Mypy reports a function is "missing return statement" when unused imports are removed

[eli-schwartz]

I know this sounds ridiculous, but I'm not sure how else to explain it. In the middle of a large refactor of code, which can be found here in its original, mypy-passing state:

https://github.com/mesonbuild/meson/blob/83d18d137dc50437a8263983e8883198c7bc41ed/mesonbuild/compilers/detect.py#L272-L352

I have a function which looks like this:

def _handle_exceptions(data: StuffICareAbout) -> typing.NoReturn:
    raise mesonlib.EnvironmentException(f'{data} happened and this is bad')

def detect_static_linker(env: 'Environment', compiler: Compiler) -> StaticLinker:
    data = do_stuff()
    if foo_condition:
        return StaticLinkerA()
    elif bar_condition:
        return StaticLinkerB()
    _handle_exceptions(data)

mypy says this is fine. detect_static_linker() doesn't always return, but if it doesn't return, it runs a function that is guaranteed to never return, always raise.

This somehow stops working during the course of this series of changes, which doesn't affect the high-level overview of that function: eli-schwartz/meson@83d18d1...6816096

After way too much troubleshooting, and separating out parts of the commit to see what caused it, I produced the above series of commits, and...

... this commit is fingered by git bisect: eli-schwartz/meson@fe42a1c...b1676e5

All it does is finally drop the unused imports rendered redundant by the previous commit. I don't understand how this could be possible.

Furthermore, if I run mypy mesonbuild/compilers/detect.py, mypy passes. If I run ./run_mypy.py, a script that contains a list of files to check, and runs mypy "list" "of" "files", it triggers the issue.

Discussion in the meson review can be seen here, in case that's interesting (two people failed to reproduce it locally at first): mesonbuild/meson#10810 (comment)

Your Environment

• Mypy version used: 0.971
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): https://github.com/mesonbuild/meson/blob/master/.mypy.ini
• Python version used: 3.10
• Operating system and version: Arch Linux

[hauntsaninja]

mesonbuild/meson#10810 (comment) sounds like there may be some mypy cache issue. This means your bisection results may be confounded, e.g. maybe changing imports just invalidated the cache. Perhaps try bisecting again with --no-incremental (disables cache)?

[eli-schwartz]

No luck. The command that run_mypy.py ultimately invokes, with --no-incremental stuck in there:

$ /usr/bin/python3 -m mypy --no-incremental mesonbuild/cmake mesonbuild/compilers mesonbuild/dependencies mesonbuild/interpreter/primitives mesonbuild/interpreterbase mesonbuild/linkers mesonbuild/scripts mesonbuild/wrap mesonbuild/arglist.py mesonbuild/backend/backends.py mesonbuild/depfile.py mesonbuild/envconfig.py mesonbuild/interpreter/compiler.py mesonbuild/interpreter/mesonmain.py mesonbuild/interpreter/interpreterobjects.py mesonbuild/interpreter/type_checking.py mesonbuild/mcompile.py mesonbuild/mdevenv.py mesonbuild/mesonlib/platform.py mesonbuild/mesonlib/universal.py mesonbuild/minit.py mesonbuild/minstall.py mesonbuild/mintro.py mesonbuild/mlog.py mesonbuild/msubprojects.py mesonbuild/modules/external_project.py mesonbuild/modules/fs.py mesonbuild/modules/gnome.py mesonbuild/modules/i18n.py mesonbuild/modules/icestorm.py mesonbuild/modules/java.py mesonbuild/modules/keyval.py mesonbuild/modules/modtest.py mesonbuild/modules/pkgconfig.py mesonbuild/modules/qt.py mesonbuild/modules/rust.py mesonbuild/modules/sourceset.py mesonbuild/modules/wayland.py mesonbuild/modules/windows.py mesonbuild/mparser.py mesonbuild/msetup.py mesonbuild/mtest.py mesonbuild/optinterpreter.py mesonbuild/programs.py run_mypy.py run_project_tests.py run_single_test.py tools docs/genrefman.py docs/refman mesonbuild/mesonlib/posix.py
mesonbuild/compilers/detect.py:160:1: error: Missing return statement
Found 1 error in 1 file (checked 173 source files)

The issue is still present.

