<int *, &m> y; f(y);
   }
 }