Infer generic type arguments for slice expressions

Author: brianschubert

mypy/checkexpr.py

@@ -5616,7 +5616,11 @@ def visit_slice_expr(self, e: SliceExpr) -> 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")
+        type_args = [
+            NoneType() if arg is None else self.accept(arg)
+            for arg in [e.begin_index, e.end_index, e.stride]
+        ]
+        return self.chk.named_generic_type("builtins.slice", type_args)
 
     def visit_list_comprehension(self, e: ListComprehension) -> Type:
         return self.check_generator_or_comprehension(

test-data/unit/check-expressions.test

@@ -1178,8 +1178,8 @@ class B: pass
 [case testSlicingWithInvalidBase]
 
 a: A
-a[1:2] # E: Invalid index type "slice" for "A"; expected type "int"
-a[:]   # E: Invalid index type "slice" for "A"; expected type "int"
+a[1:2] # E: Invalid index type "slice[int, int, None]" for "A"; expected type "int"
+a[:]   # E: Invalid index type "slice[None, None, None]" for "A"; expected type "int"
 class A:
   def __getitem__(self, n: int) -> 'A': pass
 [builtins fixtures/slice.pyi]