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simple type inference fails depending on order of trait bounds #54121

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mikebenfield opened this issue Sep 11, 2018 · 4 comments · Fixed by #79514
Closed

simple type inference fails depending on order of trait bounds #54121

mikebenfield opened this issue Sep 11, 2018 · 4 comments · Fixed by #79514
Labels
A-associated-items Area: Associated items (types, constants & functions) A-inference Area: Type inference A-traits Area: Trait system E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@mikebenfield
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mikebenfield commented Sep 11, 2018

I can't be sure that this behavior is unintended, but it definitely seems very strange and undesirable to me.

This code works fine:

$ cat trash2.rs
trait Tr<A, B> {
    fn exec(a: A, b: B);
}

trait Q {
    type T: Tr<u16, u16> + Tr<u8, u8>;
}

#[allow(dead_code)]
fn f<S: Q>()
{
    <S as Q>::T::exec(0u8, 0u8)
}

fn main() {
}
$ rustc trash2.rs -o trash

But change the order of the trait bounds in the associated type T, and it fails:

$ cat trash2.rs
trait Tr<A, B> {
    fn exec(a: A, b: B);
}

trait Q {
    type T: Tr<u8, u8> + Tr<u16, u16>;
}

#[allow(dead_code)]
fn f<S: Q>()
{
    <S as Q>::T::exec(0u8, 0u8)
}

fn main() {
}
$ rustc trash2.rs -o trash
error[E0308]: mismatched types
  --> trash2.rs:12:23
   |
12 |     <S as Q>::T::exec(0u8, 0u8)
   |                       ^^^ expected u16, found u8
help: you can cast an `u8` to `u16`, which will zero-extend the source value
   |
12 |     <S as Q>::T::exec(0u8.into(), 0u8)
   |                       ^^^^^^^^^^

error[E0308]: mismatched types
  --> trash2.rs:12:28
   |
12 |     <S as Q>::T::exec(0u8, 0u8)
   |                            ^^^ expected u16, found u8
help: you can cast an `u8` to `u16`, which will zero-extend the source value
   |
12 |     <S as Q>::T::exec(0u8, 0u8.into())
   |                            ^^^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0308`.
@estebank estebank added A-associated-items Area: Associated items (types, constants & functions) A-inference Area: Type inference A-traits Area: Trait system T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 5, 2019
@WaffleLapkin
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Probably a duplicate of #72582

@vmarkushin
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vmarkushin commented Sep 26, 2020

Here is another case of such behavior (playground).

trait Trait {
    type Type: Into<Self::Type1> + Into<Self::Type2> + Copy;
    type Type1;
    type Type2;
}

fn foo<T: Trait>(x: T::Type) {
    let _1: T::Type1 = x.into();
    let _2: T::Type2 = x.into();
}
error[E0308]: mismatched types
  --> src/lib.rs:10:24
   |
10 |     let _1: T::Type1 = x.into();
   |             --------   ^^^^^^^^ expected Trait::Type1, found Trait::Type2
   |             |
   |             expected due to this
   |
   = note: expected associated type `<T as Trait>::Type1`
              found associated type `<T as Trait>::Type2`

However, this will compile:

trait Trait {
    type Type: Into<Self::Type1> + Into<Self::Type2> + Copy;
    type Type1;
    type Type2;
}

fn foo<T: Trait>(x: T::Type) {
    let _1: T::Type1 = <T::Type as Into<T::Type1>>::into(x);
    let _2: T::Type2 = <T::Type as Into<T::Type2>>::into(x);
}

Also, an example with concrete types will compile too:

trait Trait {
    type Type: Into<Self::Type1> + Into<Self::Type2> + Copy;
    type Type1;
    type Type2;
}

impl Trait for () {
    type Type = ();
    type Type1 = ();
    type Type2 = ();  
}

fn foo(x: ()) {
    let _1: <() as Trait>::Type1 = x.into();
    let _2: <() as Trait>::Type2 = x.into();
}

Similar issues: #77010 #70944 #77159 #77159 #72582

@WaffleLapkin
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This is fixed on nightly. Presumably by #73905

@camelid
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camelid commented Nov 10, 2020

I just bisected, and indeed this was fixed by #73905. I think this needs a test, and then it can be closed.

@camelid camelid added the E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. label Nov 10, 2020
wip-sync pushed a commit to NetBSD/pkgsrc-wip that referenced this issue Nov 24, 2020
Clean up some of the pkgsrc Makefile, there's still lots in here that
should just be deleted though.  Switch SunOS to the illumos bootstrap
by default.

Version 1.48.0 (2020-11-19)
==========================

Language
--------

- [The `unsafe` keyword is now syntactically permitted on modules.][75857] This
  is still rejected *semantically*, but can now be parsed by procedural macros.

