BUG: 306212
Lambda expressions were given the type of their return expression, e.g. lambda x: 12 has type "int".

That caused calling them to get no or an incorrect return type:
a = (lamba x: 12)("arg") # a should be int, but is mixed.

They should have a function type instead.

Fixes one test. No test regressions.

TODO: Add more tests, stress-testing on real projects. It's after midnight here, too...