Add more tests

Author: dhruvmanila

crates/red_knot_python_semantic/resources/mdtest/annotations/callable.md

@@ -91,6 +91,35 @@ def _(c: Callable):
     reveal_type(c)  # revealed: (*args: Any, **kwargs: Any) -> Any
 ```
 
+## Simple
+
+A simple `Callable` with multiple parameters and a return type:
+
+```py
+from typing import Callable
+
+def _(c: Callable[[int, str], int]):
+    reveal_type(c)  # revealed: (int, str, /) -> int
+```
+
+## Nested
+
+A nested `Callable` as one of the parameter types:
+
+```py
+from typing import Callable
+
+def _(c: Callable[[Callable[[int], str]], int]):
+    reveal_type(c)  # revealed: ((int, /) -> str, /) -> int
+```
+
+And, as the return type:
+
+```py
+def _(c: Callable[[int, str], Callable[[int], int]]):
+    reveal_type(c)  # revealed: (int, str, /) -> (int, /) -> int
+```
+
 ## Gradual form
 
 The `Callable` special form supports the use of `...` in place of the list of parameter types. This is a
@@ -109,4 +138,52 @@ gradual_form(lambda x: "hello")
 gradual_form(lambda x, y: "hello")
 ```
 
+## Using `typing.Concatenate`
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+Using `Concatenate` as the first argument to `Callable`:
+
+```py
+from typing import Callable, Concatenate
+
+def _(c: Callable[Concatenate[int, str, ...], int]):
+    reveal_type(c)  # revealed: (*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> int
+```
+
+And, as one of the parameter types:
+
+```py
+def _(c: Callable[[Concatenate[int, str, ...], int], int]):
+    reveal_type(c)  # revealed: (*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> int
+```
+
+## Using `typing.ParamSpec`
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+Defining `ParamSpec` explicitly:
+
+```py
+from typing import Callable, ParamSpec
+
+P1 = ParamSpec("P1")
+
+def _(c: Callable[P1, int]):
+    reveal_type(c)  # revealed: (*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> int
+```
+
+And, defining it using generics:
+
+```py
+def _[**P2](c: Callable[P2, int]):
+    reveal_type(c)  # revealed: (*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> int
+```
+
 [gradual form]: https://typing.readthedocs.io/en/latest/spec/glossary.html#term-gradual-form

crates/red_knot_python_semantic/src/types/infer.rs

@@ -6370,11 +6370,11 @@ impl<'db> TypeInferenceBuilder<'db> {
                 }
             }
             ast::Expr::Subscript(_) => {
-                // TODO: Support `Concatenate[...]` and `Unpack[...]`
+                // TODO: Support `Concatenate[...]`
                 Parameters::todo()
             }
             ast::Expr::Name(name) => {
-                // This is mainly to handle an edge case `Callable[]`
+                // This is mainly to handle an error case: `Callable[]`
                 if !name.is_valid() {
                     return Err(());
                 }