Compiler
--------
- [Stabilised the `-C link-self-contained=<yes|no>` compiler flag.][76158] This tells
  `rustc` whether to link its own C runtime and libraries or to rely on a external
  linker to find them. (Supported only on `windows-gnu`, `linux-musl`, and `wasi` platforms.)
- [You can now use `-C target-feature=+crt-static` on `linux-gnu` targets.][77386]
  Note: If you're using cargo you must explicitly pass the `--target` flag.
- [Added tier 2\* support for `aarch64-unknown-linux-musl`.][76420]

\* Refer to Rust's [platform support page][forge-platform-support] for more
information on Rust's tiered platform support.

Libraries
---------
- [`io::Write` is now implemented for `&ChildStdin` `&Sink`, `&Stdout`,
  and `&Stderr`.][76275]
- [All arrays of any length now implement `TryFrom<Vec<T>>`.][76310]
- [The `matches!` macro now supports having a trailing comma.][74880]
- [`Vec<A>` now implements `PartialEq<[B]>` where `A: PartialEq<B>`.][74194]
- [The `RefCell::{replace, replace_with, clone}` methods now all use `#[track_caller]`.][77055]

Stabilized APIs
---------------
- [`slice::as_ptr_range`]
- [`slice::as_mut_ptr_range`]
- [`VecDeque::make_contiguous`]
- [`future::pending`]
- [`future::ready`]

The following previously stable methods are now `const fn`'s:

- [`Option::is_some`]
- [`Option::is_none`]
- [`Option::as_ref`]
- [`Result::is_ok`]
- [`Result::is_err`]
- [`Result::as_ref`]
- [`Ordering::reverse`]
- [`Ordering::then`]

Cargo
-----

Rustdoc
-------
- [You can now link to items in `rustdoc` using the intra-doc link
  syntax.][74430] E.g. ``/// Uses [`std::future`]`` will automatically generate
  a link to `std::future`'s documentation. See ["Linking to items by
  name"][intradoc-links] for more information.
- [You can now specify `#[doc(alias = "<alias>")]` on items to add search aliases
  when searching through `rustdoc`'s UI.][75740]

Compatibility Notes
-------------------
- [Promotion of references to `'static` lifetime inside `const fn` now follows the
  same rules as inside a `fn` body.][75502] In particular, `&foo()` will not be
  promoted to `'static` lifetime any more inside `const fn`s.
- [Associated type bindings on trait objects are now verified to meet the bounds
  declared on the trait when checking that they implement the trait.][27675]
- [When trait bounds on associated types or opaque types are ambiguous, the
  compiler no longer makes an arbitrary choice on which bound to use.][54121]
- [Fixed recursive nonterminals not being expanded in macros during
  pretty-print/reparse check.][77153] This may cause errors if your macro wasn't
  correctly handling recursive nonterminal tokens.
- [`&mut` references to non zero-sized types are no longer promoted.][75585]
- [`rustc` will now warn if you use attributes like `#[link_name]` or `#[cold]`
  in places where they have no effect.][73461]
- [Updated `_mm256_extract_epi8` and `_mm256_extract_epi16` signatures in
  `arch::{x86, x86_64}` to return `i32` to match the vendor signatures.][73166]
- [`mem::uninitialized` will now panic if any inner types inside a struct or enum
  disallow zero-initialization.][71274]
- [`#[target_feature]` will now error if used in a place where it has no effect.][78143]
- [Foreign exceptions are now caught by `catch_unwind` and will cause an abort.][70212]
  Note: This behaviour is not guaranteed and is still considered undefined behaviour,
  see the [`catch_unwind`] documentation for further information.

Internal Only
-------------
These changes provide no direct user facing benefits, but represent significant
improvements to the internals and overall performance of rustc and
related tools.
- [Building `rustc` from source now uses `ninja` by default over `make`.][74922]
  You can continue building with `make` by setting `ninja=false` in
  your `config.toml`.
- [cg_llvm: `fewer_names` in `uncached_llvm_type`][76030]
- [Made `ensure_sufficient_stack()` non-generic][76680]

