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Inference worse in method signature using associated type than with new generic type parameter eq-constrained to the associated type #45462

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scottmcm opened this issue Oct 23, 2017 · 1 comment
Open

Inference worse in method signature using associated type than with new generic type parameter eq-constrained to the associated type #45462

scottmcm opened this issue Oct 23, 2017 · 1 comment
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A-associated-items Area: Associated items such as associated types and consts. A-inference Area: Type inference C-enhancement Category: An issue proposing an enhancement or a PR with one. T-types Relevant to the types team, which will review and decide on the PR/issue.

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@scottmcm
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With this signature

    fn try_fold<F, T: Try>(self, init: T::Ok, mut f: F) -> T where
        Self: Sized, F: FnMut(T::Ok, Self::Item) -> T

This code

    let a = [1, 2, 3];
    let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));

Gets error[E0619]: the type of this value must be known in this context.

But it works fine with the seemingly-equivalent signature

    fn try_fold<F, U, T: Try<Ok=U>>(self, init: U, mut f: F) -> T where
        Self: Sized, F: FnMut(U, Self::Item) -> T

Since acc and init must have the same type in both cases, shouldn't both signatures work?

Full repro: https://play.rust-lang.org/?gist=f8d2baeae64b17344e1cf27673a2e905&version=nightly
@scottmcm scottmcm changed the title Inference worse in method signature using associated type than with new generic type parameter eq-constrained to the generic type Inference worse in method signature using associated type than with new generic type parameter eq-constrained to the associated type Oct 23, 2017
@TimNN TimNN added A-associated-items Area: Associated items such as associated types and consts. A-inference Area: Type inference C-enhancement Category: An issue proposing an enhancement or a PR with one. labels Oct 24, 2017
This was referenced Oct 28, 2017
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bors added a commit that referenced this issue Nov 17, 2017
@bors
Auto merge of #45595 - scottmcm:iter-try-fold, r=dtolnay
b32267f 
Short-circuiting internal iteration with Iterator::try_fold & try_rfold

These are the core methods in terms of which the other methods (`fold`, `all`, `any`, `find`, `position`, `nth`, ...) can be implemented, allowing Iterator implementors to get the full goodness of internal iteration by only overriding one method (per direction).

Based off the `Try` trait, so works with both `Result` and `Option` (:tada: #42526).  The `try_fold` rustdoc examples use `Option` and the `try_rfold` ones use `Result`.

AKA continuing in the vein of PRs #44682 & #44856 for more of `Iterator`.

New bench following the pattern from the latter of those:
```
test iter::bench_take_while_chain_ref_sum          ... bench:   1,130,843 ns/iter (+/- 25,110)
test iter::bench_take_while_chain_sum              ... bench:     362,530 ns/iter (+/- 391)
```

I also ran the benches without the `fold` & `rfold` overrides to test their new default impls, with basically no change.  I left them there, though, to take advantage of existing overrides and because `AlwaysOk` has some sub-optimality due to #43278 (which 45225 should fix).

If you're wondering why there are three type parameters, see issue #45462

Thanks for @bluss for the [original IRLO thread](https://internals.rust-lang.org/t/pre-rfc-fold-ok-is-composable-internal-iteration/4434) and the rfold PR and to @cuviper for adding so many folds, [encouraging me](#45379 (comment)) to make this PR, and finding a catastrophic bug in a pre-review.
@SimonSapin
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To me this looks like a bug or limitation in our current inference implementation. The two signatures should be equivalent.

@scottmcm scottmcm added the T-types Relevant to the types team, which will review and decide on the PR/issue. label Dec 8, 2022
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