Allow None to be inferred from context in strict Optional mode (#2233)

Fixes first part of #2230.

mypy/checkexpr.py

@@ -460,10 +460,13 @@ def infer_function_type_arguments_using_context(
                 # See also github issues #462 and #360.
                 ret_type = NoneTyp()
         args = infer_type_arguments(callable.type_var_ids(), ret_type, erased_ctx)
-        # Only substitute non-None and non-erased types.
+        # Only substitute non-Uninhabited and non-erased types.
         new_args = []  # type: List[Type]
         for arg in args:
-            if isinstance(arg, (NoneTyp, UninhabitedType)) or has_erased_component(arg):
+            if isinstance(arg, UninhabitedType) or has_erased_component(arg):
+                new_args.append(None)
+            elif not experiments.STRICT_OPTIONAL and isinstance(arg, NoneTyp):
+                # Don't substitute None types in non-strict-Optional mode.
                 new_args.append(None)
             else:
                 new_args.append(arg)

test-data/unit/check-optional.test

@@ -428,3 +428,11 @@ x + 1
 [out]
 main:2: note: In module imported here:
 tmp/a.py:3: error: Unsupported left operand type for + (some union)
+
+[case testNoneContextInference]
+from typing import Dict, List
+def f() -> List[None]:
+    return []
+def g() -> Dict[None, None]:
+    return {}
+[builtins fixtures/dict.pyi]