[clang] Reapply Handle templated operators with reversed arguments (#72213)

Re-applies #69595 with extra
[diff](79181ef)
### New changes

Further relax ambiguities with a warning for member operators of a
template class (primary templates of such ops do not match). Eg:
```cpp
template <class T>
struct S {
    template <typename OtherT>
    bool operator==(const OtherT &rhs); 
};
struct A : S<int> {};
struct B : S<bool> {};
bool x = A{} == B{}; // accepted with a warning.
```

This is important for making llvm build using previous clang versions in
C++20 mode (eg: this makes the commit
e558be5 keep working with a warning
instead of an error).

### Description from #69595

#68999 correctly computed
conversion sequence for reversed args to a template operator. This was a
breaking change as code, previously accepted in C++17, starts to break
in C++20.

Example:
```cpp
struct P {};
template<class S> bool operator==(const P&, const S &);

struct A : public P {};
struct B : public P {};
bool check(A a, B b) { return a == b; }  // This is now ambiguous in C++20.
```

In order to minimise widespread breakages, as a clang extension, we had
previously accepted such ambiguities with a warning
(`-Wambiguous-reversed-operator`) for non-template operators. Due to the
same reasons, we extend this relaxation for template operators.

Fixes #53954

Author: usx95

clang/docs/ReleaseNotes.rst

@@ -37,6 +37,29 @@ These changes are ones which we think may surprise users when upgrading to
 Clang |release| because of the opportunity they pose for disruption to existing
 code bases.
 
+- Fix a bug in reversed argument for templated operators.
+  This breaks code in C++20 which was previously accepted in C++17.
+  Clang did not properly diagnose such casese in C++20 before this change. Eg:
+
+  .. code-block:: cpp
+
+    struct P {};
+    template<class S> bool operator==(const P&, const S&);
+
+    struct A : public P {};
+    struct B : public P {};
+
+    // This equality is now ambiguous in C++20.
+    bool ambiguous(A a, B b) { return a == b; }
+
+    template<class S> bool operator!=(const P&, const S&);
+    // Ok. Found a matching operator!=.
+    bool fine(A a, B b) { return a == b; }
+
+  To reduce such widespread breakages, as an extension, Clang accepts this code
+  with an existing warning ``-Wambiguous-reversed-operator`` warning.
+  Fixes `GH <https://github.com/llvm/llvm-project/issues/53954>`_.
+
 - The CMake variable ``GCC_INSTALL_PREFIX`` (which sets the default
   ``--gcc-toolchain=``) is deprecated and will be removed. Specify
   ``--gcc-install-dir=`` or ``--gcc-triple=`` in a `configuration file

clang/lib/Sema/SemaOverload.cpp

@@ -7723,9 +7723,19 @@ bool Sema::CheckNonDependentConversions(
        ++I) {
     QualType ParamType = ParamTypes[I + Offset];
     if (!ParamType->isDependentType()) {
-      unsigned ConvIdx = PO == OverloadCandidateParamOrder::Reversed
-                             ? 0
-                             : (ThisConversions + I);
+      unsigned ConvIdx;
+      if (PO == OverloadCandidateParamOrder::Reversed) {
+        ConvIdx = Args.size() - 1 - I;
+        assert(Args.size() + ThisConversions == 2 &&
+               "number of args (including 'this') must be exactly 2 for "
+               "reversed order");
+        // For members, there would be only one arg 'Args[0]' whose ConvIdx
+        // would also be 0. 'this' got ConvIdx = 1 previously.
+        assert(!HasThisConversion || (ConvIdx == 0 && I == 0));
+      } else {
+        // For members, 'this' got ConvIdx = 0 previously.
+        ConvIdx = ThisConversions + I;
+      }
       Conversions[ConvIdx]
         = TryCopyInitialization(*this, Args[I], ParamType,
                                 SuppressUserConversions,
@@ -10121,11 +10131,23 @@ getImplicitObjectParamType(ASTContext &Context, const FunctionDecl *F) {
   return M->getFunctionObjectParameterReferenceType();
 }
 
-static bool haveSameParameterTypes(ASTContext &Context, const FunctionDecl *F1,
-                                   const FunctionDecl *F2) {
+// As a Clang extension, allow ambiguity among F1 and F2 if they represent
+// represent the same entity.
+static bool allowAmbiguity(ASTContext &Context, const FunctionDecl *F1,
+                           const FunctionDecl *F2) {
   if (declaresSameEntity(F1, F2))
     return true;
-
+  auto PT1 = F1->getPrimaryTemplate();
+  auto PT2 = F2->getPrimaryTemplate();
+  if (PT1 && PT2) {
+    if (declaresSameEntity(PT1, PT2) ||
+        declaresSameEntity(PT1->getInstantiatedFromMemberTemplate(),
+                           PT2->getInstantiatedFromMemberTemplate()))
+      return true;
+  }
+  // TODO: It is not clear whether comparing parameters is necessary (i.e.
+  // different functions with same params). Consider removing this (as no test
+  // fail w/o it).
   auto NextParam = [&](const FunctionDecl *F, unsigned &I, bool First) {
     if (First) {
       if (std::optional<QualType> T = getImplicitObjectParamType(Context, F))
@@ -10329,14 +10351,14 @@ bool clang::isBetterOverloadCandidate(
     case ImplicitConversionSequence::Worse:
       if (Cand1.Function && Cand2.Function &&
           Cand1.isReversed() != Cand2.isReversed() &&
-          haveSameParameterTypes(S.Context, Cand1.Function, Cand2.Function)) {
+          allowAmbiguity(S.Context, Cand1.Function, Cand2.Function)) {
         // Work around large-scale breakage caused by considering reversed
         // forms of operator== in C++20:
         //
-        // When comparing a function against a reversed function with the same
-        // parameter types, if we have a better conversion for one argument and
-        // a worse conversion for the other, the implicit conversion sequences
-        // are treated as being equally good.
+        // When comparing a function against a reversed function, if we have a
+        // better conversion for one argument and a worse conversion for the
+        // other, the implicit conversion sequences are treated as being equally
+        // good.
         //
         // This prevents a comparison function from being considered ambiguous
         // with a reversed form that is written in the same way.
@@ -14526,7 +14548,7 @@ ExprResult Sema::CreateOverloadedBinOp(SourceLocation OpLoc,
           llvm::SmallVector<FunctionDecl*, 4> AmbiguousWith;
           for (OverloadCandidate &Cand : CandidateSet) {
             if (Cand.Viable && Cand.Function && Cand.isReversed() &&
-                haveSameParameterTypes(Context, Cand.Function, FnDecl)) {
+                allowAmbiguity(Context, Cand.Function, FnDecl)) {
               for (unsigned ArgIdx = 0; ArgIdx < 2; ++ArgIdx) {
                 if (CompareImplicitConversionSequences(
                         *this, OpLoc, Cand.Conversions[ArgIdx],

clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp

@@ -324,6 +324,124 @@ bool x = X() == X(); // expected-warning {{ambiguous}}
 }
 } // namespace P2468R2
 
+namespace GH53954{
+namespace friend_template_1 {
+struct P {
+  template <class T>
+  friend bool operator==(const P&, const T&) { return true; } // expected-note {{candidate}} \
+                              // expected-note {{ambiguous candidate function with reversed arguments}}
+};
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a == b; } // expected-warning {{use of overloaded operator '==' (with operand types 'A' and 'B') to be ambiguous}}
+}
+
+namespace friend_template_2 {
+struct P {
+  template <class T>
+  friend bool operator==(const T&, const P&) { return true; } // expected-note {{candidate}} \
+                                              // expected-note {{ambiguous candidate function with reversed arguments}}
+};
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a == b; } // expected-warning {{use of overloaded operator '==' (with operand types 'A' and 'B') to be ambiguous}}
+}
+
+namespace friend_template_class_template {
+template<class S>
+struct P {
+  template <class T>
+  friend bool operator==(const T&, const P&) { // expected-note 2 {{candidate}}
+    return true;
+  }
+};
+struct A : public P<int> {};
+struct B : public P<bool> {};
+bool check(A a, B b) { return a == b; } // expected-warning {{ambiguous}}
+}
+
+namespace friend_template_fixme {
+// FIXME(GH70210): This should not rewrite operator== and definitely not a hard error.
+struct P {
+  template <class T>
+  friend bool operator==(const T &, const P &) { return true; } // expected-note 2 {{candidate}}
+  template <class T>
+  friend bool operator!=(const T &, const P &) { return true; } // expected-note {{candidate}}
+};
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a != b; } // expected-error{{ambiguous}}
+}
+
+namespace member_template_1 {
+struct P {
+  template<class S>
+  bool operator==(const S &) const; // expected-note {{candidate}} \
+                                    // expected-note {{ambiguous candidate function with reversed arguments}}
+};
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a == b; } // expected-warning {{use of overloaded operator '==' (with operand types 'A' and 'B') to be ambiguous}}
+} // namespace member_template
+
+namespace member_template_2{
+template <typename T>
+class Foo {
+ public:
+  template <typename U = T>
+  bool operator==(const Foo& other) const;
+};
+bool x = Foo<int>{} == Foo<int>{};
+} // namespace template_member_opeqeq
+
+namespace non_member_template_1 {
+struct P {};
+template<class S>
+bool operator==(const P&, const S &); // expected-note {{candidate}} \
+                                      // expected-note {{ambiguous candidate function with reversed arguments}}
+
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a == b; } // expected-warning {{use of overloaded operator '==' (with operand types 'A' and 'B') to be ambiguous}}
+
+template<class S> bool operator!=(const P&, const S &);
+bool fine(A a, B b) { return a == b; } // Ok. Found a matching operator!=.
+} // namespace non_member_template_1
+
+namespace non_member_template_2 {
+struct P {};
+template<class S>
+bool operator==(const S&, const P&); // expected-note {{candidate}} \
+                                     // expected-note {{ambiguous candidate function with reversed arguments}}
+
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a == b; } // expected-warning {{use of overloaded operator '==' (with operand types 'A' and 'B') to be ambiguous}}
+} // namespace non_member_template_2
+
+namespace class_and_member_template {
+template <class T>
+struct S {
+    template <typename OtherT>
+    bool operator==(const OtherT &rhs); // expected-note {{candidate}} \
+                                        // expected-note {{reversed arguments}}
+};
+struct A : S<int> {};
+struct B : S<bool> {};
+bool x = A{} == B{}; // expected-warning {{ambiguous}}
+} // namespace class_and_member_template
+
+namespace ambiguous_case {
+template <class T>
+struct Foo {};
+template <class T, class U> bool operator==(Foo<U>, Foo<T*>); // expected-note{{candidate}}
+template <class T, class U> bool operator==(Foo<T*>, Foo<U>); // expected-note{{candidate}}
+
+void test() {
+    Foo<int*>() == Foo<int*>(); // expected-error{{ambiguous}}
+}
+} // namespace ambiguous_case
+} // namespace
 namespace ADL_GH68901{
 namespace test1 {
 namespace A {