Mypy fails to type check function signature after plugin pass

Bug Report

I wrote a plugin to replace types, but Mypy fails to typecheck after my changes. I start off trying to fix the type error in this signature with a plugin:

def almost_id(a: str) -> str:
    return int(a)

almost_id("6")

with my plugin:

def replace_str(self, ctx: FunctionSigContext) -> FunctionLike:
        api = ctx.api

        new_signature = ctx.default_signature.copy_modified(
            ret_type=api.named_type('builtins.int')
        )

        un_anal = new_signature.definition.unanalyzed_type.copy_modified(
            ret_type = api.named_type('builtins.int')
        )

        new_signature.definition.unanalyzed_type = un_anal

        typ = new_signature.definition.type.copy_modified(
            ret_type = api.named_type('builtins.int')
        )

        new_signature.definition.type = typ

        return new_signature

I have tried going through the attrs of the functionsigcontext, api and the expr_checker but can't seem to find some other functionality to either:

• force mypy to proceed with type checking
• modify the existing type analysis or map
• delay type checking until the plugin mods have been completed, something like defer() in the semantic analyzer.

To Reproduce
gist: https://gist.github.com/dibrinsofor/bddcbd2eedbb233b131bb1b8d562369a

mypy tests/str_int.py

Expected Behavior

I expect mypy to pass

Actual Behavior

tests/str_int.py:2: error: Incompatible return value type (got "int", expected "str")  [return-value]

even though printing the new signature returns:

def (a: builtins.str) -> builtins.int

Your Environment

• Mypy version used: 1.14.0+dev.8c7db13f71098a6facea66ab165b6bc3ef688bdf
• Mypy command-line flags: --show-traceback
• Python version used: Python 3.11.5

Run mypy tests/str_int.py using the gist reproduction and compare the reported error with the signature printed after replace_str. Trace how the FunctionSigContext result is consumed during semantic analysis and expression checking, using the expr_checker and defer() paths mentioned in the report. Done means the plugin-modified return type is respected and the reproduction passes.