Mypy does not recheck files when follow_imports changes for dependencies

Steps to reproduce

1. Create module test.py with import from nonexistent module:

from foo import bar

2. Create mypy.ini file:

[mypy]

[mypy-foo]
ignore_missing_imports = True 

3. Run mypy

$ mypy test.py 
Success: no issues found in 1 source file

4. Comment out [mypy-foo] section in mypy.ini

[mypy]

# [mypy-foo]
# ignore_missing_imports = True 

5. Run mypy

$ mypy test.py 
Success: no issues found in 1 source file

Expected result

After step 4, mypy should detect config change and (partially?) invalidate cache, leading to a type error.

$ mypy test.py 
test.py:1: error: Cannot find module named 'foo'
test.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

Notes

• Deleting .mypy_cache/ and re-running mypy fixes the issue.
• Adding/removing ignore_missing_imports in the main [mypy] section does seem to invalidate the cache as expected. So this issue only seems to apply to [mypy-PATTERN] sections.

Versions

$ python --version
Python 3.7.3

$ mypy --version
mypy 0.740

Reproduce the issue with test.py, mypy.ini, and the .mypy_cache/ directory using the configuration steps shown. Start by tracing cache invalidation for [mypy-PATTERN] sections and compare it with changes in the main [mypy] section. Done means changing the pattern section causes mypy to report the missing foo module without manually deleting the cache.