[clang] Eliminate relational function pointer comparisons in all C++ modes

[mizvekov]

Summary

Word on the grapevine is that the committee has unanimous
agreement on eliminating relational function pointer comparisons.
There isn't any wording on it or anything, yet, but we must jump the gun here
and just do away with it pronto.

Signed-off-by: Matheus Izvekov <[e-mail]>

[mizvekov]

• Update more tests.
• Slightly improved error recovery: deduce type for non-three-way relational comparisons.

[mizvekov]

• Add one more test case.
• Formating

[mizvekov]

clang/test/Parser/cxx-template-argument.cpp

28 ↗

(On Diff #353651)

So here we are recovering from the parser error into this type check error.
Maybe there is something that could be improved as a follow up task so we don't get a double error.

[• Quuxplusone]

There isn't any wording on it or anything, yet, but we must jump the gun here and just do away with it pronto.

I think "must" is the wrong word here. "Might as well"? Anyway, I agree with this general idea, FWLIW.

clang/lib/Sema/SemaExpr.cpp

11815

Peanut gallery says: Is QualType() the right "placeholder" to return here? IIUC, this is the situation where we've diagnosed an ill-formed expression and are just trying to do error-recovery: if the expression looks like x < y then we assume the programmer wants it to return bool, and if the expression looks like x <=> y then we assume the programmer wants it to return... QualType()? Is that the same thing we'd do for e.g. x + y or undeclaredfunction(x)? (If so, good, I think.)

clang/test/Parser/cxx-template-argument.cpp

28 ↗

(On Diff #353651)

Since this is a parsing test, not a semantics test, I think it should avoid doing anything sketchy semantic-wise. It should just be rewritten as something like

struct RHS {
    friend void operator==(void(*)(), RHS) {}
    friend void operator>=(void(*)(), RHS) {}
};
(void)(&t<int>==RHS());
(void)(&t<int>>=RHS());
(void)(&t<S<int>>==RHS());
(void)(&t<S<int>>>=RHS());

clang/test/SemaCXX/compare-function-pointer.cpp

12

I believe you should also test these same cases for a OP c where c is a different function pointer type, e.g. int (*c)();

clang/test/SemaTemplate/resolve-single-template-id.cpp

73–75

Cast (void)(x < y) here to suppress one of these irrelevant warnings.
The combination of warning "expr always evaluates to false" and erroring "expr is ill-formed" is also silly, but I suppose we can't do much about it.

[mizvekov]

clang/lib/Sema/SemaExpr.cpp

11815

So right now, this is the same we are doing for three-way comparison between builtin types we do not support / recognize. Check just below in this same function. Just look for the call to getComparisonCategoryForBuiltinCmp.

I agree completely with you and I was going to raise the same point, but I would prefer we changed both places at once instead of fixing it just here, so I think this should be a follow up task: Find something more appropriate to return here.

clang/test/Parser/cxx-template-argument.cpp

28 ↗

(On Diff #353651)

I like that idea.

clang/test/SemaCXX/compare-function-pointer.cpp

12

Sounds good.

clang/test/SemaTemplate/resolve-single-template-id.cpp

73–75

I tried avoid changing the original test because I am not sure what the original intention was, but I agree in principle the two errors already give enough indication that the compiler is figuring out what is happening here correctly.

[mizvekov]

Implement 2/3 of Arthur's suggestions.

[mizvekov]

clang/test/SemaTemplate/resolve-single-template-id.cpp

73–75

Taking another look at this test, it is doing too much weird stuff like this all over the place, and I am a bit hesitant to make such a change because the test scope is really not clear here...

I would be more open to suppressing this warning by passing a command line flag, but even then I would wait for a second opinion.

Some crazy talk on my part here, but I wonder a bit if this is somehow bothering us because there are so many diagnostics thrown in such small amount of code. Perhaps a bit like how we might feel pointing someone out for too many errors is not very social?
Suppose the user writes this 10 character statement and he gets one error and two warnings, that might offend him a little bit, maybe?

