Compiler hangs when adding a `Send` trait bound to an HRTB associated type

I tried this code:

trait X<'a>
{
    type U: ;  
}

impl X<'_> for u32 
where
    for<'b> <Self as X<'b>>::U: Clone + Send + ,
{
    type U = str;
}

fn main() {}

this hang does not occur if the -Z next-solver flag is used

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rustc --version --verbose:

rustc 1.93.1 (01f6ddf75 2026-02-11)
binary: rustc
commit-hash: 01f6ddf7588f42ae2d7eb0a2f21d44e8e96674cf
commit-date: 2026-02-11
host: x86_64-unknown-linux-gnu
release: 1.93.1
LLVM version: 21.1.8

<backtrace>

Start by compiling the reproducer with rustc and compare its behavior with and without the -Z next-solver flag. No repository file or test is named, so trace the compiler path handling the HRTB associated-type bounds; done means the example no longer hangs and behavior is covered by a regression test.