[red-knot] Ban direct instantiation of generic protocols as well as non-generic ones

crates/red_knot_python_semantic/resources/mdtest/protocols.md

@@ -304,6 +304,11 @@ reveal_type(typing.Protocol is not typing_extensions.Protocol)  # revealed: bool
 
 Neither `Protocol`, nor any protocol class, can be directly instantiated:
 
+```toml
+[environment]
+python-version = "3.12"
+```
+
 ```py
 from typing_extensions import Protocol, reveal_type
 
@@ -315,6 +320,12 @@ class MyProtocol(Protocol):
 
 # error: [call-non-callable] "Cannot instantiate class `MyProtocol`"
 reveal_type(MyProtocol())  # revealed: MyProtocol
+
+class GenericProtocol[T](Protocol):
+    x: T
+
+# error: [call-non-callable] "Cannot instantiate class `GenericProtocol`"
+reveal_type(GenericProtocol[int]())  # revealed: GenericProtocol[int]
 ```
 
 But a non-protocol class can be instantiated, even if it has `Protocol` in its MRO:
@@ -323,6 +334,10 @@ But a non-protocol class can be instantiated, even if it has `Protocol` in its M
 class SubclassOfMyProtocol(MyProtocol): ...
 
 reveal_type(SubclassOfMyProtocol())  # revealed: SubclassOfMyProtocol
+
+class SubclassOfGenericProtocol[T](GenericProtocol[T]): ...
+
+reveal_type(SubclassOfGenericProtocol[int]())  # revealed: SubclassOfGenericProtocol[int]
 ```
 
 And as a corollary, `type[MyProtocol]` can also be called:

crates/red_knot_python_semantic/resources/mdtest/snapshots/protocols.md_-_Protocols_-_Calls_to_protocol_classes.snap

@@ -22,11 +22,21 @@ mdtest path: crates/red_knot_python_semantic/resources/mdtest/protocols.md
  8 | 
  9 | # error: [call-non-callable] "Cannot instantiate class `MyProtocol`"
 10 | reveal_type(MyProtocol())  # revealed: MyProtocol
-11 | class SubclassOfMyProtocol(MyProtocol): ...
-12 | 
-13 | reveal_type(SubclassOfMyProtocol())  # revealed: SubclassOfMyProtocol
-14 | def f(x: type[MyProtocol]):
-15 |     reveal_type(x())  # revealed: MyProtocol
+11 | 
+12 | class GenericProtocol[T](Protocol):
+13 |     x: T
+14 | 
+15 | # error: [call-non-callable] "Cannot instantiate class `GenericProtocol`"
+16 | reveal_type(GenericProtocol[int]())  # revealed: GenericProtocol[int]
+17 | class SubclassOfMyProtocol(MyProtocol): ...
+18 | 
+19 | reveal_type(SubclassOfMyProtocol())  # revealed: SubclassOfMyProtocol
+20 | 
+21 | class SubclassOfGenericProtocol[T](GenericProtocol[T]): ...
+22 | 
+23 | reveal_type(SubclassOfGenericProtocol[int]())  # revealed: SubclassOfGenericProtocol[int]
+24 | def f(x: type[MyProtocol]):
+25 |     reveal_type(x())  # revealed: MyProtocol
 ```
 
 # Diagnostics
@@ -64,7 +74,8 @@ error: lint:call-non-callable: Cannot instantiate class `MyProtocol`
  9 | # error: [call-non-callable] "Cannot instantiate class `MyProtocol`"
 10 | reveal_type(MyProtocol())  # revealed: MyProtocol
    |             ^^^^^^^^^^^^ This call will raise `TypeError` at runtime
-11 | class SubclassOfMyProtocol(MyProtocol): ...
+11 |
+12 | class GenericProtocol[T](Protocol):
    |
 info: Protocol classes cannot be instantiated
  --> src/mdtest_snippet.py:6:7
@@ -85,32 +96,80 @@ info: revealed-type: Revealed type
  9 | # error: [call-non-callable] "Cannot instantiate class `MyProtocol`"
 10 | reveal_type(MyProtocol())  # revealed: MyProtocol
    | ^^^^^^^^^^^^^^^^^^^^^^^^^ `MyProtocol`
-11 | class SubclassOfMyProtocol(MyProtocol): ...
+11 |
+12 | class GenericProtocol[T](Protocol):
+   |
+
+```
+
+```
+error: lint:call-non-callable: Cannot instantiate class `GenericProtocol`
+  --> src/mdtest_snippet.py:16:13
+   |
+15 | # error: [call-non-callable] "Cannot instantiate class `GenericProtocol`"
+16 | reveal_type(GenericProtocol[int]())  # revealed: GenericProtocol[int]
+   |             ^^^^^^^^^^^^^^^^^^^^^^ This call will raise `TypeError` at runtime
+17 | class SubclassOfMyProtocol(MyProtocol): ...
+   |
+info: Protocol classes cannot be instantiated
+  --> src/mdtest_snippet.py:12:7
+   |
+10 | reveal_type(MyProtocol())  # revealed: MyProtocol
+11 |
+12 | class GenericProtocol[T](Protocol):
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `GenericProtocol` declared as a protocol here
+13 |     x: T
+   |
+
+```
+
+```
+info: revealed-type: Revealed type
+  --> src/mdtest_snippet.py:16:1
+   |
+15 | # error: [call-non-callable] "Cannot instantiate class `GenericProtocol`"
+16 | reveal_type(GenericProtocol[int]())  # revealed: GenericProtocol[int]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `GenericProtocol[int]`
+17 | class SubclassOfMyProtocol(MyProtocol): ...
    |
 
 ```
 
