[red-knot] Ban direct instantiations of Protocol classes

crates/red_knot_python_semantic/resources/mdtest/protocols.md

@@ -304,18 +304,20 @@ reveal_type(typing.Protocol is not typing_extensions.Protocol)  # revealed: bool
 
 ## Calls to protocol classes
 
+<!-- snapshot-diagnostics -->
+
 Neither `Protocol`, nor any protocol class, can be directly instantiated:
 
 ```py
-from typing import Protocol
+from typing_extensions import Protocol, reveal_type
 
 # error: [call-non-callable]
 reveal_type(Protocol())  # revealed: Unknown
 
 class MyProtocol(Protocol):
     x: int
 
-# TODO: should emit error
+# error: [call-non-callable] "Cannot instantiate class `MyProtocol`"
 reveal_type(MyProtocol())  # revealed: MyProtocol
 ```
 

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

@@ -0,0 +1,117 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: protocols.md - Protocols - Calls to protocol classes
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/protocols.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import Protocol, reveal_type
+ 2 | 
+ 3 | # error: [call-non-callable]
+ 4 | reveal_type(Protocol())  # revealed: Unknown
+ 5 | 
+ 6 | class MyProtocol(Protocol):
+ 7 |     x: int
+ 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
+```
+
+# Diagnostics
+
+```
+error: lint:call-non-callable: Object of type `typing.Protocol` is not callable
+ --> /src/mdtest_snippet.py:4:13
+  |
+3 | # error: [call-non-callable]
+4 | reveal_type(Protocol())  # revealed: Unknown
+  |             ^^^^^^^^^^
+5 |
+6 | class MyProtocol(Protocol):
+  |
+
+```
+
+```
+info: revealed-type: Revealed type
+ --> /src/mdtest_snippet.py:4:1
+  |
+3 | # error: [call-non-callable]
+4 | reveal_type(Protocol())  # revealed: Unknown
+  | ^^^^^^^^^^^^^^^^^^^^^^^ `Unknown`
+5 |
+6 | class MyProtocol(Protocol):
+  |
+
+```
+
+```
+error: lint:call-non-callable: Cannot instantiate class `MyProtocol`
+  --> /src/mdtest_snippet.py:10:13
+   |
+ 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): ...
+   |
+info: Protocol classes cannot be instantiated
+ --> /src/mdtest_snippet.py:6:7
+  |
+4 | reveal_type(Protocol())  # revealed: Unknown
+5 |
+6 | class MyProtocol(Protocol):
+  |       ^^^^^^^^^^^^^^^^^^^^ `MyProtocol` declared as a protocol here
+7 |     x: int
+  |
+
+```
+
+```
+info: revealed-type: Revealed type
+  --> /src/mdtest_snippet.py:10:1
+   |
+ 9 | # error: [call-non-callable] "Cannot instantiate class `MyProtocol`"
+10 | reveal_type(MyProtocol())  # revealed: MyProtocol
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ `MyProtocol`
+11 | class SubclassOfMyProtocol(MyProtocol): ...
+   |
+
+```
+
+```
+info: revealed-type: Revealed type
+  --> /src/mdtest_snippet.py:13:1
+   |
+11 | class SubclassOfMyProtocol(MyProtocol): ...
+12 |
+13 | reveal_type(SubclassOfMyProtocol())  # revealed: SubclassOfMyProtocol
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `SubclassOfMyProtocol`
+14 | def f(x: type[MyProtocol]):
+15 |     reveal_type(x())  # revealed: MyProtocol
+   |
+
+```
+
+```
+info: revealed-type: Revealed type
+  --> /src/mdtest_snippet.py:15:5
+   |
+13 | reveal_type(SubclassOfMyProtocol())  # revealed: SubclassOfMyProtocol
+14 | def f(x: type[MyProtocol]):
+15 |     reveal_type(x())  # revealed: MyProtocol
+   |     ^^^^^^^^^^^^^^^^ `MyProtocol`
+   |
+
+```

crates/red_knot_python_semantic/src/types/class.rs

@@ -37,9 +37,11 @@ use crate::{
 };
 use indexmap::IndexSet;
 use itertools::Itertools as _;
+use ruff_db::diagnostic::Span;
 use ruff_db::files::File;
 use ruff_python_ast::name::Name;
 use ruff_python_ast::{self as ast, PythonVersion};
+use ruff_text_size::{Ranged, TextRange};
 use rustc_hash::{FxHashSet, FxHasher};
 
 type FxOrderMap<K, V> = ordermap::map::OrderMap<K, V, BuildHasherDefault<FxHasher>>;
@@ -1725,6 +1727,28 @@ impl<'db> ClassLiteralType<'db> {
     pub(super) fn into_protocol_class(self, db: &'db dyn Db) -> Option<ProtocolClassLiteral<'db>> {
         self.is_protocol(db).then_some(ProtocolClassLiteral(self))
     }
+
+    /// Returns the [`Span`] of the class's "header": the class name
+    /// and any arguments passed to the `class` statement. E.g.
+    ///
+    /// ```ignore
+    /// class Foo(Bar, metaclass=Baz): ...
+    ///       ^^^^^^^^^^^^^^^^^^^^^^^
+    /// ```
+    pub(super) fn header_span(self, db: &'db dyn Db) -> Span {
+        let class_scope = self.body_scope(db);
+        let class_node = class_scope.node(db).expect_class();
+        let class_name = &class_node.name;
+        let header_range = TextRange::new(
+            class_name.start(),
+            class_node
+                .arguments
+                .as_deref()
+                .map(Ranged::end)
+                .unwrap_or_else(|| class_name.end()),
+        );
+        Span::from(class_scope.file(db)).with_range(header_range)
+    }
 }
 
 impl<'db> From<ClassLiteralType<'db>> for Type<'db> {

crates/red_knot_python_semantic/src/types/diagnostic.rs

@@ -11,7 +11,7 @@ use crate::types::string_annotation::{
 use crate::types::{class::ProtocolClassLiteral, KnownFunction, KnownInstanceType, Type};
 use ruff_db::diagnostic::{Annotation, Diagnostic, Severity, Span, SubDiagnostic};
 use ruff_python_ast::{self as ast, AnyNodeRef};
-use ruff_text_size::{Ranged, TextRange};
+use ruff_text_size::Ranged;
 use rustc_hash::FxHashSet;
 use std::fmt::Formatter;
 
