Fix assert_type behaviour with Instance last_known_value (#15123)

Fixes #12923

assert_type(42, int) ... had issues. #12923 was a bit of a mess as the
OP was or wasn't currently accurate.

Added two tests to repo the current problem, which now pass.

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

mypy/checkexpr.py

@@ -3918,6 +3918,12 @@ def visit_assert_type_expr(self, expr: AssertTypeExpr) -> Type:
             always_allow_any=True,
         )
         target_type = expr.type
+        proper_source_type = get_proper_type(source_type)
+        if (
+            isinstance(proper_source_type, mypy.types.Instance)
+            and proper_source_type.last_known_value is not None
+        ):
+            source_type = proper_source_type.last_known_value
         if not is_same_type(source_type, target_type):
             if not self.chk.in_checked_function():
                 self.msg.note(

test-data/unit/check-expressions.test

@@ -937,6 +937,8 @@ reveal_type(returned)  # N: Revealed type is "builtins.int"
 assert_type(a, str)  # E: Expression is of type "int", not "str"
 assert_type(a, Any)  # E: Expression is of type "int", not "Any"
 assert_type(a, Literal[1])  # E: Expression is of type "int", not "Literal[1]"
+assert_type(42, Literal[42])
+assert_type(42, int)  # E: Expression is of type "Literal[42]", not "int"
 [builtins fixtures/tuple.pyi]
 
 [case testAssertTypeGeneric]