Associated Type bound in GAT Parameter incorrectly errors as unsatisfied #144127
Description
I tried this code:
trait TypeFn<T> { type Output; } trait Boo { type Arg; type CallCopy<F: TypeFn<Self::Arg, Output: Copy>>; } impl Boo for () { type Arg = (); type CallCopy<F: TypeFn<Self::Arg, Output: Copy>> = F; }I expected to see this happen: The code compiles.
Instead, this happened: The code errored with
error[E0277]: the trait bound `F: TypeFn<()>` is not satisfied --> error.rs:10:5 | 10 | type CallCopy<F: TypeFn<Self::Arg, Output: Copy>> = F; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TypeFn<()>` is not implemented for `F` error[E0276]: impl has stricter requirements than trait --> error.rs:10:22 | 6 | type CallCopy<F: TypeFn<Self::Arg, Output: Copy>>; | ------------------------------------------------- definition of `CallCopy` from trait ... 10 | type CallCopy<F: TypeFn<Self::Arg, Output: Copy>> = F; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl has extra requirement `F: TypeFn<()>` error: aborting due to 2 previous errors Some errors have detailed explanations: E0276, E0277. For more information about an error, try `rustc --explain E0276`.Meta
rustc --version --verbose:
rustc 1.88.0 (6b00bc388 2025-06-23) binary: rustc commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc commit-date: 2025-06-23 host: x86_64-unknown-linux-gnu release: 1.88.0 LLVM version: 20.1.5 The error persists on nightly:
rustc 1.90.0-nightly (e46629662 2025-07-17) binary: rustc commit-hash: e4662966273ed58b51f9ff8d682accc202aa1210 commit-date: 2025-07-17 host: x86_64-unknown-linux-gnu release: 1.90.0-nightly LLVM version: 20.1.8