Fix attrs.evolve with generics

mypy/plugins/attrs.py

@@ -9,7 +9,7 @@
 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.expandtype import expand_type, expand_type_by_instance
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.messages import format_type_bare
 from mypy.nodes import (
@@ -986,6 +986,8 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
         )
         return ctx.default_signature
 
+    attrs_init_type = expand_type_by_instance(attrs_init_type, attrs_type)
+
     # AttrClass.__init__ has the following signature (or similar, if having kw-only & defaults):
     #   def __init__(self, attr1: Type1, attr2: Type2) -> None:
     # We want to generate a signature for evolve that looks like this:

test-data/unit/check-attr.test

@@ -1970,6 +1970,26 @@ reveal_type(ret)  # N: Revealed type is "Any"
 
 [typing fixtures/typing-medium.pyi]
 
+[case testEvolveGeneric]
+import attrs
+from typing import Generic, TypeVar
+
+T = TypeVar('T')
+
+@attrs.define
+class A(Generic[T]):
+    x: T
+
+
+a = A(x=42)
+reveal_type(a)  # N: Revealed type is "__main__.A[builtins.int]"
+a2 = attrs.evolve(a, x=42)
+reveal_type(a2)  # N: Revealed type is "__main__.A[builtins.int]"
+a2 = attrs.evolve(a, x='42')  # E: Argument "x" to "evolve" of "A[int]" has incompatible type "str"; expected "int"
+reveal_type(a2)  # N: Revealed type is "__main__.A[builtins.int]"
+
+[builtins fixtures/attr.pyi]
+
 [case testEvolveTypeVarBound]
 import attrs
 from typing import TypeVar