Avoid crash on lamba (a, b): a with context. Fix #1421 (#1452)

Author: gvanrossum

mypy/checkexpr.py

@@ -1267,6 +1267,8 @@ def visit_func_expr(self, e: FuncExpr) -> Type:
         else:
             # Type context available.
             self.chk.check_func_item(e, type_override=inferred_type)
+            if e.expr() not in self.chk.type_map:
+                self.accept(e.expr())
             ret_type = self.chk.type_map[e.expr()]
             return replace_callable_return_type(inferred_type, ret_type)
 

mypy/test/data/check-python2.test

@@ -193,3 +193,11 @@ def f((x,)): return x
 def g((x)): return x
 f(0) + g(0)
 [out]
+
+[case testLambdaAsSortKeyForTuplePython2]
+from typing import Any, Tuple, Callable
+def bar(key: Callable[[Tuple[int, int]], int]) -> int:
+    pass
+def foo() -> int:
+    return bar(key=lambda (a, b): a)
+[out]