Misbehavior of `f.__code__.co_firstlineno` for decorated functions

Bug report

Bug description:

See also: #139783

Compare:

dummy = lambda f: f                 # L1
@dummy                              # L2
def f():                            # L3
    return                          # L4
print(f.__code__.co_firstlineno)    # L5  # result = 2

with

dummy = lambda f: f                 # L1
pass                                # L2
def f():                            # L3
    return                          # L4
print(f.__code__.co_firstlineno)    # L5  # result = 3

and

dummy = lambda f: f                 # L1
@\
dummy                               # L3
def f():                            # L4
    return                          # L5
print(f.__code__.co_firstlineno)    # L6  # result = 3

In the last example, the line continuation after @ is not handled correctly and the first line for f.__code__ then becomes the line at dummy. Because of that, the values returned by inspect.findsource(f) will not be consistent and inspect.getsourcelines will not use the correct source (it will only start at "dummy" and will not contain the "@" part).

@serhiy-storchaka suggested that this is an issue in the AST parser. I however don't know how we should actually fix this.

Consequences to inspect.getsource and other introspection functions:

import inspect

def outer(*args, **kwargs):
    def inner(f):
        return f
    return deco

@(
    outer(
        ...
    )
)
def f():
    return "hello"

print(inspect.getsource(f))

On Python 3.12, the output is expected:

@(
    outer(
        ...
    )
)
def func():
    return "hello"

On Python 3.13.7, the output is missing the first line @(:

    outer(
        ...
    )
)
def func():
    return "hello"

On Python 3.13.9 and main, we are missing @ and outer parentheses:

    outer(
        ...
    )

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

3つのデコレータの例とネストされたデコレータの例を再現し、その後、inspect.findsource、inspect.getsourcelines、inspect.getsourceと併せてASTパーサーの動作を調査します。デコレートされた関数が、途中で継続するデコレータや開き括弧を含めて、正しい最初のソース行を報告し、イントロスペクションがデコレータのソース全体を返せば完了です。