lazy_type_alias: trait bound no longer satisified

[matthiaskrgr]

I tried this code:
tests/ui/associated-types/param-env-normalize-cycle.rs

// Minimized case from typenum that didn't compile because:
// - We tried to normalize the ParamEnv of the second impl
// - This requires trying to normalize `GrEq<Self, Square<Square<U>>>`
// - This requires proving `Square<Square<U>>: Sized` so that the first impl
//   applies
// - This requires Providing `Square<Square<U>>` is well-formed, so that we
//   can use the `Sized` bound on `Mul::Output`
// - This requires proving `Square<U>: Mul`
// - But first we tried normalizing the whole obligation, including the
//   ParamEnv, which leads to a cycle error.

// check-pass

trait PrivateSquareRoot {}

pub trait Mul<Rhs = Self> {
    type Output;
}

pub trait IsGreaterOrEqual<Rhs> {
    type Output;
}

pub type Square<A> = <A as Mul>::Output;
pub type GrEq<A, B> = <A as IsGreaterOrEqual<B>>::Output;

impl<A, B> IsGreaterOrEqual<B> for A {
    type Output = ();
}

impl<U> PrivateSquareRoot for U
where
    U: Mul,
    Square<U>: Mul,
    GrEq<Self, Square<Square<U>>>: Sized,
{
}

fn main() {}

without `-Zcrate-attr=feature(lazy_type_alias)´: no warnings

with -Zcrate-attr=feature(lazy_type_alias):

error[E0277]: the trait bound `<U as Mul>::Output: Mul` is not satisfied
  --> tests/ui/associated-types/param-env-normalize-cycle.rs:31:1
   |
31 | impl<U> PrivateSquareRoot for U
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Mul` is not implemented for `<U as Mul>::Output`
   |
help: consider further restricting the associated type
   |
35 |     GrEq<Self, Square<Square<U>>>: Sized, <U as Mul>::Output: Mul
   |                                         ~~~~~~~~~~~~~~~~~~~~~~~~~

error[E0277]: the trait bound `<U as Mul>::Output: Mul` is not satisfied
  --> tests/ui/associated-types/param-env-normalize-cycle.rs:31:1
   |
31 | / impl<U> PrivateSquareRoot for U
32 | | where
33 | |     U: Mul,
34 | |     Square<U>: Mul,
35 | |     GrEq<Self, Square<Square<U>>>: Sized,
36 | | {
37 | | }
   | |_^ the trait `Mul` is not implemented for `<U as Mul>::Output`
   |
help: consider further restricting the associated type
   |
35 |     GrEq<Self, Square<Square<U>>>: Sized, <U as Mul>::Output: Mul
   |                                         ~~~~~~~~~~~~~~~~~~~~~~~~~

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.

rustc 1.73.0-nightly (04abc370b 2023-07-28)
binary: rustc
commit-hash: 04abc370b9f3855b28172b65a7f7d5a433f41412
commit-date: 2023-07-28
host: x86_64-unknown-linux-gnu
release: 1.73.0-nightly
LLVM version: 16.0.5

[compiler-errors]

I'm gonna mark this and all the other issues you just opened as needs-mcve, since the UI test you provided is not totally minimized

[compiler-errors]

Also, for the posterity of other folks who may want to work on these issues or repro them in the future, it would be very helpful if you copied all the -Zcrate-attr= into the code itself.

[matthiaskrgr]

script for reducing with treereduce

treereduce-rust --passes=10 --min-reduction=10 --interesting-exit-code=101 --source file.rs -- ./reduce.sh @@.rs

#!/bin/bash

if  rustc --crate-type lib --cap-lints allow $1 ; then
	if ! rustc "-Zcrate-attr=feature(lazy_type_alias)" --crate-type lib $1 ; then
		exit 101
	else
		exit 0
	fi
else
	exit 0
fi

[fmease]

Slightly reduced:

#![feature(lazy_type_alias)]

type H<A: F> = <A as F>::O;

trait F { type O; }
trait G {}

impl<A> G for A {}

fn f<U: F>()
where
    H<U>: F,
    H<H<U>>: G,
{}

fn main() {}

And as usual, repro[ty::Weak ↦ ty::Projection] has exactly the same errors.

[fmease]

Fixed by the next solver.