[rsmith]

I would prefer to split this into two changes:

1. Stop providing a builtin overload candidate R operator<=>(P, P) where P is a function pointer type, to be consistent with the behavior of the built-in <=> operator.
2. Stop providing </<=/>/>= support for function pointer types.

I think (1) is a (relatively) safe change that we can make immediately with very little roll-out concern. <=> is new, and the built-in candidate would never work anyway, so this is unlikely to cause problems for any real code, and has had clear and unanimous support in C++ committee discussions. So let's do that.

I think (2) is a lot riskier for existing code, and I think there's at least some chance that the C++ committee will end up effectively asking for change (1) but not change (2). It also looks like this part of the change will require a matching change in the standard library, because std::less on function pointer types would presumably still be required to work. So I think a safer approach here would be to add an enabled-by-default warning for relational comparisons of function pointers now, and return to this later to make the comparisons ill-formed if it still looks like the C++ committee is moving in that direction.

Does that sound reasonable?

clang/lib/Sema/SemaExpr.cpp

11807

11815

A null QualType is in this case treated as an indication that the comparison is invalid, recovery failed, and the callee has already produced a diagnostic. (It's a bit sad that we don't generally use a separate TypeResult for this kind of thing in order to distinguish between the "no type" answer and the "invalid and I've produced a diagnostic" answer, but if we wanted to change that I think we should aim to do it holistically across all of Clang.)

clang/lib/Sema/SemaOverload.cpp

9226–9228

An /*IsOrdered*/ comment here and below (or use of an enum) would aid readability.

[mizvekov]

In D104680#2839309, @rsmith wrote:

I would prefer to split this into two changes:
...
Does that sound reasonable?

Yeah that is fine, totally understand ;)

clang/lib/Sema/SemaExpr.cpp

11815

Hmm. My thinking here is that it would be totally appropriate to error recovery with a 'strong_ordering' type, since that can decay into all the other orderings, making it kind of a safe bet that it would not cause further problems.

The idea of a separate type for error recovery is intriguing. But I wonder if it would be appropriate in some cases, but not all of them.

• The cases where all the options are equally bad and we have no good guesses would fit the bill, but this is not the case here.
• When we have a good safe bet, we can in principle just use that type. But maybe it would make sense to add a bit to it that means something like "but don't take this too seriously", which could be used to selectively suppress some diagnostics downstream.

[mizvekov]

First part of the split implemented at: https://reviews.llvm.org/D104892
I will reuse this DR for the second part.

[mizvekov]

• Rebase after split.
• Add tests which I had forgotten, covering the builtin candidates for the relational operators other than spaceship.

[mizvekov]

• Fix missing period as pointed by rsmith.

[joerg]

Is there any reason for breaking C++03 code here? I don't see the advantage to that at all.

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_typecheck_three_way_comparison_of_pointer_and_zero : Error<
   "three-way comparison between pointer and zero">;
 def ext_typecheck_compare_complete_incomplete_pointers : Extension<
   "pointer comparisons before C11 "
   "need to be between two complete or two incomplete types; "
   "%0 is %select{|in}2complete and "
   "%1 is %select{|in}3complete">,
   InGroup<C11>;
-def warn_typecheck_ordered_comparison_of_function_pointers : Warning<
-  "ordered comparison of function pointers (%0 and %1)">,
-  InGroup<OrderedCompareFunctionPointers>;
 def ext_typecheck_ordered_comparison_of_function_pointers : ExtWarn<
   "ordered comparison of function pointers (%0 and %1)">,
   InGroup<OrderedCompareFunctionPointers>;
 def err_typecheck_ordered_comparison_of_function_pointers : Error<
   "ordered comparison of function pointers (%0 and %1)">;
 def ext_typecheck_comparison_of_fptr_to_void : Extension<
   "equality comparison between function pointer and void pointer (%0 and %1)">;
 def err_typecheck_comparison_of_fptr_to_void : Error<

clang/lib/Sema/SemaExpr.cpp

if (RHSIsNull)
                                    RHS.get()->getSourceRange());
     else
       DiagnoseAlwaysNonNullPointer(RHS.get(), LHSNullKind, IsEquality,
                                    LHS.get()->getSourceRange());
   }
 
