Improve match statement union narrowing/inference

[bergkvist]

Improve inference/narrowing support for union types in match statements.

Fixes #17549

Before:

var: tuple[int, int] | tuple[str, str]

# TODO: we can infer better here.
match var:
    case (42, a):
        reveal_type(a)  # N: Revealed type is "Union[builtins.int, builtins.str]"
    case ("yes", b):
        reveal_type(b)  # N: Revealed type is "Union[builtins.int, builtins.str]"

After:

var: tuple[int, int] | tuple[str, str]

match var:
    case (42, a):
        reveal_type(a)  # N: Revealed type is "builtins.int"
    case ("yes", b):
        reveal_type(b)  # N: Revealed type is "builtins.str"

Related:

• Support for python 3.10 match statement #10191

[Hnasar]

Nice!
Does your fix address any of the other match statement issues?
https://github.com/python/mypy/issues?q=sort%3Aupdated-desc+is%3Aopen+label%3Atopic-match-statement

[bergkvist]

Nice! Does your fix address any of the other match statement issues? https://github.com/python/mypy/issues?q=sort%3Aupdated-desc+is%3Aopen+label%3Atopic-match-statement

From reading through the issues, I believe it should at least solve these:

• Unable to narrow the type of a tagged union when using a match statement #15190
• Type is not being narrowed properly in match statement #17549

I notice there are a bunch of match exhaustiveness issues with narrowing of the type passed on to the next match branches, which this PR does not fix.

[hjwp]

maybe it would address this too? #16835

[bergkvist]

maybe it would address this too? #16835

I don't think so, as I return the "current type" as the "rest type", which means the remainder/rest type (which is what gets passed on to the next match case) doesn't get narrowed properly yet. I have yet to figure out how to do this.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