Fix attrs.evolve with generics

[ikonst]

Fixes attrs.evolve for generic attrs classes.

[ikonst]

👋 @JelleZijlstra you reviewed #14526 so you might have context

[vfazio]

I was looking around for MRs touching attr.evolve after a piece of our code was getting flagged after upgrading to 1.2.0 and stumbled on this.

I was hoping this MR resolved our issue, but it doesn't appear to do so. Our use case is below. I made a minor tweak to make things "work" albeit maybe not in a satisfactory way. I'm willing to open an issue and discuss the proper fix for it if it doesn't fit with this MR re evolve + Generics.

import attr
from typing import Generic, TypeVar
from dataclasses import dataclass

C_T = TypeVar("C_T", bound="Config")

@attr.s
class Config():
    name: str = attr.ib()

@dataclass
class ConfigFactory(Generic[C_T]):
    _configs: list[C_T]

    def create_configs(self) -> None:
        for current_config in self._configs:
            x = attr.evolve(current_config, name="abc")
            print(x)

@dataclass
class TestCF(ConfigFactory[Config]):
    pass

def test() -> None:
    factory = TestCF(list())
    factory._configs.extend([Config(name="test")])
    factory.create_configs()

Where attr.evolve gets flagged: error: Argument 1 to "evolve" has incompatible type "C_T"; expected an attrs class [misc]

I needed to add:

diff --git a/mypy/plugins/attrs.py b/mypy/plugins/attrs.py
index 98090cbeb..668293fad 100644
--- a/mypy/plugins/attrs.py
+++ b/mypy/plugins/attrs.py
@@ -963,7 +963,10 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
     # </hack>
 
     inst_type = get_proper_type(inst_type)
+    if isinstance(inst_type, TypeVarType):
+        inst_type = inst_type.upper_bound
     inst_type_str = format_type_bare(inst_type)
+

(mypy-3.11.2) vfazio@vfazio4 /mnt/development/mypy $ python3 -m mypy -V
mypy 1.3.0+dev.edbcb217e6acdb1851a18d63a06f756fe28f2aa5 (compiled: no)
(mypy-3.11.2) vfazio@vfazio4 /mnt/development/mypy $ python3 -m mypy test.py 
test.py:14: error: Argument 1 to "evolve" has incompatible type "C_T"; expected an attrs class  [misc]
Found 1 error in 1 file (checked 1 source file)
(mypy-3.11.2) vfazio@vfazio4 /mnt/development/mypy $ git stash pop
HEAD detached at edbcb217e
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   mypy/plugins/attrs.py

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        test.py
        test2.py

no changes added to commit (use "git add" and/or "git commit -a")
Dropped refs/stash@{0} (3f973a5d6dda869c9e05bd358da9bfd1984d0a94)
(mypy-3.11.2) vfazio@vfazio4 /mnt/development/mypy $ python3 -m mypy -V
mypy 1.3.0+dev.edbcb217e6acdb1851a18d63a06f756fe28f2aa5.dirty (compiled: no)
(mypy-3.11.2) vfazio@vfazio4 /mnt/development/mypy $ python3 -m mypy test.py 
Success: no issues found in 1 source file

[ikonst]

@vfazio tried addressing it in the next commits

I basically did what you suggested, but added an attrs_type in parallel with inst_type:

• inst_type and inst_type_str are used for messages (so that we refer to T rather than the upper bound type)
• inst_type (the T) is the return type
• attrs_type is used to look up the attrs class and its fields

[vfazio]

@vfazio tried addressing it in the next commits

I basically did what you suggested, but added an attrs_type in parallel with inst_type:

* `inst_type` and `inst_type_str` are used for messages (so that we refer to `T` rather than the upper bound type)

* `inst_type` (the `T`) is the return type

* `attrs_type` is used to look up the attrs class and its fields

Awesome, thanks!

[ikonst]

Should this be more friendly, e.g.

has a variable type "T" that is not necessarily an attrs class; consider adding bounds= to your TypeVar definition

I can't tell where we stand between succinct and instructive.

[ikonst]

Split the TypeVar fix into #15022, to be merged before this.

[ikonst]

@hauntsaninja while I have your attention...

(There's a flood of comments above but it's basically around split-out #15022 which we already merged.)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[ikonst]

Closed in favor of #15050 which also addresses unions.