   if (IsOrdered && LHSType->isFunctionPointerType() &&
       RHSType->isFunctionPointerType()) {
-    // Valid unless a relational comparison of function pointers
-    bool IsError = Opc == BO_Cmp;
-    auto DiagID =
-        IsError ? diag::err_typecheck_ordered_comparison_of_function_pointers
-        : getLangOpts().CPlusPlus
-            ? diag::warn_typecheck_ordered_comparison_of_function_pointers
-            : diag::ext_typecheck_ordered_comparison_of_function_pointers;
-    Diag(Loc, DiagID) << LHSType << RHSType << LHS.get()->getSourceRange()
-                      << RHS.get()->getSourceRange();
+    // Valid unless a relational comparison of function pointers.
+    bool IsError = getLangOpts().CPlusPlus;
+    Diag(Loc, IsError
+                  ? diag::err_typecheck_ordered_comparison_of_function_pointers
+                  : diag::ext_typecheck_ordered_comparison_of_function_pointers)
+        << LHSType << RHSType << LHS.get()->getSourceRange()
     if (IsError)
-      return QualType();
+      return Opc != BO_Cmp ? Context.getLogicalOperationType() : QualType();
   }
 
   if ((LHSType->isIntegerType() && !LHSIsNull) ||
       (RHSType->isIntegerType() && !RHSIsNull)) {
     // Skip normal pointer conversion checks in this case; we have better
     // diagnostics for this below.
   } else if (getLangOpts().CPlusPlus) {
     // Equality comparison of a function pointer to a void pointer is invalid,

clang/lib/Sema/SemaOverload.cpp

public:
   //
   //        bool       operator<(T, T);
   //        bool       operator>(T, T);
   //        bool       operator<=(T, T);
   //        bool       operator>=(T, T);
   //        bool       operator==(T, T);
   //        bool       operator!=(T, T);
   //           R       operator<=>(T, T)
-  void addGenericBinaryPointerOrEnumeralOverloads(bool IsSpaceship) {
+  void addGenericBinaryPointerOrEnumeralOverloads(bool IsOrdered) {
     // C++ [over.match.oper]p3:
     //   [...]the built-in candidates include all of the candidate operator
     //   functions defined in 13.6 that, compared to the given operator, [...]
     //   do not have the same parameter-type-list as any non-template non-member
     //   candidate.
     //
     // Note that in practice, this only affects enumeration types because there
     // aren't any built-in candidates of record type, and a user-defined operator
void addGenericBinaryPointerOrEnumeralOverloads(bool IsOrdered) {
     /// Set of (canonical) types that we've already handled.
     llvm::SmallPtrSet<QualType, 8> AddedTypes;
 
     for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
       for (QualType PtrTy : CandidateTypes[ArgIdx].pointer_types()) {
         // Don't add the same builtin candidate twice.
         if (!AddedTypes.insert(S.Context.getCanonicalType(PtrTy)).second)
           continue;
-        if (IsSpaceship && PtrTy->isFunctionPointerType())
+        if (IsOrdered && PtrTy->isFunctionPointerType())
           continue;
 
         QualType ParamTypes[2] = {PtrTy, PtrTy};
         S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
       }
       for (QualType EnumTy : CandidateTypes[ArgIdx].enumeration_types()) {
         CanQualType CanonType = S.Context.getCanonicalType(EnumTy);
 
void Sema::AddBuiltinOperatorCandidates(OverloadedOperatorKind Op,
   case OO_MinusMinus:
     OpBuilder.addPlusPlusMinusMinusArithmeticOverloads(Op);
     OpBuilder.addPlusPlusMinusMinusPointerOverloads();
     break;
 
   case OO_EqualEqual:
   case OO_ExclaimEqual:
     OpBuilder.addEqualEqualOrNotEqualMemberPointerOrNullptrOverloads();
-    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsSpaceship=*/false);
+    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsOrdered=*/false);
     OpBuilder.addGenericBinaryArithmeticOverloads();
     break;
 
