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Make None promotable to object, not a subclass, when resolving overloads #3763

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Make None promotable to object, not a subclass, when resolving overloads #3763

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Test case
benkuhn Jul 22, 2017
bd913ad
Fix?
benkuhn Jul 22, 2017
84956f3
Merge remote-tracking branch 'upstream/master' into bug-3757
benkuhn Nov 26, 2017
e38b303
Improve documentation of promotion hack
benkuhn Nov 26, 2017
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5 changes: 3 additions & 2 deletions mypy/checkexpr.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2650,9 +2650,10 @@ def overload_arg_similarity(actual: Type, formal: Type) -> int:
# NoneTyp matches anything if we're not doing strict Optional checking
return 2
else:
# NoneType is a subtype of object
# NoneType can be promoted to object, but isn't actually a subtype (otherwise you
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The comment doesn't sound quite right. Maybe something like this: "As a special case, we don't consider NoneType as a subtype of object here, as otherwise you wouldn't be able to define overloads between object and None."

# wouldn't be able to define overloads between object and None).
if isinstance(formal, Instance) and formal.type.fullname() == "builtins.object":
return 2
return 1
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Update the docstring above to mention that we treat None -> object as a promotion though it's actually treated as a real subtype elsewhere.

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Oh, I forgot that this was true. That makes this seem more hacky.

A general solution (not specific to None or strict-optional) would be to add a fourth level of overload arg similarity for subclasses, e.g.

0: No match
1: Actual can be promoted to formal
2: Actual is a subclass of formal
3: Actual is the same as formal

Would that be preferable? I'd be happy to implement (optionally including refactoring this function to return an Enum) if so. It seems less hacky but much more likely to have unforseen consequences/introduce new errors.

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TBH, I was also thinking that three levels of similarity might be not enough, see e.g. #3656. But introducing a new level of similarity is a big change, so that I think we should wait until @JukkaL is back.

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Treating promotions as different from other forms of compatibility is already unsafe, and adding more levels of similarity would add more room for unsafety. I'd prefer to use the hack since its effect is limited. Basically we'd only surgically enable a specific use case that we know to be important, but otherwise overload resolution would remain the same.

if isinstance(actual, UnionType):
return max(overload_arg_similarity(item, formal)
for item in actual.relevant_items())
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11 changes: 11 additions & 0 deletions test-data/unit/check-optional.test
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Expand Up @@ -238,6 +238,17 @@ def f(x: int) -> int: pass
reveal_type(f(None)) # E: Revealed type is 'builtins.str'
reveal_type(f(0)) # E: Revealed type is 'builtins.int'

[case testOverloadWithObject]
from foo import *
[file foo.pyi]
from typing import overload
@overload
def f(x: None) -> str: pass
@overload
def f(x: object) -> int: pass
reveal_type(f(None)) # E: Revealed type is 'builtins.str'
reveal_type(f(0)) # E: Revealed type is 'builtins.int'

[case testOptionalTypeOrTypePlain]
from typing import Optional
def f(a: Optional[int]) -> int:
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