Better message for "Function ... could always be true"

[JukkaL]

Consider this example:

def f(x: int) -> str: pass

if f:
    pass

It generates this error:

t.py:3: error: Function "Callable[[int], str]" could always be true in boolean context  [truthy-function]

Instead, if we have a direct reference to a function definition, we could generate a message like this which would be clearer, especially if the signature is complex:

t.py:3: error: Function "f" could always be true in boolean context  [truthy-function]

There are at least these different cases to consider:

1. Direct reference to a function using a short name of a function (similar to the above example).
2. Direct reference to a function using a module prefix (e.g. if mod.func:).
3. Reference to a method (e.g. if obj.method:).
4. Reference to a variable/attribute (Var node) with type Callable[...] (in this case we could show the name of the variable/attribute or the callable type -- both would be okay).
5. Any other expression that produces a callable value, such as if func(): where func returns a callable. Here the best option is to show the callable type as we do now.

[ashaik4]

@JukkaL This is my initial attempt to make the message better. Please let me know if that looks good.
I will proceed with other cases as mentioned above.

[Aryanryn09]

can we implement this behaviour instead

def f(x: int) -> str: pass

if callable(f):
pass

[ikonst]

While at it, would it favor usability by adding the presumptuous "; did you mean to call it?" ...?

[spatinom]

can this be assigned to me?

[Neo77-cyber]

Is this issue still open?

[JukkaL]

This issue is still open, since there is no PR with test cases so we don't whether any attempts are working or not. PRs are welcome!