Union should work for constrained type variable

[hannes-ucsc]

https://mypy-play.net/?mypy=master&python=3.12&gist=f0db7bce0a6d929cff485bd50c76a158

def truncate[S:(str, bytes)](s: S) -> S:
    return s[:10]
    
def log(v: str | bytes):
    print(truncate(v))  # error: Value of type variable "S" of "truncate" cannot be "str | bytes"  [type-var]

How can I avoid the error without narrowing to every possible alternative of the union (note how the if and else branches are identical) …

def log2(v: str | bytes):
    if isinstance(v, str):
        print(truncate(v))
    else:
        print(truncate(v))

… or adding overloads to truncate? Both seem like inelegant and tedious workarounds.

[Daverball]

If you want to be able to pass in an union you should use an upper bound of str | bytes, rather than a constrained type variable.

It's rare that you actually need a constrained type variable. They're not very well specced out currently, so you can expect many bugs and weird corner cases.

def truncate[S: str | bytes](s: S) -> S:
    return s[:10]
    
def log(v: str | bytes):
    print(truncate(v))

I see that this version leads to a different error in mypy for return s[:10], probably because bytes.__getitem__ is overloaded. This error is a bug however, so you can safely type ignore it and open an issue in the mypy bug tracker, if there isn't already one.