[ty] Reduce false positives when subscripting classes generic over TypeVarTuples

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/annotations/starred.md

@@ -18,5 +18,5 @@ def append_int(*args: *Ts) -> tuple[*Ts, int]:
     return (*args, 1)
 
 # TODO should be tuple[Literal[True], Literal["a"], int]
-reveal_type(append_int(True, "a"))  # revealed: tuple[@Todo(PEP 646), ...]
+reveal_type(append_int(True, "a"))  # revealed: tuple[@Todo(TypeVarTuple), ...]
 ```

crates/ty_python_semantic/resources/mdtest/annotations/unsupported_special_types.md

@@ -1,5 +1,7 @@
 # Unsupported special types
 
+## Functional classes
+
 We do not understand the functional syntax for creating `TypedDict`s or `Enum`s yet. But we also do
 not emit false positives when these are used in type expressions.
 
@@ -14,3 +16,32 @@ MyTypedDict = typing.TypedDict("MyTypedDict", {"foo": int})
 
 def f(a: MyEnum, b: MyTypedDict): ...
 ```
+
+## No false positives for subscripting a class generic over a `TypeVarTuple`
+
+We don't support `TypeVarTuple` yet, but we also try to avoid emitting false positives when you
+subscript classes generic over a `TypeVarTuple`:
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+from typing import Generic, TypeVarTuple, Unpack
+
+Ts = TypeVarTuple("Ts")
+
+class Foo(Generic[Unpack[Ts]]): ...
+
+x: Foo[int, str, bytes]  # fine
+
+class Bar(Generic[*Ts]): ...
+
+y: Bar[int, str, bytes]  # fine
+
+class Baz[*Ts]: ...
+
+# TODO: false positive
+z: Baz[int, str, bytes]  # error: [not-subscriptable]
+```

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -69,7 +69,7 @@ reveal_type(d)  # revealed: tuple[tuple[str, str], tuple[int, int]]
 reveal_type(e)  # revealed: tuple[str, ...]
 
 reveal_type(f)  # revealed: tuple[str, *tuple[int, ...], bytes]
-reveal_type(g)  # revealed: tuple[@Todo(PEP 646), ...]
+reveal_type(g)  # revealed: tuple[@Todo(TypeVarTuple), ...]
 
 reveal_type(h)  # revealed: tuple[list[int], list[int]]
 reveal_type(i)  # revealed: tuple[str | int, str | int]

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -15098,7 +15098,15 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         }
 
         if let Some(first_excess_type_argument_index) = first_excess_type_argument_index {
-            if typevars_len == 0 {
+            if let Type::GenericAlias(alias) = value_ty
+                && let spec = alias.specialization(self.db())
+                && spec
+                    .types(self.db())
+                    .contains(&Type::Dynamic(DynamicType::TodoTypeVarTuple))
+            {
+                // Avoid false-positive errors when specializing a class
+                // that's generic over a legacy TypeVarTuple
+            } else if typevars_len == 0 {
                 // Type parameter list cannot be empty, so if we reach here, `value_ty` is not a generic type.
                 if let Some(builder) = self
                     .context

crates/ty_python_semantic/src/types/infer/builder/type_expression.rs

@@ -713,7 +713,10 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                 }
 
                 let ty = if return_todo {
-                    Some(TupleType::homogeneous(self.db(), todo_type!("PEP 646")))
+                    Some(TupleType::homogeneous(
+                        self.db(),
+                        Type::Dynamic(DynamicType::TodoTypeVarTuple),
+                    ))
                 } else {
                     TupleType::new(self.db(), &element_types.build())
                 };
@@ -738,7 +741,10 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                 let single_element_ty = self.infer_type_expression(single_element);
                 if element_could_alter_type_of_whole_tuple(single_element, single_element_ty, self)
                 {
-                    Some(TupleType::homogeneous(self.db(), todo_type!("PEP 646")))
+                    Some(TupleType::homogeneous(
+                        self.db(),
+                        Type::Dynamic(DynamicType::TodoTypeVarTuple),
+                    ))
                 } else {
                     TupleType::heterogeneous(self.db(), std::iter::once(single_element_ty))
                 }

crates/ty_test/src/matcher.rs

@@ -208,8 +208,11 @@ fn discard_todo_metadata(ty: &str) -> Cow<'_, str> {
     {
         /// `@Todo` variants that are hardcoded and always display their message,
         /// even in release mode.
-        const PRESERVED_TODO_VARIANTS: &[&str] =
-            &["@Todo(StarredExpression)", "@Todo(typing.Unpack)"];
+        const PRESERVED_TODO_VARIANTS: &[&str] = &[
+            "@Todo(StarredExpression)",
+            "@Todo(typing.Unpack)",
+            "@Todo(TypeVarTuple)",
+        ];
 
         static TODO_METADATA_REGEX: LazyLock<regex::Regex> =
             LazyLock::new(|| regex::Regex::new(r"@Todo\([^)]*\)").unwrap());