f | Functional stuff for Python | Functional Programming library

pip install "pip<22"

https://github.com/jazzband/pip-tools/issues/1558

• both overloads are applicable, and;
• neither overload is more specific than the other

Notice that both the f(int, A) overload and the f(float, B) overload can be called with the parameters (i, b), since there is an implicit conversion from int to float, and an implicit conversion from B to A.

What happens when there are more than one applicable method? Java is supposed to choose the most specific method. This is described in §15.12.2.5 of the language spec. It turns out that it is not the case that one of these overloads are more specific than the other.

One applicable method m1 is more specific than another applicable method m2, for an invocation with argument expressions e1, ..., ek, if any of the following are true:

• m2 is generic [...]

• m2 is not generic, and m1 and m2 are applicable by strict or loose invocation, and where m1 has formal parameter types S1, ..., Sn and m2 has formal parameter types T1, ..., Tn, the type Si is more specific than Ti for argument ei for all i (1 ≤ i ≤ n, n = k).

• m2 is not generic, and m1 and m2 are applicable by variable arity invocation [...]

Only the second point applies to the two overloads of f. For one of the overloads to be more specific than the other, every parameter type of one overload has to be more specific than the corresponding parameter type in the other overload.

A type S is more specific than a type T for any expression if S <: T (§4.10).

Note that"<:" is the subtyping relationship. B is clearly a subtype of A. float is actually a supertype (not subtype!) of int. This can be derived from the direct subtyping relations listed in §4.10.1. Therefore, neither of the overloads is more specific than the other.

The language spec goes on to talk about maximally specific methods, which doesn't really apply to f here. Finally, it says:

Otherwise, the method invocation is ambiguous, and a compile-time error occurs.

Note that this even simpler example does not compile too:

substitute is a "special" whose arguments are not evaluated (doc).

[dcl.fct.def.general]

2 The type of a parameter or the return type for a function definition shall not be a (possibly cv-qualified) class type that is incomplete or abstract within the function body unless the function is deleted ([dcl.fct.def.delete]).

That's pretty clear I think. A deleted definition allows for an incomplete class type. It's not like the function can actually be called in a well-formed program, or the body is actually using the incomplete type in some way. The function is a placeholder to signify an invalid result to overload resolution.

Granted, the parameter types are more interesting in the case of actual overload resolution (and the return type can be anything), but there is no reason to restrict the return type into being complete here either.

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