Just running that single file, nothing is reported:

$ /usr/bin/python3 -m mypy --no-incremental mesonbuild/compilers/detect.py
Success: no issues found in 1 source file

Plugging this into git bisect:

$ git bisect start HEAD origin/master 
Bisecting: 5 revisions left to test after this (roughly 3 steps)
[f537c308efce427ed863004331a4d67aa7db3c4b] compilers: remove dead code
$ git bisect run /usr/bin/python3 -m mypy --no-incremental mesonbuild/cmake mesonbuild/compilers mesonbuild/dependencies mesonbuild/interpreter/primitives mesonbuild/interpreterbase mesonbuild/linkers mesonbuild/scripts mesonbuild/wrap mesonbuild/arglist.py mesonbuild/backend/backends.py mesonbuild/depfile.py mesonbuild/envconfig.py mesonbuild/interpreter/compiler.py mesonbuild/interpreter/mesonmain.py mesonbuild/interpreter/interpreterobjects.py mesonbuild/interpreter/type_checking.py mesonbuild/mcompile.py mesonbuild/mdevenv.py mesonbuild/mesonlib/platform.py mesonbuild/mesonlib/universal.py mesonbuild/minit.py mesonbuild/minstall.py mesonbuild/mintro.py mesonbuild/mlog.py mesonbuild/msubprojects.py mesonbuild/modules/external_project.py mesonbuild/modules/fs.py mesonbuild/modules/gnome.py mesonbuild/modules/i18n.py mesonbuild/modules/icestorm.py mesonbuild/modules/java.py mesonbuild/modules/keyval.py mesonbuild/modules/modtest.py mesonbuild/modules/pkgconfig.py mesonbuild/modules/qt.py mesonbuild/modules/rust.py mesonbuild/modules/sourceset.py mesonbuild/modules/wayland.py mesonbuild/modules/windows.py mesonbuild/mparser.py mesonbuild/msetup.py mesonbuild/mtest.py mesonbuild/optinterpreter.py mesonbuild/programs.py run_mypy.py run_project_tests.py run_single_test.py tools docs/genrefman.py docs/refman mesonbuild/mesonlib/posix.py
running  '/usr/bin/python3' '-m' 'mypy' '--no-incremental' 'mesonbuild/cmake' 'mesonbuild/compilers' 'mesonbuild/dependencies' 'mesonbuild/interpreter/primitives' 'mesonbuild/interpreterbase' 'mesonbuild/linkers' 'mesonbuild/scripts' 'mesonbuild/wrap' 'mesonbuild/arglist.py' 'mesonbuild/backend/backends.py' 'mesonbuild/depfile.py' 'mesonbuild/envconfig.py' 'mesonbuild/interpreter/compiler.py' 'mesonbuild/interpreter/mesonmain.py' 'mesonbuild/interpreter/interpreterobjects.py' 'mesonbuild/interpreter/type_checking.py' 'mesonbuild/mcompile.py' 'mesonbuild/mdevenv.py' 'mesonbuild/mesonlib/platform.py' 'mesonbuild/mesonlib/universal.py' 'mesonbuild/minit.py' 'mesonbuild/minstall.py' 'mesonbuild/mintro.py' 'mesonbuild/mlog.py' 'mesonbuild/msubprojects.py' 'mesonbuild/modules/external_project.py' 'mesonbuild/modules/fs.py' 'mesonbuild/modules/gnome.py' 'mesonbuild/modules/i18n.py' 'mesonbuild/modules/icestorm.py' 'mesonbuild/modules/java.py' 'mesonbuild/modules/keyval.py' 'mesonbuild/modules/modtest.py' 'mesonbuild/modules/pkgconfig.py' 'mesonbuild/modules/qt.py' 'mesonbuild/modules/rust.py' 'mesonbuild/modules/sourceset.py' 'mesonbuild/modules/wayland.py' 'mesonbuild/modules/windows.py' 'mesonbuild/mparser.py' 'mesonbuild/msetup.py' 'mesonbuild/mtest.py' 'mesonbuild/optinterpreter.py' 'mesonbuild/programs.py' 'run_mypy.py' 'run_project_tests.py' 'run_single_test.py' 'tools' 'docs/genrefman.py' 'docs/refman' 'mesonbuild/mesonlib/posix.py'