[78143]: rust-lang/rust#78143
[76680]: rust-lang/rust#76680
[76030]: rust-lang/rust#76030
[70212]: rust-lang/rust#70212
[27675]: rust-lang/rust#27675
[54121]: rust-lang/rust#54121
[71274]: rust-lang/rust#71274
[77386]: rust-lang/rust#77386
[77153]: rust-lang/rust#77153
[77055]: rust-lang/rust#77055
[76275]: rust-lang/rust#76275
[76310]: rust-lang/rust#76310
[76420]: rust-lang/rust#76420
[76158]: rust-lang/rust#76158
[75857]: rust-lang/rust#75857
[75585]: rust-lang/rust#75585
[75740]: rust-lang/rust#75740
[75502]: rust-lang/rust#75502
[74880]: rust-lang/rust#74880
[74922]: rust-lang/rust#74922
[74430]: rust-lang/rust#74430
[74194]: rust-lang/rust#74194
[73461]: rust-lang/rust#73461
[73166]: rust-lang/rust#73166
[intradoc-links]: https://doc.rust-lang.org/rustdoc/linking-to-items-by-name.html
[`catch_unwind`]: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html
[`Option::is_some`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_some
[`Option::is_none`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_none
[`Option::as_ref`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.as_ref
[`Result::is_ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_ok
[`Result::is_err`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err
[`Result::as_ref`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.as_ref
[`Ordering::reverse`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.reverse
[`Ordering::then`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.then
[`slice::as_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_ptr_range
[`slice::as_mut_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_mut_ptr_range
[`VecDeque::make_contiguous`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.make_contiguous
[`future::pending`]: https://doc.rust-lang.org/std/future/fn.pending.html
[`future::ready`]: https://doc.rust-lang.org/std/future/fn.ready.html
Julian-Wollersberger pushed a commit to Julian-Wollersberger/rust that referenced this issue Nov 28, 2020
"simple type inference fails depending on order of trait bounds"
bors added a commit to rust-lang-ci/rust that referenced this issue Nov 29, 2020
Rollup of 11 pull requests

Successful merges:

 - rust-lang#79327 (Require allocator to be static for boxed `Pin`-API)
 - rust-lang#79340 (Rename "stability" CSS class to "item-info" and combine `document_stability` with `document_short`)
 - rust-lang#79363 (BTreeMap: try to enhance various comments)
 - rust-lang#79395 (Move ui if tests from top-level into `expr/if`)
 - rust-lang#79443 (Improve rustdoc JS tests error output)
 - rust-lang#79464 (Extend doc keyword feature by allowing any ident)
 - rust-lang#79484 (add enable-full-tools to freebsd builds to prevent occasional link er…)
 - rust-lang#79505 (Cleanup: shorter and faster code)
 - rust-lang#79514 (Add test for issue rust-lang#54121: order dependent trait bounds)
 - rust-lang#79516 (Remove unnecessary `mut` binding)
 - rust-lang#79528 (Fix a bootstrap comment)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors closed this as completed in f0e41ce Nov 29, 2020
netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this issue Jan 1, 2021
Pkgsrc changes:
 * Compensate for files being moved around upstream.
 * Introduce optional, on-by-default semi-static building of cargo,
   using the internal curl and openssl sources.  This reduces the dynamic
   dependencies of cargo and therefore the rust package itself.
   Ref. options.mk.
 * The 1.47.0 bootstrap kits have been re-built with the above option
   turned on, so no longer depends on curl or openssl from pkgsrc and/or
   from earlier OS or pkgsrc versions.  This should hopefully fix
   installation of rust with non-default PREFIX, ref. PR#54453.


Upstream changes:

Version 1.48.0 (2020-11-19)
==========================

Language
--------
- [The `unsafe` keyword is now syntactically permitted on modules.][75857] This
  is still rejected *semantically*, but can now be parsed by procedural macros.

Compiler
--------
- [Stabilised the `-C link-self-contained=<yes|no>` compiler flag.][76158]
  This tells `rustc` whether to link its own C runtime and libraries
  or to rely on a external linker to find them. (Supported only on
  `windows-gnu`, `linux-musl`, and `wasi` platforms.)
- [You can now use `-C target-feature=+crt-static` on `linux-gnu` targets.]
  [77386]
  Note: If you're using cargo you must explicitly pass the `--target` flag.
- [Added tier 2\* support for `aarch64-unknown-linux-musl`.][76420]

\* Refer to Rust's [platform support page][forge-platform-support] for more
information on Rust's tiered platform support.