   case OO_Less:
   case OO_Greater:
   case OO_LessEqual:
   case OO_GreaterEqual:
-    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsSpaceship=*/false);
+    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsOrdered=*/true);
     OpBuilder.addGenericBinaryArithmeticOverloads();
     break;
 
   case OO_Spaceship:
-    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsSpaceship=*/true);
+    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsOrdered=*/true);
     OpBuilder.addThreeWayArithmeticOverloads();
     break;
 
   case OO_Percent:
   case OO_Caret:
   case OO_Pipe:
   case OO_LessLess:
   case OO_GreaterGreater:

clang/test/CXX/drs/dr15xx.cpp

template<typename A, typename B, typename C> void composite_pointer_type_is_base() {
     typedef __typeof(val<A>() == val<B>()) cmp;
     typedef __typeof(val<A>() != val<B>()) cmp;
     typedef bool cmp;
   }
 
   template<typename A, typename B, typename C> void composite_pointer_type_is_ord() {
     composite_pointer_type_is_base<A, B, C>();
 
-    typedef __typeof(val<A>() < val<B>()) cmp;  // cxx17-warning 2 {{ordered comparison of function pointers}}
-    typedef __typeof(val<A>() <= val<B>()) cmp; // cxx17-warning 2 {{ordered comparison of function pointers}}
-    typedef __typeof(val<A>() > val<B>()) cmp;  // cxx17-warning 2 {{ordered comparison of function pointers}}
-    typedef __typeof(val<A>() >= val<B>()) cmp; // cxx17-warning 2 {{ordered comparison of function pointers}}
+    typedef __typeof(val<A>() < val<B>()) cmp;
+    typedef __typeof(val<A>() <= val<B>()) cmp;
+    typedef __typeof(val<A>() > val<B>()) cmp;
+    typedef __typeof(val<A>() >= val<B>()) cmp;
     typedef bool cmp;
   }
 
   template <typename A, typename B, typename C>
   void composite_pointer_type_is_unord(int = 0) {
     composite_pointer_type_is_base<A, B, C>();
   }
   template <typename A, typename B, typename C>
#endif
     composite_pointer_type_is_ord<const B*, volatile A*, const volatile A*>();
 
     composite_pointer_type_is_unord<const int *A::*, volatile int *B::*, const volatile int *const B::*>();
     composite_pointer_type_is_unord<const int *B::*, volatile int *A::*, const volatile int *const B::*>();
     no_composite_pointer_type<int (A::*)(), int (C::*)()>();
     no_composite_pointer_type<const int (A::*)(), volatile int (C::*)()>();
 
 #if __cplusplus > 201402
-    composite_pointer_type_is_ord<int (*)() noexcept, int (*)(), int (*)()>(); // expected-note {{requested here}}
-    composite_pointer_type_is_ord<int (*)(), int (*)() noexcept, int (*)()>(); // expected-note {{requested here}}
+    composite_pointer_type_is_unord<int (*)() noexcept, int (*)(), int (*)()>();
+    composite_pointer_type_is_unord<int (*)(), int (*)() noexcept, int (*)()>();
     composite_pointer_type_is_unord<int (A::*)() noexcept, int (A::*)(), int (A::*)()>();
     composite_pointer_type_is_unord<int (A::*)(), int (A::*)() noexcept, int (A::*)()>();
     // FIXME: This looks like a standard defect; these should probably all have type 'int (B::*)()'.
     composite_pointer_type_is_unord<int (B::*)(), int (A::*)() noexcept, int (B::*)()>();
     composite_pointer_type_is_unord<int (A::*)() noexcept, int (B::*)(), int (B::*)()>();
     composite_pointer_type_is_unord<int (B::*)() noexcept, int (A::*)(), int (B::*)()>();
     composite_pointer_type_is_unord<int (A::*)(), int (B::*)() noexcept, int (B::*)()>();
 

clang/test/CXX/drs/dr3xx.cpp

 
 namespace dr301 { // dr301: yes
   // see also dr38
   struct S;
   template<typename T> void operator+(T, T);
   void operator-(S, S);
 
