fastparse and native parser assign different line numbers to nested subscripts in multi-line type annotations #21480

Bug Report

For a multi-line type annotation containing an error in a nested subscript, fastparse and nativeparse assign different line numbers to the resulting UnboundType. fastparse uses the outermost annotation line (self.line fixed at TypeConverter construction); nativeparse uses the actual per-node source line. Downstream, type-arg / arity errors reported against that node show up on different lines depending on which parser was used.

I ran into this as # type: ignore comments in serial mode became unused-ignore when trying parallel mode due to the line number difference.

To Reproduce

producer.py:

def foo() -> dict[
    str,
    dict[int, int, int],
]:
    return {}

mypy.ini:

[mypy]
python_version = 3.12

rm -rf .mypy_cache && mypy --num-workers=0 producer.py                       # fastparse
rm -rf .mypy_cache && mypy --num-workers=0 --native-parser producer.py        # nativeparse, no workers

Expected Behavior

Both parsers should agree on the reported line — either both line 1 (outer annotation) or both line 3 (inner offending node).

Actual Behavior

Same error in both: "dict" expects 2 type arguments, but 3 given [type-arg].

Bisection notes:

• The divergence requires the error-bearing node to be a nested sub-expression. Top-level multi-line wrong-arity (x: (\n dict[str, str, str]\n ) = {}) reports line 1 in both parsers.
• Single-line wrong-arity has nothing to diverge on; both parsers agree.

Your Environment

• Mypy version used: 2.1.0 (PyPI). Also reproduces on current master (2.2.0+dev.e53693be05a2).
• Mypy command-line flags: --native-parser selects the inner-line behavior; default (--num-workers=0, no --native-parser`) selects the outer-line behavior.
• Mypy configuration: python_version = 3.12.
• Python version used: 3.12.13