__auto_type does not inherit nullability to inferred type

Number:rdar://27062504 Date Originated:28-Jun-2016
Status:Open Resolved:
Product:Developer Tools Product Version:Xcode-beta (8S128d)
Classification:Other Bug Reproducible:Always

When you use `__auto_type` to declare a variable, it does not inherit any nullability definitions marked by the return type of the value being assigned.

Steps to Reproduce:

It is easy to reproduce this with any _Nullable return type in Foundation, for example -[NSBundle pathForResource:ofType:], which is defined as such: `- (nullable NSString *)pathForResource:(nullable NSString *)name ofType:(nullable NSString *)ext;`

This can easily be setup:

__auto_type path = [[NSBundle mainBundle] pathForResource:@“foo” ofType:@“txt”];

Expected Results:
`path` should be the type `NSString * _Nullable` because __auto_type should inherit nullability.

Actual Results:
`path` has the type `NSString *` and is assumed _Nonnull

This can affect the use nullability warnings, since __auto_type will mark variables as assumed _Nonnull and therefore not be caught by `-Wnullable-to-nonnull-conversion`

This is only available in the first beta of Xcode 8


Engineering has provided the following feedback regarding this issue:

We can’t implement this warning in clang as implementing it would introduce a lot of complexity. Use the static analyzer instead, which already warns about this case.

By kyle.hickinson at Aug. 31, 2016, 3:19 p.m. (reply...)

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