@@ -1331,24 +1331,14 @@ pub(crate) fn report_bad_argument_to_get_protocol_members(
     diagnostic.set_primary_message("This call will raise `TypeError` at runtime");
     diagnostic.info("Only protocol classes can be passed to `get_protocol_members`");
 
-    let class_scope = class.body_scope(db);
-    let class_node = class_scope.node(db).expect_class();
-    let class_name = &class_node.name;
-    let class_def_diagnostic_range = TextRange::new(
-        class_name.start(),
-        class_node
-            .arguments
-            .as_deref()
-            .map(Ranged::end)
-            .unwrap_or_else(|| class_name.end()),
-    );
     let mut class_def_diagnostic = SubDiagnostic::new(
         Severity::Info,
-        format_args!("`{class_name}` is declared here, but it is not a protocol class:"),
+        format_args!(
+            "`{}` is declared here, but it is not a protocol class:",
+            class.name(db)
+        ),
     );
-    class_def_diagnostic.annotate(Annotation::primary(
-        Span::from(class_scope.file(db)).with_range(class_def_diagnostic_range),
-    ));
+    class_def_diagnostic.annotate(Annotation::primary(class.header_span(db)));
     diagnostic.sub(class_def_diagnostic);
 
     diagnostic.info(
@@ -1393,12 +1383,6 @@ pub(crate) fn report_runtime_check_against_non_runtime_checkable_protocol(
     ));
     diagnostic.set_primary_message("This call will raise `TypeError` at runtime");
 
-    let class_scope = protocol.body_scope(db);
-    let class_node = class_scope.node(db).expect_class();
-    let class_def_arguments = class_node
-        .arguments
-        .as_ref()
-        .expect("A `Protocol` class should always have at least one explicit base");
     let mut class_def_diagnostic = SubDiagnostic::new(
         Severity::Info,
         format_args!(
@@ -1407,11 +1391,8 @@ pub(crate) fn report_runtime_check_against_non_runtime_checkable_protocol(
         ),
     );
     class_def_diagnostic.annotate(
-        Annotation::primary(Span::from(class_scope.file(db)).with_range(TextRange::new(
-            class_node.name.start(),
-            class_def_arguments.end(),
-        )))
-        .message(format_args!("`{class_name}` declared here")),
+        Annotation::primary(protocol.header_span(db))
+            .message(format_args!("`{class_name}` declared here")),
     );
     diagnostic.sub(class_def_diagnostic);
 
@@ -1421,3 +1402,28 @@ pub(crate) fn report_runtime_check_against_non_runtime_checkable_protocol(
     ));
     diagnostic.info("See https://docs.python.org/3/library/typing.html#typing.runtime_checkable");
 }
+
+pub(crate) fn report_attempted_protocol_instantiation(
+    context: &InferContext,
+    call: &ast::ExprCall,
+    protocol: ProtocolClassLiteral,
+) {
+    let Some(builder) = context.report_lint(&CALL_NON_CALLABLE, call) else {
+        return;
+    };
+    let db = context.db();
+    let class_name = protocol.name(db);
+    let mut diagnostic =
+        builder.into_diagnostic(format_args!("Cannot instantiate class `{class_name}`",));
+    diagnostic.set_primary_message("This call will raise `TypeError` at runtime");
+
+    let mut class_def_diagnostic = SubDiagnostic::new(
+        Severity::Info,
+        format_args!("Protocol classes cannot be instantiated"),
+    );
+    class_def_diagnostic.annotate(
+        Annotation::primary(protocol.header_span(db))
+            .message(format_args!("`{class_name}` declared as a protocol here")),
+    );
+    diagnostic.sub(class_def_diagnostic);
+}

crates/red_knot_python_semantic/src/types/infer.rs

@@ -96,8 +96,8 @@ use crate::Db;
 
 use super::context::{InNoTypeCheck, InferContext};
 use super::diagnostic::{
-    report_bad_argument_to_get_protocol_members, report_index_out_of_bounds,
-    report_invalid_exception_caught, report_invalid_exception_cause,
+    report_attempted_protocol_instantiation, report_bad_argument_to_get_protocol_members,
+    report_index_out_of_bounds, report_invalid_exception_caught, report_invalid_exception_cause,
     report_invalid_exception_raised, report_invalid_type_checking_constant,
     report_non_subscriptable, report_possibly_unresolved_reference,
     report_runtime_check_against_non_runtime_checkable_protocol, report_slice_step_size_zero,
@@ -4280,6 +4280,20 @@ impl<'db> TypeInferenceBuilder<'db> {
         let mut call_arguments = Self::parse_arguments(arguments);
         let callable_type = self.infer_expression(func);
 
+        // 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]`.
+        // <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()))
+        {
+            report_attempted_protocol_instantiation(&self.context, call_expression, protocol_class);
+        }
+
         // For class literals we model the entire class instantiation logic, so it is handled
         // in a separate function. For some known classes we have manual signatures defined and use
         // the `try_call` path below.