Success: no issues found in 173 source files
Bisecting: 2 revisions left to test after this (roughly 2 steps)
[ed8b6dd193f7ba98e12ee71dd9eb9fe2508be3ee] compilers: use more direct checks for what kind of compiler we have
running  '/usr/bin/python3' '-m' 'mypy' '--no-incremental' 'mesonbuild/cmake' 'mesonbuild/compilers' 'mesonbuild/dependencies' 'mesonbuild/interpreter/primitives' 'mesonbuild/interpreterbase' 'mesonbuild/linkers' 'mesonbuild/scripts' 'mesonbuild/wrap' 'mesonbuild/arglist.py' 'mesonbuild/backend/backends.py' 'mesonbuild/depfile.py' 'mesonbuild/envconfig.py' 'mesonbuild/interpreter/compiler.py' 'mesonbuild/interpreter/mesonmain.py' 'mesonbuild/interpreter/interpreterobjects.py' 'mesonbuild/interpreter/type_checking.py' 'mesonbuild/mcompile.py' 'mesonbuild/mdevenv.py' 'mesonbuild/mesonlib/platform.py' 'mesonbuild/mesonlib/universal.py' 'mesonbuild/minit.py' 'mesonbuild/minstall.py' 'mesonbuild/mintro.py' 'mesonbuild/mlog.py' 'mesonbuild/msubprojects.py' 'mesonbuild/modules/external_project.py' 'mesonbuild/modules/fs.py' 'mesonbuild/modules/gnome.py' 'mesonbuild/modules/i18n.py' 'mesonbuild/modules/icestorm.py' 'mesonbuild/modules/java.py' 'mesonbuild/modules/keyval.py' 'mesonbuild/modules/modtest.py' 'mesonbuild/modules/pkgconfig.py' 'mesonbuild/modules/qt.py' 'mesonbuild/modules/rust.py' 'mesonbuild/modules/sourceset.py' 'mesonbuild/modules/wayland.py' 'mesonbuild/modules/windows.py' 'mesonbuild/mparser.py' 'mesonbuild/msetup.py' 'mesonbuild/mtest.py' 'mesonbuild/optinterpreter.py' 'mesonbuild/programs.py' 'run_mypy.py' 'run_project_tests.py' 'run_single_test.py' 'tools' 'docs/genrefman.py' 'docs/refman' 'mesonbuild/mesonlib/posix.py'
Success: no issues found in 173 source files
Bisecting: 0 revisions left to test after this (roughly 1 step)
[b1676e57f60c6a1fb7533096402ae5c8ff713a35] compilers: avoid importing compilers upfront for detect
running  '/usr/bin/python3' '-m' 'mypy' '--no-incremental' 'mesonbuild/cmake' 'mesonbuild/compilers' 'mesonbuild/dependencies' 'mesonbuild/interpreter/primitives' 'mesonbuild/interpreterbase' 'mesonbuild/linkers' 'mesonbuild/scripts' 'mesonbuild/wrap' 'mesonbuild/arglist.py' 'mesonbuild/backend/backends.py' 'mesonbuild/depfile.py' 'mesonbuild/envconfig.py' 'mesonbuild/interpreter/compiler.py' 'mesonbuild/interpreter/mesonmain.py' 'mesonbuild/interpreter/interpreterobjects.py' 'mesonbuild/interpreter/type_checking.py' 'mesonbuild/mcompile.py' 