(🐞) errors from imported modules aren't reported when project is installed

given

-src
 |- __init__.py
 |- test.py
 |- test2.py

# src/test.py
from src import test2

reveal_type(test2.a)

# src/test2.py
1 + ""
a = 1

then

> mypy -m src.test
src/test.py:4: note: Revealed type is "builtins.int"
Success: no issues found in 1 source file
> mypy src/test
src/test.py:4: note: Revealed type is "builtins.int"
Success: no issues found in 1 source file

expected

reference

Mypy is designed to doggedly follow all imports, even if the imported module is not a file you explicitly wanted mypy to check.

--follow-imports=normal follows all imports normally and type checks all top level code (as well as the bodies of all functions and methods with at least one type annotation in the signature).

> mypy src/test.py
src/test2.py:1: error: Unsupported operand types for + ("int" and "str")  [operator]
src/test.py:4: note: Revealed type is "builtins.int"
Found 1 error in 1 file (checked 3 source files)

type checks all code

where are the errors from this typechecking?

i found them

> mypy --no-silence-site-packages -m src.test
src/test2.py:1: error: Unsupported operand types for + ("int" and "str")  [operator]
src/test.py:4: note: Revealed type is "builtins.int"

uhhhh, what?

Enables reporting error messages generated within installed packages (see PEP 561 for more details on distributing type information). Those error messages are suppressed by default, since you are usually not able to control errors in 3rd party code.

> mypy -m src.test
src/test.py:4: note: Revealed type is "builtins.int"
Success: no issues found in 1 source file
> uv pip uninstall -e .
...
> mypy -m src.test
src/test2.py:1: error: Unsupported operand types for + ("int" and "str")  [operator]
src/test.py:4: note: Revealed type is "builtins.int"
Found 1 error in 1 file (checked 1 source file)

Reproduce the behavior with the provided src/test.py and src/test2.py examples, comparing mypy -m src.test, mypy src/test.py, and --no-silence-site-packages before and after installation. Trace import following and installed-package error suppression, including the PEP 561 behavior referenced in the issue. Done means errors from the imported module are reported in the intended case without breaking normal third-party suppression.