Infer generic type arguments for slice expressions

[brianschubert]

Fixes #18149

Slices were made generic in python/typeshed#11637. Currently, all slice expressions are inferred to have type slice[Any, Any, Any]. This PR fills in the generic type arguments more appropriately using start/stop/stride expression types.

Given

class Foo:
    def __getitem__[T](self, item: T) -> T: return item

x = Foo()
reveal_type(x[1:])

Before:

main.py:5: note: Revealed type is "builtins.slice[Any, Any, Any]"

After:

main.py:5: note: Revealed type is "builtins.slice[builtins.int, None, None]"

[brianschubert]

As an alternative approach, I toyed with using the slice.__new__ overloads from typeshed to infer the type arguments. However, this had a big impact on the tests (since all tests involving slice expressions now needed a fixture defining slice), and I'm not sure if there are any benefits from doing it that way.

--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -5616,7 +5616,12 @@ class ExpressionChecker(ExpressionVisitor[Type]):
             if index:
                 t = self.accept(index)
                 self.chk.check_subtype(t, expected, index, message_registry.INVALID_SLICE_INDEX)
-        return self.named_type("builtins.slice")
+        args = [
+            TempNode(NoneType()) if arg is None else arg
+            for arg in [e.begin_index, e.end_index, e.stride]
+        ]
+        slice_type = TypeType(self.named_type("builtins.slice"))
+        return self.check_call(slice_type, args, [nodes.ARG_POS] * 3, e, [None] * 3)[0]

[hauntsaninja]

Nice! Can we record t in the loop so we avoid type checking the index expressions twice?

[brianschubert]

Sure, done!

(FWIW, that's how I originally wrote it 😉. I thought to try separating this logic from that loop since I figured it may be removed soon, now that generic slices let us do better checking on slice indices than the hardcoded SupportsIndex/None checks in that loop)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas-stubs (https://github.com/pandas-dev/pandas-stubs)
+ tests/test_frame.py:2246: error: Expression is of type "tuple[Index[int], slice[None, None, None]]", not "tuple[Index[int], slice[Any, Any, Any]]"  [assert-type]
- tests/test_frame.py:226: error: Expression is of type "Any", not "DataFrame"  [assert-type]
- tests/test_frame.py:227: error: Expression is of type "Any", not "DataFrame"  [assert-type]