Allow inferring lambda type from Callable[..., T] context (#1522)

Fixes #1517.

Author: rwbarton

mypy/checkexpr.py

@@ -1299,6 +1299,14 @@ def infer_lambda_type_using_context(self, e: FuncExpr) -> CallableType:
 
         arg_kinds = [arg.kind for arg in e.arguments]
 
+        if callable_ctx.is_ellipsis_args:
+            # Fill in Any arguments to match the arguments of the lambda.
+            callable_ctx = callable_ctx.copy_modified(
+                is_ellipsis_args=False,
+                arg_types=[AnyType()] * len(arg_kinds),
+                arg_kinds=arg_kinds
+            )
+
         if callable_ctx.arg_kinds != arg_kinds:
             # Incompatible context; cannot use it to infer types.
             self.chk.fail(messages.CANNOT_INFER_LAMBDA_TYPE, e)

mypy/test/data/check-inference-context.test

@@ -593,6 +593,24 @@ main:2: error: Cannot infer type of lambda
 main:3: error: Cannot infer type of lambda
 main:3: error: Incompatible types in assignment (expression has type Callable[[], A], variable has type Callable[[A], A])
 
+[case testEllipsisContextForLambda]
+from typing import Callable
+f1 = lambda x: 1 # type: Callable[..., int]
+f2 = lambda: 1 # type: Callable[..., int]
+f3 = lambda *args, **kwargs: 1 # type: Callable[..., int]
+f4 = lambda x: x # type: Callable[..., int]
+g = lambda x: 1 # type: Callable[..., str]
+[builtins fixtures/dict.py]
+[out]
+main:6: error: Incompatible return value type: expected builtins.str, got builtins.int
+main:6: error: Incompatible types in assignment (expression has type Callable[[Any], int], variable has type Callable[..., str])
+
+[case testEllipsisContextForLambda2]
+from typing import TypeVar, Callable
+T = TypeVar('T')
+def foo(arg: Callable[..., T]) -> None: pass
+foo(lambda: 1)
+
 
 -- Overloads + generic functions
 -- -----------------------------