Fix mypy plugin crash when dekind gets a non-KindN argument

The dekind plugin hook asserted that the argument type is an Instance and then read its first type argument. For an argument like 1 or an unbound TypeVar the assert or the index lookup raised, which mypy reports as an INTERNAL ERROR. Fall back to the default return type instead, so only the regular arg-type error is shown.

Closes #2555

Signed-off-by: mayuriphad <mayuriphad656@gmail.com>

Author: mayuriphad

CHANGELOG.md

@@ -17,6 +17,9 @@ See [0Ver](https://0ver.org/).
 
 ### Bugfixes
 
+- Fixes the `mypy` plugin crashing with `INTERNAL ERROR` when `dekind`
+  is called with an argument that is not a `KindN` instance;
+  now only the regular `arg-type` error is reported
 - Relaxes `future` and `future_safe` decorator argument types from
   `Coroutine` to `Awaitable`, so plain `async def` functions wrapping
   another awaitable (instead of being coroutine functions themselves)

returns/contrib/mypy/_features/kind.py

@@ -85,7 +85,9 @@ def dekind(ctx: FunctionContext) -> MypyType:
     So, ``dekind(KindN[T, int])`` will fail.
     """
     kind = get_proper_type(ctx.arg_types[0][0])
-    assert isinstance(kind, Instance)  # mypy requires these lines
+    if not isinstance(kind, Instance) or not kind.args:
+        # Not a `KindN` instance: mypy already reports the `arg-type` error.
+        return ctx.default_return_type
 
     kind_inst = get_proper_type(kind.args[0])
 

typesafety/test_primitives/test_hkt/test_dekind/test_dekind.yml

@@ -70,3 +70,29 @@
   out: |
     main:8: error: dekind must be used with Instance as the first type argument  [misc]
     main:8: note: Revealed type is "Any"
+
+
+- case: dekind_wrong_non_kind_argument
+  disable_cache: false
+  main: |
+    from returns.primitives.hkt import dekind
+
+    reveal_type(dekind(1))
+  out: |
+    main:3: note: Revealed type is "Never"
+    main:3: error: Argument 1 to "dekind" has incompatible type "int"; expected "KindN[Never, Never, Never, Never]"  [arg-type]
+
+
+- case: dekind_wrong_typevar_argument
+  disable_cache: false
+  main: |
+    from typing import TypeVar
+    from returns.primitives.hkt import dekind
+
+    T = TypeVar('T')
+
+    def some(arg: T) -> None:
+        reveal_type(dekind(arg))
+  out: |
+    main:7: note: Revealed type is "Never"
+    main:7: error: Argument 1 to "dekind" has incompatible type "T"; expected "KindN[Never, Never, Never, Never]"  [arg-type]