mypy fails to recognize functions as generators when sys.platform is used in conditions

[marcinsulikowski]

mypy seems to be too thorough when interpreting if sys.platform == ... conditions. Consider the following file:

import sys
from typing import Generator

def f() -> Generator[int, None, None]:
    if sys.platform == "darwin":
        yield 1

I'm trying to run mypy for this file on Linux using mypy gen.py

Expected behavior:
mypy test passes for this file on Linux

Actual behavior:
I get this on Linux:

$ mypy gen.py 
gen.py:4: error: Missing return statement

At the same time, mypy on macOS does not complain. I think that this is related to #698

Platform information:

$ uname -sr; mypy --version; python --version
Linux 4.15.0-34-generic
mypy 0.630
Python 2.7.15rc1

[gvanrossum]

Agreed. It looks like the analysis that determines whether a function body contains any yields runs after the version/platform-based dead code elimination; it ought to run before that. Are you interested in trying to fix this?

[marcinsulikowski]

I could try although I'm not familiar with this codebase so I cannot guarantee anything.

After quick look into the source code, it seems to me that the following:

    def visit_block(self, b: Block) -> None:
        if b.is_unreachable:
            return
        self.block_depth[-1] += 1
        for s in b.body:
            self.accept(s)
        self.block_depth[-1] -= 1

in class SemanticAnalyzerPass2 probably needs some change in the if b.is_unreachable: case – unreachable blocks within a function scope may require a quick scan to look for yields instead of being skipped altogether.