[ty] Remove hack in protocol satisfiability check (#20568)

## Summary

This removes a hack in the protocol satisfiability check that was
previously needed to work around missing assignability-modeling of
inferable type variables. Assignability of type variables is not
implemented fully, but some recent changes allow us to remove that hack
with limited impact on the ecosystem (and the test suite). The change in
the typing conformance test is favorable.

## Test Plan

* Adapted Markdown tests
* Made sure that this change works in combination with
https://github.com/astral-sh/ruff/pull/20517

crates/ty_python_semantic/resources/mdtest/protocols.md

@@ -1928,21 +1928,23 @@ from typing_extensions import TypeVar, Self, Protocol
 from ty_extensions import is_equivalent_to, static_assert, is_assignable_to, is_subtype_of
 
 class NewStyleClassScoped[T](Protocol):
-    def method(self, input: T) -> None: ...
+    def method(self: Self, input: T) -> None: ...
 
 S = TypeVar("S")
 
 class LegacyClassScoped(Protocol[S]):
-    def method(self, input: S) -> None: ...
+    def method(self: Self, input: S) -> None: ...
 
-static_assert(is_equivalent_to(NewStyleClassScoped, LegacyClassScoped))
+# TODO: these should pass
-static_assert(is_equivalent_to(NewStyleClassScoped[int], LegacyClassScoped[int]))
+static_assert(is_equivalent_to(NewStyleClassScoped, LegacyClassScoped))  # error: [static-assert-error]
+static_assert(is_equivalent_to(NewStyleClassScoped[int], LegacyClassScoped[int]))  # error: [static-assert-error]
 
 class NominalGeneric[T]:
     def method(self, input: T) -> None: ...
 
 def _[T](x: T) -> T:
-    static_assert(is_equivalent_to(NewStyleClassScoped[T], LegacyClassScoped[T]))
+    # TODO: should pass
+    static_assert(is_equivalent_to(NewStyleClassScoped[T], LegacyClassScoped[T]))  # error: [static-assert-error]
     static_assert(is_subtype_of(NominalGeneric[T], NewStyleClassScoped[T]))
     static_assert(is_subtype_of(NominalGeneric[T], LegacyClassScoped[T]))
     return x
@@ -2016,17 +2018,27 @@ class NominalReturningSelfNotGeneric:
 # TODO: should pass
 static_assert(is_equivalent_to(LegacyFunctionScoped, NewStyleFunctionScoped))  # error: [static-assert-error]
 
-static_assert(is_subtype_of(NominalNewStyle, NewStyleFunctionScoped))
+static_assert(is_assignable_to(NominalNewStyle, NewStyleFunctionScoped))
-static_assert(is_subtype_of(NominalNewStyle, LegacyFunctionScoped))
+static_assert(is_assignable_to(NominalNewStyle, LegacyFunctionScoped))
+# TODO: should pass
+static_assert(is_subtype_of(NominalNewStyle, NewStyleFunctionScoped))  # error: [static-assert-error]
+# TODO: should pass
+static_assert(is_subtype_of(NominalNewStyle, LegacyFunctionScoped))  # error: [static-assert-error]
 static_assert(not is_assignable_to(NominalNewStyle, UsesSelf))
 
-static_assert(is_subtype_of(NominalLegacy, NewStyleFunctionScoped))
+static_assert(is_assignable_to(NominalLegacy, NewStyleFunctionScoped))
-static_assert(is_subtype_of(NominalLegacy, LegacyFunctionScoped))
+static_assert(is_assignable_to(NominalLegacy, LegacyFunctionScoped))
+# TODO: should pass
+static_assert(is_subtype_of(NominalLegacy, NewStyleFunctionScoped))  # error: [static-assert-error]
+# TODO: should pass
+static_assert(is_subtype_of(NominalLegacy, LegacyFunctionScoped))  # error: [static-assert-error]
 static_assert(not is_assignable_to(NominalLegacy, UsesSelf))
 
 static_assert(not is_assignable_to(NominalWithSelf, NewStyleFunctionScoped))
 static_assert(not is_assignable_to(NominalWithSelf, LegacyFunctionScoped))
-static_assert(is_subtype_of(NominalWithSelf, UsesSelf))
+static_assert(is_assignable_to(NominalWithSelf, UsesSelf))
+# TODO: should pass
+static_assert(is_subtype_of(NominalWithSelf, UsesSelf))  # error: [static-assert-error]
 
 # TODO: these should pass
 static_assert(not is_assignable_to(NominalNotGeneric, NewStyleFunctionScoped))  # error: [static-assert-error]
@@ -2035,8 +2047,23 @@ static_assert(not is_assignable_to(NominalNotGeneric, UsesSelf))
 
 static_assert(not is_assignable_to(NominalReturningSelfNotGeneric, NewStyleFunctionScoped))
 static_assert(not is_assignable_to(NominalReturningSelfNotGeneric, LegacyFunctionScoped))
+
 # TODO: should pass
 static_assert(not is_assignable_to(NominalReturningSelfNotGeneric, UsesSelf))  # error: [static-assert-error]
+
+# These test cases are taken from the typing conformance suite:
+class ShapeProtocolImplicitSelf(Protocol):
+    def set_scale(self, scale: float) -> Self: ...
+
+class ShapeProtocolExplicitSelf(Protocol):
+    def set_scale(self: Self, scale: float) -> Self: ...
+
+class BadReturnType:
+    def set_scale(self, scale: float) -> int:
+        return 42
+
+static_assert(not is_assignable_to(BadReturnType, ShapeProtocolImplicitSelf))
+static_assert(not is_assignable_to(BadReturnType, ShapeProtocolExplicitSelf))
 ```
 
 ## Subtyping of protocols with `@classmethod` or `@staticmethod` members

crates/ty_python_semantic/src/types/protocol_class.rs

@@ -23,7 +23,6 @@ use crate::{
         diagnostic::report_undeclared_protocol_member,
         signatures::{Parameter, Parameters},
         todo_type,
-        visitor::any_over_type,
     },
 };
 
@@ -571,24 +570,7 @@ impl<'a, 'db> ProtocolMember<'a, 'db> {
                     attribute_type
                 };
 
-                let proto_member_as_bound_method = method.bind_self(db);
+                attribute_type.has_relation_to_impl(db, method.bind_self(db), relation, visitor)
-
-                if any_over_type(
-                    db,
-                    proto_member_as_bound_method,
-                    &|t| matches!(t, Type::TypeVar(_)),
-                    true,
-                ) {
-                    // TODO: proper validation for generic methods on protocols
-                    return ConstraintSet::from(true);
-                }
-
-                attribute_type.has_relation_to_impl(
-                    db,
-                    proto_member_as_bound_method,
-                    relation,
-                    visitor,
-                )
             }
             // TODO: consider the types of the attribute on `other` for property members
             ProtocolMemberKind::Property(_) => ConstraintSet::from(matches!(