[red-knot] Add Type::TypeVar variant (#17102)

This adds a new `Type` variant for holding an instance of a typevar
inside of a generic function or class. We don't handle specializing the
typevars yet, but this should implement most of the typing rules for
inside the generic function/class, where we don't know yet which
specific type the typevar will be specialized to.

This PR does _not_ yet handle the constraint that multiple occurrences
of the typevar must be specialized to the _same_ time. (There is an
existing test case for this in `generics/functions.md` which is still
marked as TODO.)

---------

Co-authored-by: Alex Waygood <[email protected]>
Co-authored-by: Carl Meyer <[email protected]>

Author: 3 people

crates/red_knot_ide/src/lib.rs

@@ -162,6 +162,17 @@ impl HasNavigationTargets for Type<'_> {
             | Type::PropertyInstance(_)
             | Type::Tuple(_) => self.to_meta_type(db.upcast()).navigation_targets(db),
 
+            Type::TypeVar(var) => {
+                let definition = var.definition(db);
+                let full_range = definition.full_range(db.upcast());
+
+                NavigationTargets::single(NavigationTarget {
+                    file: full_range.file(),
+                    focus_range: definition.focus_range(db.upcast()).range(),
+                    full_range: full_range.range(),
+                })
+            }
+
             Type::Intersection(intersection) => intersection.navigation_targets(db),
 
             Type::Dynamic(_)

crates/red_knot_python_semantic/resources/mdtest/generics/functions.md

@@ -107,8 +107,7 @@ def good_return[T: int](x: T) -> T:
     return x
 
 def bad_return[T: int](x: T) -> T:
-    # TODO: error: int is not assignable to T
-    # error: [unsupported-operator] "Operator `+` is unsupported between objects of type `T` and `Literal[1]`"
+    # error: [invalid-return-type] "Object of type `int` is not assignable to return type `T`"
     return x + 1
 ```