Compiler wants to add bound which already exists to generic. #123513
Description
It looks like the compiler can't unify associated types with their actual types when given a concrete type inside an impl. Additionally, the diagnostics are very misleading. Here's a minimum reproducible example:
#![feature(trait_alias)] trait Something {} trait Test<A> { type T; } trait Alias<A, T: Test<A>> = Test<A> where T::T: Something; trait ArgHaver { type Arg; } impl<T> ArgHaver for T { type Arg = T; } trait Q<A: ArgHaver> { fn f<X, T: Test<A::Arg>>() where X: Alias<A::Arg, T>; } impl<F> Q<()> for F { // both versions don't work // fn f<X, T: Test<<() as ArgHaver>::Arg>>() where X: Alias<<() as ArgHaver>::Arg, T> {} fn f<X, T: Test<()>>() where X: Alias<(), T> {} }I expected to see this happen: The program compiles successfully.
Instead, this happened:
error[E0277]: the trait bound `T: Test<()>` is not satisfied --> <source>:19:5 | 19 | fn f<X, T: Test<()>>() where X: Alias<(), T> {} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Test<()>` is not implemented for `T` | help: consider further restricting this bound | 19 | fn f<X, T: Test<()> + Test<()>>() where X: Alias<(), T> {} | ++++++++++ error[E0276]: impl has stricter requirements than trait --> <source>:19:16 | 13 | fn f<X, T: Test<A::Arg>>() where X: Alias<A::Arg, T>; | ----------------------------------------------------- definition of `f` from trait ... 19 | fn f<X, T: Test<()>>() where X: Alias<(), T> {} | ^^^^^^^^ impl has extra requirement `T: Test<()>` error[E0277]: the trait bound `X: Alias<(), T>` is not satisfied --> <source>:19:37 | 19 | fn f<X, T: Test<()>>() where X: Alias<(), T> {} | ^^^^^^^^^^^^ the trait `Test<()>` is not implemented for `X`, which is required by `X: Alias<(), T>` | = note: required for `X` to implement `Alias<(), T>` note: the requirement `X: Alias<(), T>` appears on the `impl`'s method `f` but not on the corresponding trait's method --> <source>:13:8 | 12 | trait Q<A: ArgHaver> { | - in this trait 13 | fn f<X, T: Test<A::Arg>>() where X: Alias<A::Arg, T>; | ^ this trait's method doesn't have the requirement `X: Alias<(), T>` help: consider further restricting this bound | 19 | fn f<X, T: Test<()>>() where X: Alias<(), T> + Test<()> {} | ++++++++++ Meta
rustc --version --verbose:
rustc 1.79.0-nightly (88c2f4f5f 2024-04-02) binary: rustc commit-hash: 88c2f4f5f50ace5ddc7655ea311435104d3659bd commit-date: 2024-04-02 host: x86_64-unknown-linux-gnu release: 1.79.0-nightly LLVM version: 18.1.2