Fix type context for assert_type() (#12612)

Noticed in python/typeshed#7655 that it was incorrectly inferring list[Any]
in all cases. This is because I incorrectly put Any as the type context
in the assert_type implementation. Use the current context instead, like
for reveal_type().

Author: JelleZijlstra

Diff for: mypy/checkexpr.py

@@ -3145,7 +3145,7 @@ def visit_cast_expr(self, expr: CastExpr) -> Type:
         return target_type
 
     def visit_assert_type_expr(self, expr: AssertTypeExpr) -> Type:
-        source_type = self.accept(expr.expr, type_context=AnyType(TypeOfAny.special_form),
+        source_type = self.accept(expr.expr, type_context=self.type_context[-1],
                                   allow_none_return=True, always_allow_any=True)
         target_type = expr.type
         if not is_same_type(source_type, target_type):

Diff for: test-data/unit/check-expressions.test

@@ -1049,6 +1049,20 @@ 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]"
 [builtins fixtures/tuple.pyi]
 
+[case testAssertTypeGeneric]
+from typing import assert_type, TypeVar, Generic
+from typing_extensions import Literal
+T = TypeVar("T")
+def f(x: T) -> T: return x
+assert_type(f(1), int)
+class Gen(Generic[T]):
+    def __new__(cls, obj: T) -> Gen[T]: ...
+assert_type(Gen(1), Gen[int])
+# With type context, it infers Gen[Literal[1]] instead.
+y: Gen[Literal[1]] = assert_type(Gen(1), Gen[Literal[1]])
+
+[builtins fixtures/tuple.pyi]
+
 -- None return type
 -- ----------------