Support attribute access on enum members correctly (#19422)

Fixes #11368 (apparently canonical). 
Fixes #10910. 
Fixes #12107. 
Fixes #13841.
Fixes #15186. 
Fixes #15454. 
Fixes #19418.

`mypy` now understands attribute access on enum members - the
"recursive" behaviour of supporting access of almost-all enum members
from members. "Almost", because `.name` and `.value` take precedence
even if a member of the same name exists.

```python
from enum import Enum

class E(Enum):
    FOO = 1
    BAR = 1

# The following is still a `E.BAR` instance:
E.FOO.FOO.BAR.BAR
```

Looks like this is a much wanted feature.

Author: sterliakov

mypy/checkmember.py

@@ -921,6 +921,18 @@ def analyze_var(
         result = AnyType(TypeOfAny.special_form)
     fullname = f"{var.info.fullname}.{name}"
     hook = mx.chk.plugin.get_attribute_hook(fullname)
+
+    if var.info.is_enum and not mx.is_lvalue:
+        if name in var.info.enum_members and name not in {"name", "value"}:
+            enum_literal = LiteralType(name, fallback=itype)
+            result = itype.copy_modified(last_known_value=enum_literal)
+        elif (
+            isinstance(p_result := get_proper_type(result), Instance)
+            and p_result.type.fullname == "enum.nonmember"
+            and p_result.args
+        ):
+            # Unwrap nonmember similar to class-level access
+            result = p_result.args[0]
     if result and not (implicit or var.info.is_protocol and is_instance_var(var)):
         result = analyze_descriptor_access(result, mx)
     if hook:

mypy/plugins/enums.py

@@ -144,7 +144,7 @@ def _implements_new(info: TypeInfo) -> bool:
 def enum_member_callback(ctx: mypy.plugin.FunctionContext) -> Type:
     """By default `member(1)` will be inferred as `member[int]`,
     we want to improve the inference to be `Literal[1]` here."""
-    if ctx.arg_types or ctx.arg_types[0]:
+    if ctx.arg_types and ctx.arg_types[0]:
         arg = get_proper_type(ctx.arg_types[0][0])
         proper_return = get_proper_type(ctx.default_return_type)
         if (

test-data/unit/check-enum.test

@@ -1953,7 +1953,8 @@ class A(Enum):
     x: int
     def method(self) -> int: pass
 class B(A):
-    x = 1  # E: Cannot override writable attribute "x" with a final one
+    x = 1  # E: Cannot override writable attribute "x" with a final one \
+           # E: Incompatible types in assignment (expression has type "B", base class "A" defined the type as "int")
 
 class A1(Enum):
     x: int = 1  # E: Enum members must be left unannotated \
@@ -1971,8 +1972,8 @@ class B2(A2):  # E: Cannot extend enum with existing members: "A2"
 class A3(Enum):
     x: Final[int]  # type: ignore
 class B3(A3):
-    x = 1  # E: Cannot override final attribute "x" (previously declared in base class "A3")
-
+    x = 1  # E: Cannot override final attribute "x" (previously declared in base class "A3") \
+           # E: Incompatible types in assignment (expression has type "B3", base class "A3" defined the type as "int")
 [builtins fixtures/bool.pyi]
 
 [case testEnumNotFinalWithMethodsAndUninitializedValuesStub]
@@ -1984,14 +1985,16 @@ class A(Enum):  # E: Detected enum "lib.A" in a type stub with zero members. The
                 # N: See https://typing.readthedocs.io/en/latest/spec/enums.html#defining-members
     x: int
 class B(A):
-    x = 1    # E: Cannot override writable attribute "x" with a final one
+    x = 1    # E: Cannot override writable attribute "x" with a final one \
+             # E: Incompatible types in assignment (expression has type "B", base class "A" defined the type as "int")
 
 class C(Enum):
     x = 1
 class D(C):  # E: Cannot extend enum with existing members: "C" \
              # E: Detected enum "lib.D" in a type stub with zero members. There is a chance this is due to a recent change in the semantics of enum membership. If so, use `member = value` to mark an enum member, instead of `member: type` \
              # N: See https://typing.readthedocs.io/en/latest/spec/enums.html#defining-members
-    x: int   # E: Cannot assign to final name "x"
+    x: int   # E: Incompatible types in assignment (expression has type "int", base class "C" defined the type as "C") \
+             # E: Cannot assign to final name "x"
 [builtins fixtures/bool.pyi]
 
