Fixes, #15339

Use TypeVar defaults to resolve fallback return type of functions.

**Note**: Defaults for TypeVarTuples don't yet work, probably a result
of the limited support for `Unpack` / `TypeVarTuple`.

Ref: #14851

This adds support for `Literal` as return type of `__bool__` in the
reachability analysis.

Fixes #7008

Fixes #15658.

Source commit:

python/typeshed@fe2ebd6

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

Fixes #14142

Provide a concrete example of what file level comments would look like.
Sort text into sections a little better.

Fixes #15343

Looks like `capi_version < 3.7` is not supported, so I changed the
lowest version to be `3.7`.

Based on the discord discussion.

See #14740

My PR was pretty old and predates the nice check to ensure error codes
are documented.

Resolves #9789

Users could use `--disable-error-code=import-untyped` to only ignore
errors about libraries not having stubs, but continue to get errors
about e.g. typos in an import name.

The error subcode mechanism is new from #14570. Note that users will now
get a different error code depending on whether or not a package is
installed, and may not know that they can use the parent error code to
ignore the issue regardless. I think this is okay, in general type
checking results can change if you run them in two different
environments. Note also that with `--warn-unused-ignore` / `--strict`
mypy will complain about not having the most specific error code

This is a third PR in series following
#15287 and
#15754. This one is quite simple: I
just add basic support for polymorphic inference involving type
variables with upper bounds and values. A complete support would be
quite complicated, and it will be a corner case to already rare
situation. Finally, it is written in a way that is easy to tune in the
future.

I also use this PR to add some unit tests for all three PRs so far,
other two PRs only added integration tests (and I clean up existing unit
tests as well).

#15832)

…5272)"

This reverts commit 2aaeda4.

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(Explain how this PR changes mypy.)

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- Fixes #15037
- Fixes #15065
- Fixes #15073
- Fixes #15388
- Fixes #15086

Yet another part of #14903 that's
finally been extracted!

The black pre-commit mirror is now hosted at:
https://github.com/psf/black-pre-commit-mirror

This is a first follow-up for #15287 (I like how my PR titles sound like
research paper titles, LOL)

This PR completes the new type inference foundations by switching to a
complete and well founded algorithm [1] for transitive closure (that
replaces more ad hoc initial algorithm that covered 80% of cases and was
good for experimenting with new inference scheme). In particular the
algorithm in this PR covers two important edge cases (see tests). Some
comments:
* I don't intend to switch the default for `--new-type-inference`, I
just want to see the effect of the switch on `mypy_primer`, I will
switch back to false before merging
* This flag is still not ready to be publicly announced, I am going to
make another 2-3 PRs from the list in #15287 before making this public.
* I am not adding yet the unit tests as discussed in previous PR. This
PR is already quite big, and the next one (support for upper bounds and
values) should be much smaller. I am going to add unit tests only for
`transitive_closure()` which is the core of new logic.
* While working on this I fixed couple bugs exposed in `TypeVarTuple`
support: one is rare technical corner case, another one is serious,
template and actual where swapped during constraint inference,
effectively causing outer/return context to be completely ignored for
instances.
* It is better to review the PR with "ignore whitespace" option turned
on (there is big chunk in solve.py that is just change of indentation).
* There is one questionable design choice I am making in this PR, I am
adding `extra_tvars` as an attribute of `Constraint` class, while it
logically should not be attributed to any individual constraint, but
rather to the full list of constrains. However, doing this properly
would require changing the return type of `infer_constrains()` and all
related functions, which would be a really big refactoring.

[1] Definition 7.1 in https://inria.hal.science/inria-00073205/document

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Closes #15526

Closes #15782

It solves two problems:
1. `fix_annotate` and `async_matrix` were removed in
#15728
2. It is better to reuse stuff like `runtests.py` not to rewrite the
same command we already have

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

While working on #15776 I've noticed
that some `RuntimeError` do not have enough metadata to understand what
is going on.
CI:
https://github.com/python/mypy/actions/runs/5700479199/job/15450345887

This PR adds more context to error messages.

Remove the Python 37 environment from the `tox.ini` since Python 3.7 is
now end of life.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

This is a test case for #15680 from
`stubtest`'s point of view.

`flatten_types` forgot about the second way we represent `tuple` inside.

Closes #15443

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Co-authored-by: Ilya Priven <ilya.konstantinov@gmail.com>

…values (#15680)

Closes #15638

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Completes a `TODO` item :)

Refs #15724

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Followup to #15681