Add support for converters with TypeVars on generic attrs classes (#14908)

When creating generic classes using `attrs`, converters with type vars
are not properly integrated into the generated `__init__`:

```python
from typing import TypeVar, Generic, List, Iterable, Iterator
import attr

T = TypeVar('T')

def int_gen() -> Iterator[int]:
    yield 1

def list_converter(x: Iterable[T]) -> List[T]:
    return list(x)

@attr.s(auto_attribs=True)
class A(Generic[T]):
    x: List[T] = attr.ib(converter=list_converter)
    y: T = attr.ib()

a1 = A([1], 2)   # E: Argument 1 to "A" has incompatible type "Iterator[int]"; expected "Iterable[T]"
```

This MR fixes the bug by copying type vars from the field/attrib into
the type extracted from the converter.

Author: chadrik

mypy/plugins/attrs.py

@@ -6,8 +6,10 @@
 from typing_extensions import Final, Literal
 
 import mypy.plugin  # To avoid circular imports.
+from mypy.applytype import apply_generic_arguments
 from mypy.checker import TypeChecker
 from mypy.errorcodes import LITERAL_REQ
+from mypy.expandtype import expand_type
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.messages import format_type_bare
 from mypy.nodes import (
@@ -23,6 +25,7 @@
     Decorator,
     Expression,
     FuncDef,
+    IndexExpr,
     JsonDict,
     LambdaExpr,
     ListExpr,
@@ -34,6 +37,7 @@
     SymbolTableNode,
     TempNode,
     TupleExpr,
+    TypeApplication,
     TypeInfo,
     TypeVarExpr,
     Var,
@@ -49,7 +53,7 @@
     deserialize_and_fixup_type,
 )
 from mypy.server.trigger import make_wildcard_trigger
-from mypy.typeops import make_simplified_union, map_type_from_supertype
+from mypy.typeops import get_type_vars, make_simplified_union, map_type_from_supertype
 from mypy.types import (
     AnyType,
     CallableType,
@@ -85,8 +89,9 @@
 class Converter:
     """Holds information about a `converter=` argument"""
 
-    def __init__(self, init_type: Type | None = None) -> None:
+    def __init__(self, init_type: Type | None = None, ret_type: Type | None = None) -> None:
         self.init_type = init_type
+        self.ret_type = ret_type
 
 
 class Attribute:
@@ -115,11 +120,20 @@ def __init__(
     def argument(self, ctx: mypy.plugin.ClassDefContext) -> Argument:
         """Return this attribute as an argument to __init__."""
         assert self.init
-
         init_type: Type | None = None
         if self.converter:
             if self.converter.init_type:
                 init_type = self.converter.init_type
+                if init_type and self.init_type and self.converter.ret_type:
+                    # The converter return type should be the same type as the attribute type.
+                    # Copy type vars from attr type to converter.
+                    converter_vars = get_type_vars(self.converter.ret_type)
+                    init_vars = get_type_vars(self.init_type)
+                    if converter_vars and len(converter_vars) == len(init_vars):
+                        variables = {
+                            binder.id: arg for binder, arg in zip(converter_vars, init_vars)
+                        }
+                        init_type = expand_type(init_type, variables)
             else:
                 ctx.api.fail("Cannot determine __init__ type from converter", self.context)
                 init_type = AnyType(TypeOfAny.from_error)
@@ -653,6 +667,26 @@ def _parse_converter(
             from mypy.checkmember import type_object_type  # To avoid import cycle.
 
             converter_type = type_object_type(converter_expr.node, ctx.api.named_type)
+    elif (
+        isinstance(converter_expr, IndexExpr)
+        and isinstance(converter_expr.analyzed, TypeApplication)
+        and isinstance(converter_expr.base, RefExpr)
+        and isinstance(converter_expr.base.node, TypeInfo)
+    ):
+        # The converter is a generic type.
+        from mypy.checkmember import type_object_type  # To avoid import cycle.
+
+        converter_type = type_object_type(converter_expr.base.node, ctx.api.named_type)
+        if isinstance(converter_type, CallableType):
+            converter_type = apply_generic_arguments(
+                converter_type,
+                converter_expr.analyzed.types,
+                ctx.api.msg.incompatible_typevar_value,
+                converter_type,
+            )
+        else:
+            converter_type = None
+
     if isinstance(converter_expr, LambdaExpr):
         # TODO: should we send a fail if converter_expr.min_args > 1?
         converter_info.init_type = AnyType(TypeOfAny.unannotated)
@@ -671,6 +705,8 @@ def _parse_converter(
     converter_type = get_proper_type(converter_type)
     if isinstance(converter_type, CallableType) and converter_type.arg_types:
         converter_info.init_type = converter_type.arg_types[0]
+        if not is_attr_converters_optional:
+            converter_info.ret_type = converter_type.ret_type
     elif isinstance(converter_type, Overloaded):
         types: list[Type] = []
         for item in converter_type.items:

test-data/unit/check-attr.test

@@ -469,6 +469,56 @@ A([1], '2')  # E: Cannot infer type argument 1 of "A"
 
 [builtins fixtures/list.pyi]
 
+[case testAttrsGenericWithConverter]
+from typing import TypeVar, Generic, List, Iterable, Iterator, Callable
+import attr
+T = TypeVar('T')
+
+def int_gen() -> Iterator[int]:
+    yield 1
+
+def list_converter(x: Iterable[T]) -> List[T]:
+    return list(x)
+
+@attr.s(auto_attribs=True)
+class A(Generic[T]):
+    x: List[T] = attr.ib(converter=list_converter)
+    y: T = attr.ib()
+    def foo(self) -> List[T]:
+        return [self.y]
+    def bar(self) -> T:
+        return self.x[0]
+    def problem(self) -> T:
+        return self.x  # E: Incompatible return value type (got "List[T]", expected "T")
+reveal_type(A) # N: Revealed type is "def [T] (x: typing.Iterable[T`1], y: T`1) -> __main__.A[T`1]"
+a1 = A([1], 2)
+reveal_type(a1)  # N: Revealed type is "__main__.A[builtins.int]"
+reveal_type(a1.x)  # N: Revealed type is "builtins.list[builtins.int]"
+reveal_type(a1.y)  # N: Revealed type is "builtins.int"
+
+a2 = A(int_gen(), 2)
+reveal_type(a2)  # N: Revealed type is "__main__.A[builtins.int]"
+reveal_type(a2.x)  # N: Revealed type is "builtins.list[builtins.int]"
+reveal_type(a2.y)  # N: Revealed type is "builtins.int"
+
+
+def get_int() -> int:
+    return 1
+
+class Other(Generic[T]):
+    def __init__(self, x: T) -> None:
+        pass
+
+@attr.s(auto_attribs=True)
+class B(Generic[T]):
+    x: Other[Callable[..., T]] = attr.ib(converter=Other[Callable[..., T]])
+
+b1 = B(get_int)
+reveal_type(b1)  # N: Revealed type is "__main__.B[builtins.int]"
+reveal_type(b1.x)  # N: Revealed type is "__main__.Other[def (*Any, **Any) -> builtins.int]"
+
+[builtins fixtures/list.pyi]
+
 
 [case testAttrsUntypedGenericInheritance]
 from typing import Generic, TypeVar