'mesonbuild/mdevenv.py' 'mesonbuild/mesonlib/platform.py' 'mesonbuild/mesonlib/universal.py' 'mesonbuild/minit.py' 'mesonbuild/minstall.py' 'mesonbuild/mintro.py' 'mesonbuild/mlog.py' 'mesonbuild/msubprojects.py' 'mesonbuild/modules/external_project.py' 'mesonbuild/modules/fs.py' 'mesonbuild/modules/gnome.py' 'mesonbuild/modules/i18n.py' 'mesonbuild/modules/icestorm.py' 'mesonbuild/modules/java.py' 'mesonbuild/modules/keyval.py' 'mesonbuild/modules/modtest.py' 'mesonbuild/modules/pkgconfig.py' 'mesonbuild/modules/qt.py' 'mesonbuild/modules/rust.py' 'mesonbuild/modules/sourceset.py' 'mesonbuild/modules/wayland.py' 'mesonbuild/modules/windows.py' 'mesonbuild/mparser.py' 'mesonbuild/msetup.py' 'mesonbuild/mtest.py' 'mesonbuild/optinterpreter.py' 'mesonbuild/programs.py' 'run_mypy.py' 'run_project_tests.py' 'run_single_test.py' 'tools' 'docs/genrefman.py' 'docs/refman' 'mesonbuild/mesonlib/posix.py'
mesonbuild/compilers/detect.py:160:1: error: Missing return statement
Found 1 error in 1 file (checked 173 source files)
Bisecting: 0 revisions left to test after this (roughly 0 steps)
[fe42a1cadade94cb39e2761500a1f5849ad7d2e8] compilers: perform targeted imports for detect
running  '/usr/bin/python3' '-m' 'mypy' '--no-incremental' 'mesonbuild/cmake' 'mesonbuild/compilers' 'mesonbuild/dependencies' 'mesonbuild/interpreter/primitives' 'mesonbuild/interpreterbase' 'mesonbuild/linkers' 'mesonbuild/scripts' 'mesonbuild/wrap' 'mesonbuild/arglist.py' 'mesonbuild/backend/backends.py' 'mesonbuild/depfile.py' 'mesonbuild/envconfig.py' 'mesonbuild/interpreter/compiler.py' 'mesonbuild/interpreter/mesonmain.py' 'mesonbuild/interpreter/interpreterobjects.py' 'mesonbuild/interpreter/type_checking.py' 'mesonbuild/mcompile.py' 'mesonbuild/mdevenv.py' 'mesonbuild/mesonlib/platform.py' 'mesonbuild/mesonlib/universal.py' 'mesonbuild/minit.py' 'mesonbuild/minstall.py' 'mesonbuild/mintro.py' 'mesonbuild/mlog.py' 'mesonbuild/msubprojects.py' 'mesonbuild/modules/external_project.py' 'mesonbuild/modules/fs.py' 'mesonbuild/modules/gnome.py' 'mesonbuild/modules/i18n.py' 'mesonbuild/modules/icestorm.py' 'mesonbuild/modules/java.py' 'mesonbuild/modules/keyval.py' 'mesonbuild/modules/modtest.py' 'mesonbuild/modules/pkgconfig.py' 'mesonbuild/modules/qt.py' 'mesonbuild/modules/rust.py' 'mesonbuild/modules/sourceset.py' 'mesonbuild/modules/wayland.py' 'mesonbuild/modules/windows.py' 'mesonbuild/mparser.py' 'mesonbuild/msetup.py' 'mesonbuild/mtest.py' 'mesonbuild/optinterpreter.py' 'mesonbuild/programs.py' 'run_mypy.py' 'run_project_tests.py' 'run_single_test.py' 'tools' 'docs/genrefman.py' 'docs/refman' 'mesonbuild/mesonlib/posix.py'
Success: no issues found in 173 source files
b1676e57f60c6a1fb7533096402ae5c8ff713a35 is the first bad commit

Still says that it's all the fault of eli-schwartz/meson@b1676e5