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Problem With Comparison Operator <=> in G++

988 Recognized Expert Contributor
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers, it seems that the internal comparison operator "<=>" tries to promote arguments from unsigned to signed.
This is as boiled down as I can make it.
Here is my compilation command:
Expand|Select|Wrap|Line Numbers
  1. g++-12 -std=c++20 -Wnarrowing bit_field.cpp
Here is the code in bit_field.cpp:
Expand|Select|Wrap|Line Numbers
  1. //
  2. //  bit_field.cpp
  3. //  - demonstrate problem with bit field comparisons
  4. //
  6. #include <cstdint>
  7. #include <iostream>
  9. struct thing_t
  10. {
  11.   ::std::uint32_t value : 4;
  12.   auto operator<=>(thing_t const &that) const
  13.   {
  14.     return  (this->value <=> that.value);
  15.   }
  16. };
  18. int main()
  19. {
  20.   thing_t s0{1};
  21.   thing_t s1{2};
  23.   ::std::cout
  24.     << ::std::boolalpha
  25.     << "s0 -- s1 ? " << ((s0 <=> s1) == 0) << "\n";
  27.   return 0;
  28. }
And, here are the errors which I see:
Expand|Select|Wrap|Line Numbers
  1. bit_field.cpp: In member function ‘auto thing_t::operator<=>(const thing_t&) const’:
  2. bit_field.cpp:14:20: warning: narrowing conversion of ‘((const thing_t*)this)->thing_t::value’ from ‘const uint32_t’ {aka ‘const unsigned int’} to ‘int’ [-Wnarrowing]
  3.    14 |     return  (this->value <=> that.value);
  4.       |              ~~~~~~^~~~~
  5. bit_field.cpp:14:35: warning: narrowing conversion of ‘that.thing_t::value’ from ‘const uint32_t’ {aka ‘const unsigned int’} to ‘int’ [-Wnarrowing]
  6.    14 |     return  (this->value <=> that.value);
  7.       |                              ~~~~~^~~~~
Needless to say, I am confused.
My understanding from what I have read is that "value" would be promoted to a fully unsigned int32_t value, which ought be comparable to another of the same type.
My gut reaction is that I have encountered a compiler bug, however I have found only a few in my career, so likely not.
Obviously I have done something stupid, as I can usually read multiple sources of documentation and ascertain where I encroached into undefined behaviour.
Can any of you provide explanatory links or enlighten me?
Thank You,
4 Weeks Ago #1
0 7582

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