AlexWaygood · Mar 5, 2026 · Feb 27, 2026 · Feb 27, 2026 · Feb 27, 2026 · Feb 27, 2026
@@ -2519,10 +2519,10 @@ def function():
         // TODO: This should just be `**AB@Alias2 (<variance>)`
         // https://github.com/astral-sh/ty/issues/1581
         assert_snapshot!(test.hover(), @"
-        (**AB@Alias2) -> tuple[AB@Alias2]
+        (**AB@Alias2) -> tuple[Unknown]
         ---------------------------------------------
         ```python
-        (**AB@Alias2) -> tuple[AB@Alias2]
+        (**AB@Alias2) -> tuple[Unknown]
         ```
         ---------------------------------------------
         info[hover]: Hovered content is

@@ -153,8 +153,21 @@ class B: ...
 In type annotations, `ParamSpec` is only valid as the first element to `Callable`, the final element
 to `Concatenate`, or as a type parameter to `Protocol` or `Generic`.
 
+<!-- snapshot-diagnostics -->
+
+`library.py`:
+
+```py
+from typing import ParamSpec
+
+LibraryP = ParamSpec("LibraryP")
+```
+
+`main.py`:
+
 ```py
-from typing import ParamSpec, Callable, Concatenate, Protocol, Generic
+import library
+from typing import Any, Final, ParamSpec, Callable, Concatenate, Protocol, Generic, Union, Optional, Annotated
 
 P = ParamSpec("P")
 
@@ -167,19 +180,61 @@ class ValidGeneric(Generic[P]):
 def valid(
     a1: Callable[P, int],
     a2: Callable[Concatenate[int, P], int],
+    a3: Callable["P", int],
+    a4: Callable[Concatenate[int, "P"], int],
+    a5: Callable[library.LibraryP, int],
+    a6: Callable["Concatenate[int, P]", int],
+    a7: Callable["library.LibraryP", int],
 ) -> None: ...
 def invalid(
-    # TODO: error
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
     a1: P,
-    # TODO: error
+    # TODO: this should cause us to emit an error because a `ParamSpec` type argument
+    # cannot be used to specialize a non-`ParamSpec` type parameter
     a2: list[P],
-    # TODO: error
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
     a3: Callable[[P], int],
-    # TODO: error
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
     a4: Callable[..., P],
-    # TODO: error
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
     a5: Callable[Concatenate[P, ...], int],
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a6: P | int,
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a7: Union[P, int],
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a8: Optional[P],
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a9: Annotated[P, "metadata"],
+    # error: [invalid-type-form] "The first argument to `Callable` must be either a list of types, ParamSpec, Concatenate, or `...`"
+    a10: Callable["[int, str]", str],
+    # error: [invalid-type-form] "The first argument to `Callable` must be either a list of types, ParamSpec, Concatenate, or `...`"
+    a11: Callable["...", int],
+) -> None: ...
+
+# error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+def invalid_return() -> P:
+    raise NotImplementedError
+
+def invalid_variable_annotation(y: Any) -> None:
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    x: P = y
+
+def invalid_with_qualifier(y: Any) -> None:
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    x: Final[P] = y
+
+# error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+def invalid_stringified_return() -> "P":
+    raise NotImplementedError
+
+def invalid_stringified_annotation(
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a: "P",
 ) -> None: ...
+def invalid_stringified_variable_annotation(y: Any) -> None:
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    x: "P" = y
 ```
 
 ## Validating `P.args` and `P.kwargs` usage

@@ -223,11 +223,11 @@ If a `ParamSpec` appears in `Concatenate`, it must be the last element.
 ```py
 from typing import Callable, Concatenate
 
-# TODO: Should be an error - ParamSpec not in last position
+# error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
 def invalid1[**P](c: Callable[Concatenate[P, int], bool]):
     reveal_type(c)  # revealed: (...) -> bool
 
-# TODO: Should be an error - ParamSpec not in last position
+# error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
 def invalid2[**P](c: Callable[Concatenate[P, ...], bool]):
     reveal_type(c)  # revealed: (...) -> bool
 

@@ -67,25 +67,65 @@ def foo[**P = int]() -> None:
 
 `ParamSpec` is only valid as the first element to `Callable` or the final element to `Concatenate`.
 
+<!-- snapshot-diagnostics -->
+
 ```py
-from typing import ParamSpec, Callable, Concatenate
+from typing import Any, Final, ParamSpec, Callable, Concatenate, Union, Optional, Annotated
 
 def valid[**P](
     a1: Callable[P, int],
     a2: Callable[Concatenate[int, P], int],
+    a3: Callable["P", int],
+    a4: Callable[Concatenate[int, "P"], int],
 ) -> None: ...
 def invalid[**P](
-    # TODO: error
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
     a1: P,
-    # TODO: error
+    # TODO: this should cause us to emit an error because a `ParamSpec` type argument
+    # cannot be used to specialize a non-`ParamSpec` type parameter
     a2: list[P],
-    # TODO: error
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
     a3: Callable[[P], int],
-    # TODO: error
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
     a4: Callable[..., P],
-    # TODO: error
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
     a5: Callable[Concatenate[P, ...], int],
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a6: P | int,
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a7: Union[P, int],
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a8: Optional[P],
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a9: Annotated[P, "metadata"],
+) -> None: ...
+
+# error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+def invalid_return[**P]() -> P:
+    raise NotImplementedError
+
+# error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+type Alias[**P] = P
+
+def invalid_variable_annotation[**P](y: Any) -> None:
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    x: P = y
+
+def invalid_with_qualifier[**P](y: Any) -> None:
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    x: Final[P] = y
+
+# error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+def invalid_stringified_return[**P]() -> "P":
+    raise NotImplementedError
+
+def invalid_stringified_annotation[**P](
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    a: "P",
 ) -> None: ...
+def invalid_stringified_variable_annotation[**P](y: Any) -> None:
+    # error: [invalid-type-form] "Bare ParamSpec `P` is not valid in this context"
+    x: "P" = y
 ```
 
 ## Validating `P.args` and `P.kwargs` usage