Output a note about use of assert_type in untyped functions (#13626)

`reveal_type` provides a useful note to users that it will always reveal
`Any` within an unchecked function. Similarly, `assert_type` always
checks against `Any`, but does not notify the user about its behavior.

This PR adds said note, which is displayed when `assert_type` raises an
error in an unchecked function

mypy/checkexpr.py

@@ -3724,6 +3724,11 @@ def visit_assert_type_expr(self, expr: AssertTypeExpr) -> Type:
         )
         target_type = expr.type
         if not is_same_type(source_type, target_type):
+            if not self.chk.in_checked_function():
+                self.msg.note(
+                    '"assert_type" expects everything to be "Any" in unchecked functions',
+                    expr.expr,
+                )
             self.msg.assert_type_fail(source_type, target_type, expr)
         return source_type
 

test-data/unit/check-expressions.test

@@ -952,6 +952,28 @@ y: Gen[Literal[1]] = assert_type(Gen(1), Gen[Literal[1]])
 
 [builtins fixtures/tuple.pyi]
 
+[case testAssertTypeUncheckedFunction]
+from typing import assert_type
+from typing_extensions import Literal
+def f():
+    x = 42
+    assert_type(x, Literal[42])
+[out]
+main:5: error: Expression is of type "Any", not "Literal[42]"
+main:5: note: "assert_type" expects everything to be "Any" in unchecked functions
+[builtins fixtures/tuple.pyi]
+
+[case testAssertTypeUncheckedFunctionWithUntypedCheck]
+# flags: --check-untyped-defs
+from typing import assert_type
+from typing_extensions import Literal
+def f():
+    x = 42
+    assert_type(x, Literal[42])
+[out]
+main:6: error: Expression is of type "int", not "Literal[42]"
+[builtins fixtures/tuple.pyi]
+
 -- None return type
 -- ----------------