Emit warning for invalid use of quoted types in union syntax

[brianschubert]

Using quoted types in PEP 604 union syntax can be problematic, since at runtime, using | between a type and string produces an error:

>>> x: int | "str"
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    x: int | "str"
       ~~~~^~~~~~~
TypeError: unsupported operand type(s) for |: 'type' and 'str'


>>> U = int | float
>>> x: U | "str"
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    x: U | "str"
       ~~^~~~~~~
TypeError: unsupported operand type(s) for |: 'types.UnionType' and 'str'

The exception is when the type is a TypeVar or a _UnionGenericAlias, which can both be safely |'d with strings:

>>> T = TypeVar("T")
>>> U = T | int

>>> T | "str"
typing.Union[~T, ForwardRef('str')]
>>> U | "str"
typing.Union[~T, int, ForwardRef('str')]

This PR makes mypy emit a warning for the former cases (where using | with a string will produce a runtime error) while permitting the latter cases (where using | with a string is safe).

A few notes about the implementation:

• No errors are emitted inside stub files (where quoted types are redundant but harmless).
• No errors are emitted for annotations inside files that use from __future__ import annotations. Like in stubs, quoted types in these cases are redundant but mostly harmless (aside from issues with runtime inspection). It might be a good idea to emit errors for these cases too (pyright does), but I wasn't sure if the benefit is enough to justify the churn. Without this exception, this PR has ~20 mypy_primer hits across 4 projects.
• I've verified that all test cases that emit errors do indeed raise exceptions at runtime, and all cases that don't, don't. (checked with 3.10.12 and 3.13.0).

[brianschubert]

Conformance results diff for this PR (improves conformance in annotations_forward_refs)

:...skipping...
diff --git a/conformance/results/mypy/annotations_forward_refs.toml b/conformance/results/mypy/annotations_forward_refs.toml
index bee7ebb..6763a18 100644
--- a/conformance/results/mypy/annotations_forward_refs.toml
+++ b/conformance/results/mypy/annotations_forward_refs.toml
@@ -5,6 +5,8 @@ Does not report error for use of quoted type with "|" operator (runtime error).
 Incorrectly generates error for quoted type defined in class scope.
 """
 output = """
+annotations_forward_refs.py:24: error: X | Y syntax for unions cannot use quoted operands; use quotes around the entire expression instead  [misc]
+annotations_forward_refs.py:25: error: X | Y syntax for unions cannot use quoted operands; use quotes around the entire expression instead  [misc]
 annotations_forward_refs.py:41: error: Invalid type comment or annotation  [valid-type]
 annotations_forward_refs.py:42: error: Invalid type comment or annotation  [valid-type]
 annotations_forward_refs.py:43: error: Invalid type comment or annotation  [valid-type]
@@ -33,8 +35,6 @@ conformance_automated = "Fail"
 errors_diff = """
 Line 22: Expected 1 errors
 Line 23: Expected 1 errors
-Line 24: Expected 1 errors
-Line 25: Expected 1 errors
 Line 66: Expected 1 errors
 Line 87: Unexpected errors ['annotations_forward_refs.py:87: error: Function "annotations_forward_refs.ClassD.int" is not valid as a type  [valid-type]']
 Line 96: Unexpected errors ['annotations_forward_refs.py:96: error: Expression is of type int?, not "int"  [assert-type]']
diff --git a/conformance/results/mypy/version.toml b/conformance/results/mypy/version.toml
index 51d97b4..b1035e1 100644
--- a/conformance/results/mypy/version.toml
+++ b/conformance/results/mypy/version.toml
@@ -1,2 +1,2 @@
-version = "mypy 1.14.0+dev.499adaed8adbded1a180e30d071438fef81779ec"
-test_duration = 2.4
+version = "mypy 1.14.0+dev.ce34db689d8a6cab6f540a2c38ecc32952534b0a"
+test_duration = 4.6

[github-actions]

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