 ```
 info: revealed-type: Revealed type
-  --> src/mdtest_snippet.py:13:1
+  --> src/mdtest_snippet.py:19:1
    |
-11 | class SubclassOfMyProtocol(MyProtocol): ...
-12 |
-13 | reveal_type(SubclassOfMyProtocol())  # revealed: SubclassOfMyProtocol
+17 | class SubclassOfMyProtocol(MyProtocol): ...
+18 |
+19 | reveal_type(SubclassOfMyProtocol())  # revealed: SubclassOfMyProtocol
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `SubclassOfMyProtocol`
-14 | def f(x: type[MyProtocol]):
-15 |     reveal_type(x())  # revealed: MyProtocol
+20 |
+21 | class SubclassOfGenericProtocol[T](GenericProtocol[T]): ...
+   |
+
+```
+
+```
+info: revealed-type: Revealed type
+  --> src/mdtest_snippet.py:23:1
+   |
+21 | class SubclassOfGenericProtocol[T](GenericProtocol[T]): ...
+22 |
+23 | reveal_type(SubclassOfGenericProtocol[int]())  # revealed: SubclassOfGenericProtocol[int]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `SubclassOfGenericProtocol[int]`
+24 | def f(x: type[MyProtocol]):
+25 |     reveal_type(x())  # revealed: MyProtocol
    |
 
 ```
 
 ```
 info: revealed-type: Revealed type
-  --> src/mdtest_snippet.py:15:5
+  --> src/mdtest_snippet.py:25:5
    |
-13 | reveal_type(SubclassOfMyProtocol())  # revealed: SubclassOfMyProtocol
-14 | def f(x: type[MyProtocol]):
-15 |     reveal_type(x())  # revealed: MyProtocol
+23 | reveal_type(SubclassOfGenericProtocol[int]())  # revealed: SubclassOfGenericProtocol[int]
+24 | def f(x: type[MyProtocol]):
+25 |     reveal_type(x())  # revealed: MyProtocol
    |     ^^^^^^^^^^^^^^^^ `MyProtocol`
    |
 

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4555,18 +4555,23 @@ impl<'db> TypeInferenceBuilder<'db> {
             }
         }
 
-        // It might look odd here that we emit an error for class-literals but not `type[]` types.
-        // But it's deliberate! The typing spec explicitly mandates that `type[]` types can be called
-        // even though class-literals cannot. This is because even though a protocol class `SomeProtocol`
-        // is always an abstract class, `type[SomeProtocol]` can be a concrete subclass of that protocol
-        // -- and indeed, according to the spec, type checkers must disallow abstract subclasses of the
-        // protocol to be passed to parameters that accept `type[SomeProtocol]`.
+        // It might look odd here that we emit an error for class-literals and generic aliases but not
+        // `type[]` types. But it's deliberate! The typing spec explicitly mandates that `type[]` types
+        // can be called even though class-literals cannot. This is because even though a protocol class
+        // `SomeProtocol` is always an abstract class, `type[SomeProtocol]` can be a concrete subclass of
+        // that protocol -- and indeed, according to the spec, type checkers must disallow abstract
+        // subclasses of the protocol to be passed to parameters that accept `type[SomeProtocol]`.
         // <https://typing.python.org/en/latest/spec/protocol.html#type-and-class-objects-vs-protocols>.
-        if let Some(protocol_class) = callable_type
-            .into_class_literal()
-            .and_then(|class| class.into_protocol_class(self.db()))
+        let possible_protocol_class = match callable_type {
+            Type::ClassLiteral(class) => Some(class),
+            Type::GenericAlias(generic) => Some(generic.origin(self.db())),
+            _ => None,
+        };
+
+        if let Some(protocol) =
+            possible_protocol_class.and_then(|class| class.into_protocol_class(self.db()))
         {
-            report_attempted_protocol_instantiation(&self.context, call_expression, protocol_class);
+            report_attempted_protocol_instantiation(&self.context, call_expression, protocol);
         }
 
         // For class literals we model the entire class instantiation logic, so it is handled