   void f() {
-    bool a = (void(*)(S, S))operator+<S> < // expected-warning {{ordered comparison of function pointers}}
+    bool a = (void(*)(S, S))operator+<S> < // expected-error {{ordered comparison of function pointers}}
              (void(*)(S, S))operator+<S>;
-    bool b = (void(*)(S, S))operator- < // cxx20_2b-note {{to match this '<'}} cxx98_17-warning {{ordered comparison of function pointers}}
+    bool b = (void(*)(S, S))operator- < // cxx20_2b-note {{to match this '<'}} cxx98_17-error {{ordered comparison of function pointers}}
              (void(*)(S, S))operator-; // cxx20_2b-error {{expected '>'}}
     bool c = (void(*)(S, S))operator+ < // expected-note {{to match this '<'}}
              (void(*)(S, S))operator-; // expected-error {{expected '>'}}
   }
 
   template<typename T> void f() {
     typename T::template operator+<int> a; // expected-error {{typename specifier refers to a non-type template}} expected-error +{{}}
     // FIXME: This shouldn't say (null).

clang/test/CXX/expr/expr.const/p2-0x.cpp

namespace UnspecifiedRelations {
   constexpr bool u8 = p >= (int*)0; // expected-error {{constant expression}} expected-note {{comparison has unspecified value}}
   constexpr bool u9 = (int*)0 < q; // expected-error {{constant expression}} expected-note {{comparison has unspecified value}}
   constexpr bool u10 = (int*)0 <= q; // expected-error {{constant expression}} expected-note {{comparison has unspecified value}}
   constexpr bool u11 = (int*)0 > q; // expected-error {{constant expression}} expected-note {{comparison has unspecified value}}
   constexpr bool u12 = (int*)0 >= q; // expected-error {{constant expression}} expected-note {{comparison has unspecified value}}
   void f(), g();
 
   constexpr void (*pf)() = &f, (*pg)() = &g;
-  constexpr bool u13 = pf < pg; // expected-error {{constant expression}} expected-note {{comparison has unspecified value}}
-                                // expected-warning@-1 {{ordered comparison of function pointers}}
-  constexpr bool u14 = pf == pg;
+  constexpr bool u13 = pf == pg;
 
   // If two pointers point to non-static data members of the same object with
   // different access control, the result is unspecified.
   struct A {
   public:
     constexpr A() : a(0), b(0) {}
     int a;
     constexpr bool cmp() const { return &a < &b; } // expected-note {{comparison of address of fields 'a' and 'b' of 'A' with differing access specifiers (public vs private) has unspecified value}}

clang/test/SemaCXX/compare-function-pointer.cpp

 // RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
 
 using fp0_t = void (*)();
 using fp1_t = int (*)();
 
+namespace test_builtin {
+
 extern fp0_t a, b;
 extern fp1_t c;
 
 bool eq0 = a == b;
 bool ne0 = a != b;
-bool lt0 = a < b;   // expected-warning {{ordered comparison of function pointers ('fp0_t' (aka 'void (*)()') and 'fp0_t')}}
-bool le0 = a <= b;  // expected-warning {{ordered comparison of function pointers}}
-bool gt0 = a > b;   // expected-warning {{ordered comparison of function pointers}}
-bool ge0 = a >= b;  // expected-warning {{ordered comparison of function pointers}}
-auto tw0 = a <=> b; // expected-error {{ordered comparison of function pointers}}
+bool lt0 = a < b;   // expected-error {{ordered comparison of function pointers ('fp0_t' (aka 'void (*)()') and 'fp0_t')}}
+bool le0 = a <= b;  // expected-error {{ordered comparison of function pointers}}
+bool gt0 = a > b;   // expected-error {{ordered comparison of function pointers}}
+bool ge0 = a >= b;  // expected-error {{ordered comparison of function pointers}}
 
