Match statement syntax error does not obey version guards

[wyattscarpenter]

This phenomenon was originally discovered by @dmarteau in #19892.

I wrote and ran the following two tests under Python 3.12 and mypy master (1.19.0+dev) and they succeeded.

[case testUnreachableAfterToplevelAssertFlaggy]
# flags: --python-version=3.9 --show-error-codes
# This test may false negative succeed if you are actually running python 3.9
import sys
reveal_type(0)  # N: Revealed type is "Literal[0]?"
x: int | tuple[int]  # E: X | Y syntax for unions requires Python 3.10  [syntax]
"a"+1  # E: Unsupported operand types for + ("str" and "int")  [operator]
assert sys.version_info > (3,9)
reveal_type('')  # No error here :-)
x: int | tuple[int] # a little 3.10 syntax to check
[builtins fixtures/ops.pyi]

[case testUnreachableAfterToplevelAssertConfigy]
# This test may false negative succeed if you are act>
# flags: --config-file=tmp/foo.ini --show-error-codes
import sys
reveal_type(0)  # N: Revealed type is "Literal[0]?"
x: int | tuple[int]  # E: X | Y syntax for unions requires Python 3.10  [syntax]
"a"+1  # E: Unsupported operand types for + ("str" and "int")  [operator]
assert sys.version_info > (3,9)
reveal_type('')  # No error here :-)
x: int | tuple[int] # a little 3.10 syntax to check

[file foo.ini]
\[mypy]
python_version=3.9

[builtins fixtures/ops.pyi]

However, swapping in a match statement instead of a union bar gets rid of all the other errors and produces

error: Pattern matching is only supported in Python 3.10 and greater  [syntax]

Weird!

[wyattscarpenter]

Possible clues: the union syntax error comes from

mypy/mypy/typeanal.py

Line 1386 in 16cd4c5

self.fail("X | Y syntax for unions requires Python 3.10", t, code=codes.SYNTAX)

, but I can't even figure out where the match syntax error comes from from its string. In semanal.py there is some stuff

mypy/mypy/semanal.py

Line 5541 in 16cd4c5

def visit_match_stmt(self, s: MatchStmt) -> None:

(Maybe we don't handle the syntax error, and it bubbles out of our parser and we report it as a fallback? Just a guess.)

There is historical context that might be of use https://github.com/python/mypy/pull/10191/files

[sterliakov]

Yes, exactly: if a parser cannot parse the complete file, we don't know anything about your version guards, we don't even have a tree to look for them.

• X | Y is not a parsing SyntaxError, it's just a gentle reminder from us. We do have a full parsed file, we just see something that will raise at runtime (but not a SyntaxError). If there's a preceding version guard, this is fine.
• match on py3.9 is a hard SyntaxError raised by parser, because the file does not contain valid py3.9 code. This error is blocking, and we stop the checking right there - we don't have any tree to actually check.

[wyattscarpenter]

Dang! Well, maybe after mypy moves to a new faster parser #19776 this will possible.

Thank you for the excellent explanation 🙂