ternary of Tuple[builtins.str] / Tuple[] falling to builtins.object

[asottile]

version

$ mypy --version
mypy 0.600

code

import sys

tty = ('--tty',) if sys.stdout.isatty() else ()
#reveal_type(tty)
#reveal_type(('--tty',))
#reveal_type(())
cmd = ('docker', 'run', *tty, 'ubuntu:bionic', 'bash')

run (without revealed types)

$ mypy test.py
test.py:7: error: List or tuple expected as variable arguments

run (with revealed types)

$ mypy test.py
test.py:4: error: Revealed type is 'builtins.object'
test.py:5: error: Revealed type is 'Tuple[builtins.str]'
test.py:6: error: Revealed type is 'Tuple[]'
test.py:7: error: List or tuple expected as variable arguments

Possibly related: #5134

[emmatyping]

I feel that tuple if bool else tuple should result in some kind of tuple. I'm not sure which, probably Tuple[str, ...] in this case?

[asottile]

I think Tuple[str, ...] would be ideal, though even the Union[...] would probably make this succeed

[ilevkivskyi]

This is essentially a duplicate of #4975, we just need to produce better joins for tuple types.