[cfe-dev] -Wambiguous-reversed-operator with equal types

Nuno Lopes via cfe-dev cfe-dev at lists.llvm.org
Thu Dec 10 13:50:53 PST 2020

Thanks, Richard. That works great!

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






From: Richard Smith <richard at metafoo.co.uk> 
Sent: 10 December 2020 21:39
To: Nuno Lopes <nunoplopes at sapo.pt>
Cc: Clang Dev <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] -Wambiguous-reversed-operator with equal types


On Thu, 10 Dec 2020 at 06:46, Nuno Lopes via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> > wrote:


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

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
      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

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


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; }




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



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