Ambiguous associated type when using associated_type_bounds feature #76593
Description
#![feature(associated_type_bounds)] trait Load : Sized { type Blob; } trait Primitive : Load<Blob = Self> { } trait BlobPtr : Primitive { } trait CleanPtr : Load<Blob : BlobPtr> { fn to_blob(&self) -> Self::Blob; } impl Load for () { type Blob = Self; } impl Primitive for () { } impl BlobPtr for () { } fn main() { }produces an ambiguous, and nonsensical, associated type error even though Blob is introduced only once via Load:
$ rustc test.rs error[E0221]: ambiguous associated type `Blob` in bounds of `Self` --> test.rs:12:26 | 4 | type Blob; | ---------- | | | ambiguous `Blob` from `Load` | ambiguous `Blob` from `Load` ... 12 | fn to_blob(&self) -> Self::Blob; | ^^^^^^^^^^ ambiguous associated type `Blob` | help: use fully qualified syntax to disambiguate | 12 | fn to_blob(&self) -> <Self as Load>::Blob; | ^^^^^^^^^^^^^^^^^^^^ help: use fully qualified syntax to disambiguate | 12 | fn to_blob(&self) -> <Self as Load>::Blob; | ^^^^^^^^^^^^^^^^^^^^ error: aborting due to previous error For more information about this error, try `rustc --explain E0221`. If the use of associated type bounds is removed and replaced by equivalent where clauses, the problem goes away. This version compiles without error:
trait Load : Sized { type Blob; } trait Primitive : Load<Blob = Self> { } trait BlobPtr : Primitive { } trait CleanPtr : Load where Self::Blob: BlobPtr { fn to_blob(&self) -> Self::Blob; } impl Load for () { type Blob = Self; } impl Primitive for () { } impl BlobPtr for () { } fn _use_ptr<P: CleanPtr>(_ptr: P) where P::Blob: BlobPtr { } fn main() { }FWIW in this codebase it looks like associated types could save quite a lot of boilerplate, as these will be quite common type bounds. So if you have a better idea how to do this, I'd love to hear!
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rustc --version --verbose:
rustc 1.48.0-nightly (a1947b3f9 2020-09-10) binary: rustc commit-hash: a1947b3f9e2831e2060bc42f0c78e4a2bb67930a commit-date: 2020-09-10 host: x86_64-unknown-linux-gnu release: 1.48.0-nightly LLVM version: 11.0