-Wambiguous-reversed-operator with equal types

Hi,

I've been trying to fix a bunch of LLVM headers so they are compatible with
C++20. (so I can include those from Alive2 source code)

I'm currently stuck with -Wambiguous-reversed-operator. I've fixed a couple,
but this one (a few similar) left me wondering if the warning is correct or
not:

In file included from llvm/include/llvm/Passes/PassBuilder.h:19:
In file included from llvm/include/llvm/Analysis/CGSCCPassManager.h:98:
llvm/include/llvm/Analysis/LazyCallGraph.h:1117:22: error: ISO C++20
considers use of overloaded operator '!=' (with operand types
'llvm::User::value_op_iterator' and 'llvm::User::value_op_iterator') to be
ambiguous despite there being a unique best viable function
[-Werror,-Wambiguous-reversed-operator]
      for (Value *Op : C->operand_values())
                     ^
llvm/include/llvm/ADT/iterator.h:263:8: note: ambiguity is between a regular
call to this operator and a call with the argument order reversed
  bool operator==(const DerivedT &RHS) const { return I == RHS.I; }
       ^

Note that it's complaining about comparing 'llvm::User::value_op_iterator'
and 'llvm::User::value_op_iterator' (i.e., same type). Since both arguments
are of the same type and the comparison function has const on both LHS/RHS,
reversing them shouldn't have any impact, right?
Unless there's some type conversion going on that the error message is
hiding.

Could someone please confirm if the warning is correct and/or the code needs
fixing?

Thanks,
Nuno

Hi,

I’ve been trying to fix a bunch of LLVM headers so they are compatible with
C++20. (so I can include those from Alive2 source code)

I’m currently stuck with -Wambiguous-reversed-operator. I’ve fixed a couple,
but this one (a few similar) left me wondering if the warning is correct or
not:

In file included from llvm/include/llvm/Passes/PassBuilder.h:19:
In file included from llvm/include/llvm/Analysis/CGSCCPassManager.h:98:
llvm/include/llvm/Analysis/LazyCallGraph.h:1117:22: error: ISO C++20
considers use of overloaded operator ‘!=’ (with operand types
‘llvm::User::value_op_iterator’ and ‘llvm::User::value_op_iterator’) to be
ambiguous despite there being a unique best viable function
[-Werror,-Wambiguous-reversed-operator]
for (Value *Op : C->operand_values())
^
llvm/include/llvm/ADT/iterator.h:263:8: note: ambiguity is between a regular
call to this operator and a call with the argument order reversed
bool operator==(const DerivedT &RHS) const { return I == RHS.I; }
^

Note that it’s complaining about comparing ‘llvm::User::value_op_iterator’
and ‘llvm::User::value_op_iterator’ (i.e., same type). Since both arguments
are of the same type and the comparison function has const on both LHS/RHS,
reversing them shouldn’t have any impact, right?
Unless there’s some type conversion going on that the error message is
hiding.

Could someone please confirm if the warning is correct and/or the code needs
fixing?

The warning is correct. In C++20 there are four ways to perform this != comparison:

Using iterator_adaptor_base::operator==:

1: !((iterator_adaptor_base&)a == b) [synthesized != from operator==]
2: !((iterator_adaptor_base&)b == a) [synthesized != from operator==, reversed]

Using iterator_facade_base::operator!=:

3: (iterator_facade_base&)a != b [regular operator!=]
4: (iterator_facade_base&)b != a [regular operator!=, reversed]

Of these: 1 beats 3 (better conversion for a), 2 beats 4 (better conversion for b), and they’re otherwise unordered, so the result is an ambiguity.

Probably the cleanest solution would be to replace:

bool operator==(const DerivedT &RHS) const { return I == RHS.I; }

with:

friend bool operator==(const DerivedT &LHS, const DerivedT &RHS) const { return LHS.I == RHS.I; }

Thanks, Richard. That works great!

Ok, I’ll try to fix the remaining issues on my own :blush:

Thanks,

Nuno