 [case testEnumNotFinalWithMethodsAndUninitializedValuesStubMember]
@@ -2419,6 +2422,49 @@ def some_a(a: A):
         reveal_type(a)  # N: Revealed type is "Literal[__main__.A.x]"
 [builtins fixtures/dict.pyi]
 
+[case testEnumAccessFromInstance]
+# flags: --python-version 3.11 --warn-unreachable
+# This was added in 3.11
+from enum import Enum, member, nonmember
+
+class A(Enum):
+    x = 1
+    y = member(2)
+    z = nonmember(3)
+
+reveal_type(A.x)  # N: Revealed type is "Literal[__main__.A.x]?"
+reveal_type(A.y)  # N: Revealed type is "Literal[__main__.A.y]?"
+reveal_type(A.z)  # N: Revealed type is "builtins.int"
+
+reveal_type(A.x.x)  # N: Revealed type is "Literal[__main__.A.x]?"
+reveal_type(A.x.x.x)  # N: Revealed type is "Literal[__main__.A.x]?"
+reveal_type(A.x.y)  # N: Revealed type is "Literal[__main__.A.y]?"
+reveal_type(A.x.y.y)  # N: Revealed type is "Literal[__main__.A.y]?"
+reveal_type(A.x.z)  # N: Revealed type is "builtins.int"
+
+reveal_type(A.y.x)  # N: Revealed type is "Literal[__main__.A.x]?"
+reveal_type(A.y.y)  # N: Revealed type is "Literal[__main__.A.y]?"
+reveal_type(A.y.z)  # N: Revealed type is "builtins.int"
+
+A.z.x  # E: "int" has no attribute "x"
+
+class B(Enum):
+    x = 1
+    value = 2
+
+reveal_type(B.x)  # N: Revealed type is "Literal[__main__.B.x]?"
+reveal_type(B.x.value)  # N: Revealed type is "Literal[2]?"
+reveal_type(B.x.x.value)  # N: Revealed type is "Literal[2]?"
+B.x.value.value  # E: "int" has no attribute "value"
+B.x.value.value.value  # E: "int" has no attribute "value"
+reveal_type(B.value)  # N: Revealed type is "Literal[__main__.B.value]?"
+reveal_type(B.value.x)  # N: Revealed type is "Literal[__main__.B.x]?"
+reveal_type(B.value.x.x)  # N: Revealed type is "Literal[__main__.B.x]?"
+reveal_type(B.value.x.value)  # N: Revealed type is "Literal[2]?"
+B.value.x.value.value  # E: "int" has no attribute "value"
+B.value.value.value  # E: "int" has no attribute "value"
+[builtins fixtures/dict.pyi]
+
 
 [case testErrorOnAnnotatedMember]
 from enum import Enum

test-data/unit/check-incremental.test

@@ -5675,10 +5675,12 @@ class FinalEnum(Enum):
 [builtins fixtures/isinstance.pyi]
 [out]
 main:3: error: Cannot override writable attribute "x" with a final one
+main:3: error: Incompatible types in assignment (expression has type "Ok", base class "RegularEnum" defined the type as "int")
 main:4: error: Cannot extend enum with existing members: "FinalEnum"
 main:5: error: Cannot override final attribute "x" (previously declared in base class "FinalEnum")
 [out2]
 main:3: error: Cannot override writable attribute "x" with a final one
+main:3: error: Incompatible types in assignment (expression has type "Ok", base class "RegularEnum" defined the type as "int")
 main:4: error: Cannot extend enum with existing members: "FinalEnum"
 main:5: error: Cannot override final attribute "x" (previously declared in base class "FinalEnum")