Fix attrs.evolve on bound TypeVar (#15022)

Fixes the error on the last line of this example:
```python
@attrs.define
class A:
  x: int

T = TypeVar('T', bound=A)

def f(t: T) -> None:
   _ = attrs.evolve(t, x=42)  # E: Argument 1 to "evolve" has incompatible type "T"; expected an attrs class
```

Since `T` is bounded by `A`, we know it can be treated as `A`.

Author: ikonst

mypy/plugins/attrs.py

@@ -62,6 +62,7 @@
     LiteralType,
     NoneType,
     Overloaded,
+    ProperType,
     TupleType,
     Type,
     TypeOfAny,
@@ -929,13 +930,10 @@ def add_method(
         add_method(self.ctx, method_name, args, ret_type, self_type, tvd)
 
 
-def _get_attrs_init_type(typ: Type) -> CallableType | None:
+def _get_attrs_init_type(typ: Instance) -> CallableType | None:
     """
     If `typ` refers to an attrs class, gets the type of its initializer method.
     """
-    typ = get_proper_type(typ)
-    if not isinstance(typ, Instance):
-        return None
     magic_attr = typ.type.get(MAGIC_ATTR_NAME)
     if magic_attr is None or not magic_attr.plugin_generated:
         return None
@@ -945,6 +943,14 @@ def _get_attrs_init_type(typ: Type) -> CallableType | None:
     return init_method.type
 
 
+def _get_attrs_cls_and_init(typ: ProperType) -> tuple[Instance | None, CallableType | None]:
+    if isinstance(typ, TypeVarType):
+        typ = get_proper_type(typ.upper_bound)
+    if not isinstance(typ, Instance):
+        return None, None
+    return typ, _get_attrs_init_type(typ)
+
+
 def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> CallableType:
     """
     Generates a signature for the 'attr.evolve' function that's specific to the call site
@@ -967,13 +973,15 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
 
     inst_type = get_proper_type(inst_type)
     if isinstance(inst_type, AnyType):
-        return ctx.default_signature
+        return ctx.default_signature  # evolve(Any, ....) -> Any
     inst_type_str = format_type_bare(inst_type)
 
-    attrs_init_type = _get_attrs_init_type(inst_type)
-    if not attrs_init_type:
+    attrs_type, attrs_init_type = _get_attrs_cls_and_init(inst_type)
+    if attrs_type is None or attrs_init_type is None:
         ctx.api.fail(
-            f'Argument 1 to "evolve" has incompatible type "{inst_type_str}"; expected an attrs class',
+            f'Argument 1 to "evolve" has a variable type "{inst_type_str}" not bound to an attrs class'
+            if isinstance(inst_type, TypeVarType)
+            else f'Argument 1 to "evolve" has incompatible type "{inst_type_str}"; expected an attrs class',
             ctx.context,
         )
         return ctx.default_signature

test-data/unit/check-attr.test

@@ -1970,6 +1970,75 @@ reveal_type(ret)  # N: Revealed type is "Any"
 
 [typing fixtures/typing-medium.pyi]
 
+[case testEvolveTypeVarBound]
+import attrs
+from typing import TypeVar
+
+@attrs.define
+class A:
+    x: int
+
+@attrs.define
+class B(A):
+    pass
+
+TA = TypeVar('TA', bound=A)
+
+def f(t: TA) -> TA:
+    t2 = attrs.evolve(t, x=42)
+    reveal_type(t2)  # N: Revealed type is "TA`-1"
+    t3 = attrs.evolve(t, x='42')  # E: Argument "x" to "evolve" of "TA" has incompatible type "str"; expected "int"
+    return t2
+
+f(A(x=42))
+f(B(x=42))
+
+[builtins fixtures/attr.pyi]
+
+[case testEvolveTypeVarBoundNonAttrs]
+import attrs
+from typing import TypeVar
+
+TInt = TypeVar('TInt', bound=int)
+TAny = TypeVar('TAny')
+TNone = TypeVar('TNone', bound=None)
+
+def f(t: TInt) -> None:
+    _ = attrs.evolve(t, x=42)  # E: Argument 1 to "evolve" has a variable type "TInt" not bound to an attrs class
+
+def g(t: TAny) -> None:
+    _ = attrs.evolve(t, x=42)  # E: Argument 1 to "evolve" has a variable type "TAny" not bound to an attrs class
+
+def h(t: TNone) -> None:
+    _ = attrs.evolve(t, x=42)  # E: Argument 1 to "evolve" has a variable type "TNone" not bound to an attrs class
+
+[builtins fixtures/attr.pyi]
+
+[case testEvolveTypeVarConstrained]
+import attrs
+from typing import TypeVar
+
+@attrs.define
+class A:
+    x: int
+
+@attrs.define
+class B:
+    x: str  # conflicting with A.x
+
+T = TypeVar('T', A, B)
+
+def f(t: T) -> T:
+    t2 = attrs.evolve(t, x=42)  # E: Argument "x" to "evolve" of "B" has incompatible type "int"; expected "str"
+    reveal_type(t2)  # N: Revealed type is "__main__.A"  # N: Revealed type is "__main__.B"
+    t2 = attrs.evolve(t, x='42')  # E: Argument "x" to "evolve" of "A" has incompatible type "str"; expected "int"
+    return t2
+
+f(A(x=42))
+f(B(x='42'))
+
+[builtins fixtures/attr.pyi]
+
 [case testEvolveVariants]
 from typing import Any
 import attr