candid | Candid Library for the Internet Computer | Machine Learning library

overload resolution […] chooses a single best constructor […] to perform the initialization of an object of type X from the argument initializer list

so the fact that the initialization of B from {1} would be ill-formed is irrelevant.

There are several such places where implicit conversion sequences are more liberal than actual initialization, causing certain cases to be ambiguous even though some of the possibilities wouldn’t actually work. If the programmer was confused and thought one of those near misses was actually a better match, it’s a feature that the ambiguity is reported.

Translating to your particular example, when comparing operator==(int, int) and operator==(float, int) to determine which is the better candidate, we have to determine which one has the better implicit conversion sequence for the first argument (obviously in the second argument, no conversion is required). For the first argument of operator==(int, int), we just use A::operator int. For the first argument of operator==(float, int), there is no way to decide whether to use A::operator int or A::operator char, so we get the "ambiguous conversion sequence". The overload resolution rules say that the ambiguous conversion sequence is no better or worse than any other user-defined conversion sequence. Therefore, the straightforward conversion from A{} to int (via A::operator int) is not considered better than the ambiguous conversion from A{} to float. This means neither operator== candidate is better than the other.

Clang is apparently following the letter of the standard whereas GCC and MSVC are probably doing something else because of the standard seeming to be broken here. "Which compiler is right" depends on your opinion about what the standard should say. There is no proposed resolution on the issues page.

I would suggest removing operator char unless you really, really need it, in which case you will have to think about what else you're willing to give up.

Basically,

sizeof(T)>0 is an atomic constraint, so [temp.constr.atomic]/3 applies:

To determine if an atomic constraint is satisfied, the parameter mapping and template arguments are first substituted into its expression. If substitution results in an invalid type or expression, the constraint is not satisfied. [...]

sizeof(void)>0 is an invalid expression, so that constraint is not satisfied, and constraint evaluation proceeds to sizeof(U)>0.

As in the linked question, an alternative workaround is to use "requires requires requires"; demo: