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impl trait from unboxed_closure does not accept closures #58708

CodeSandwich opened this Issue Feb 24, 2019 · 2 comments


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CodeSandwich commented Feb 24, 2019

It's impossible to pass closure to a generic fn with impl trait argument when the trait comes from unboxed_closure feature.

This compiles (uses impl Fn with arrow):

fn main() {
    baz(|x| x);

fn baz(_: impl Fn(bool) -> bool) {}

But this fails (uses impl trait from unboxed_closure):

fn main() {
    bar(|x| x);

fn bar(_: impl Fn<bool, Output = bool>) {}

Returns error:

found signature of `fn(_) -> _`
expected signature of `fn(bool) -> _`

Precise definition of closure's types does not help.


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sfackler commented Feb 25, 2019

The desugared version of Fn(bool) -> bool is Fn<(bool,), Output = bool> - the first generic argument has to be a tuple not a bare value.


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CodeSandwich commented Feb 25, 2019

Oh my goodness, of course! Sorry for the noise.

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