 bool eq1 = a == c;  // expected-error {{comparison of distinct pointer types}}
 bool ne1 = a != c;  // expected-error {{comparison of distinct pointer types}}
-bool lt1 = a < c;   // expected-warning {{ordered comparison of function pointers ('fp0_t' (aka 'void (*)()') and 'fp1_t' (aka 'int (*)()'))}}
-                    // expected-error@-1 {{comparison of distinct pointer types}}
-bool le1 = a <= c;  // expected-warning {{ordered comparison of function pointers}}
-                    // expected-error@-1 {{comparison of distinct pointer types}}
-bool gt1 = a > c;   // expected-warning {{ordered comparison of function pointers}}
-                    // expected-error@-1 {{comparison of distinct pointer types}}
-bool ge1 = a >= c;  // expected-warning {{ordered comparison of function pointers}}
-                    // expected-error@-1 {{comparison of distinct pointer types}}
+bool lt1 = a < c;   // expected-error {{ordered comparison of function pointers ('fp0_t' (aka 'void (*)()') and 'fp1_t' (aka 'int (*)()'))}}
+bool le1 = a <= c;  // expected-error {{ordered comparison of function pointers}}
+bool gt1 = a > c;   // expected-error {{ordered comparison of function pointers}}
+bool ge1 = a >= c;  // expected-error {{ordered comparison of function pointers}}
 auto tw1 = a <=> c; // expected-error {{ordered comparison of function pointers}}
+
+} // namespace test_builtin
+
+namespace test_overload {
+
+template<class T> struct wrapper { operator T(); };
+
+extern wrapper<fp0_t> a, b;
+extern wrapper<fp1_t> c;
+
+bool eq0 = a == b;
+bool ne0 = a != b;
+bool lt0 = a < b;   // expected-error {{invalid operands to binary expression}}
+bool le0 = a <= b;  // expected-error {{invalid operands to binary expression}}
+bool gt0 = a > b;   // expected-error {{invalid operands to binary expression}}
+bool ge0 = a >= b;  // expected-error {{invalid operands to binary expression}}
+
+bool eq1 = a == c;  // expected-error {{invalid operands to binary expression}}
+bool ne1 = a != c;  // expected-error {{invalid operands to binary expression}}
+bool lt1 = a < c;   // expected-error {{invalid operands to binary expression}}
+bool le1 = a <= c;  // expected-error {{invalid operands to binary expression}}
+bool gt1 = a > c;   // expected-error {{invalid operands to binary expression}}
+bool ge1 = a >= c;  // expected-error {{invalid operands to binary expression}}
+auto tw1 = a <=> c; // expected-error {{invalid operands to binary expression}}
+
+} // namespace test_overload

clang/test/SemaTemplate/resolve-single-template-id.cpp

int main()
   void (*p)() = oneT<int>;
   test<oneT<int> > ti;
   void (*u)(int) = oneT<int>;
 
   b = (void (*)()) twoT<int>;
 
   one < one; // expected-warning {{self-comparison always evaluates to false}} \
              // expected-warning {{relational comparison result unused}}       \
-             // expected-warning {{ordered comparison of function pointers}}
+             // expected-error   {{ordered comparison of function pointers}}
 
   oneT<int> < oneT<int>; // expected-warning {{self-comparison always evaluates to false}} \
                          // expected-warning {{relational comparison result unused}}       \
-                         // expected-warning {{ordered comparison of function pointers}}
+                         // expected-error   {{ordered comparison of function pointers}}
 
   two < two; //expected-error 2 {{reference to overloaded function could not be resolved; did you mean to call it with no arguments?}} expected-error {{invalid operands to binary expression ('void' and 'void')}}
   twoT<int> < twoT<int>; //expected-error {{reference to overloaded function could not be resolved; did you mean to call it?}} {{cannot resolve overloaded function 'twoT' from context}}
   oneT<int> == 0;   // expected-warning {{equality comparison result unused}} \
                     // expected-note {{use '=' to turn this equality comparison into an assignment}} \
                     // expected-warning {{comparison of function 'oneT<int>' equal to a null pointer is always false}} \
                     // expected-note {{prefix with the address-of operator to silence this warning}}