treat varargs as legal context (#2770)

Fix #2764.

This is somewhat of an ad-hoc fix, but I think it's an improvement.

See also #2634.

Author: elazarg

mypy/checkexpr.py

@@ -1799,7 +1799,7 @@ def infer_lambda_type_using_context(self, e: FuncExpr) -> Optional[CallableType]
 
         if ARG_STAR in arg_kinds or ARG_STAR2 in arg_kinds:
             # TODO treat this case appropriately
-            return None
+            return callable_ctx
         if callable_ctx.arg_kinds != arg_kinds:
             # Incompatible context; cannot use it to infer types.
             self.chk.fail(messages.CANNOT_INFER_LAMBDA_TYPE, e)

test-data/unit/check-inference.test

@@ -1033,7 +1033,18 @@ b = lambda: None  # type: Callable[[], None]
 f(b)
 g(b) # E: Argument 1 to "g" has incompatible type Callable[[], None]; expected Callable[[], int]
 
+[case testLambdaDefaultContext]
+# flags: --strict-optional
+from typing import Callable
+def f(a: Callable[..., None] = lambda *a, **k: None):
+    pass
+
+def g(a: Callable[..., None] = lambda *a, **k: 1):  # E: Incompatible return value type (got "int", expected None)
+    pass
+[out]
+main:6: error: Incompatible types in assignment (expression has type Callable[[StarArg(Any), KwArg(Any)], int], variable has type Callable[..., None])
 
+[builtins fixtures/dict.pyi]
 -- Boolean operators
 -- -----------------