Libraries
---------
- [`io::Write` is now implemented for `&ChildStdin` `&Sink`, `&Stdout`,
  and `&Stderr`.][76275]
- [All arrays of any length now implement `TryFrom<Vec<T>>`.][76310]
- [The `matches!` macro now supports having a trailing comma.][74880]
- [`Vec<A>` now implements `PartialEq<[B]>` where `A: PartialEq<B>`.][74194]
- [The `RefCell::{replace, replace_with, clone}` methods now all use
  `#[track_caller]`.][77055]

Stabilized APIs
---------------
- [`slice::as_ptr_range`]
- [`slice::as_mut_ptr_range`]
- [`VecDeque::make_contiguous`]
- [`future::pending`]
- [`future::ready`]

The following previously stable methods are now `const fn`'s:

- [`Option::is_some`]
- [`Option::is_none`]
- [`Option::as_ref`]
- [`Result::is_ok`]
- [`Result::is_err`]
- [`Result::as_ref`]
- [`Ordering::reverse`]
- [`Ordering::then`]

Cargo
-----

Rustdoc
-------
- [You can now link to items in `rustdoc` using the intra-doc link
  syntax.][74430] E.g. ``/// Uses [`std::future`]`` will automatically generate
  a link to `std::future`'s documentation. See ["Linking to items by
  name"][intradoc-links] for more information.
- [You can now specify `#[doc(alias = "<alias>")]` on items to add
  search aliases when searching through `rustdoc`'s UI.][75740]

Compatibility Notes
-------------------
- [Promotion of references to `'static` lifetime inside `const fn`
  now follows the same rules as inside a `fn` body.][75502] In
  particular, `&foo()` will not be promoted to `'static` lifetime
  any more inside `const fn`s.
- [Associated type bindings on trait objects are now verified to meet the bounds
  declared on the trait when checking that they implement the trait.][27675]
- [When trait bounds on associated types or opaque types are ambiguous, the
  compiler no longer makes an arbitrary choice on which bound to use.][54121]
- [Fixed recursive nonterminals not being expanded in macros during
  pretty-print/reparse check.][77153] This may cause errors if your macro wasn't
  correctly handling recursive nonterminal tokens.
- [`&mut` references to non zero-sized types are no longer promoted.][75585]
- [`rustc` will now warn if you use attributes like `#[link_name]` or `#[cold]`
  in places where they have no effect.][73461]
- [Updated `_mm256_extract_epi8` and `_mm256_extract_epi16` signatures in
  `arch::{x86, x86_64}` to return `i32` to match the vendor signatures.][73166]
- [`mem::uninitialized` will now panic if any inner types inside
  a struct or enum disallow zero-initialization.][71274]
- [`#[target_feature]` will now error if used in a place where it
  has no effect.][78143]
- [Foreign exceptions are now caught by `catch_unwind` and will
  cause an abort.][70212] Note: This behaviour is not guaranteed
  and is still considered undefined behaviour, see the [`catch_unwind`]
  documentation for further information.

Internal Only
-------------
These changes provide no direct user facing benefits, but represent significant
improvements to the internals and overall performance of rustc and
related tools.

- [Building `rustc` from source now uses `ninja` by default over `make`.][74922]
  You can continue building with `make` by setting `ninja=false` in
  your `config.toml`.
- [cg_llvm: `fewer_names` in `uncached_llvm_type`][76030]
- [Made `ensure_sufficient_stack()` non-generic][76680]

[78143]: rust-lang/rust#78143
[76680]: rust-lang/rust#76680
[76030]: rust-lang/rust#76030
[70212]: rust-lang/rust#70212
[27675]: rust-lang/rust#27675
[54121]: rust-lang/rust#54121
[71274]: rust-lang/rust#71274
[77386]: rust-lang/rust#77386
[77153]: rust-lang/rust#77153
[77055]: rust-lang/rust#77055
[76275]: rust-lang/rust#76275
[76310]: rust-lang/rust#76310
[76420]: rust-lang/rust#76420
[76158]: rust-lang/rust#76158
[75857]: rust-lang/rust#75857
[75585]: rust-lang/rust#75585
[75740]: rust-lang/rust#75740
[75502]: rust-lang/rust#75502
[74880]: rust-lang/rust#74880
[74922]: rust-lang/rust#74922
[74430]: rust-lang/rust#74430
[74194]: rust-lang/rust#74194
[73461]: rust-lang/rust#73461
[73166]: rust-lang/rust#73166
[intradoc-links]: https://doc.rust-lang.org/rustdoc/linking-to-items-by-name.html
[`catch_unwind`]: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html
[`Option::is_some`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_some
[`Option::is_none`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_none
[`Option::as_ref`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.as_ref
[`Result::is_ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_ok
[`Result::is_err`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err
[`Result::as_ref`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.as_ref
[`Ordering::reverse`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.reverse
[`Ordering::then`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.then
[`slice::as_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_ptr_range
[`slice::as_mut_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_mut_ptr_range
[`VecDeque::make_contiguous`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.make_contiguous
[`future::pending`]: https://doc.rust-lang.org/std/future/fn.pending.html
[`future::ready`]: https://doc.rust-lang.org/std/future/fn.ready.html
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Labels
A-associated-items Area: Associated items (types, constants & functions) A-inference Area: Type inference A-traits Area: Trait system E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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