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Compiler hangs during execution #139197

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Compiler hangs during execution#139197
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A-coherenceArea: CoherenceA-trait-systemArea: Trait systemC-bugCategory: This is a bug.I-hangIssue: The compiler never terminates, due to infinite loops, deadlock, livelock, etc.P-mediumMedium priorityS-has-bisectionStatus: A bisection has been found for this issueS-has-mcveStatus: A Minimal Complete and Verifiable Example has been found for this issueT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.T-typesRelevant to the types team, which will review and decide on the PR/issue.fixed-by-next-solverFixed by the next-generation trait solver, `-Znext-solver`.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.
@jjl9807

Description

@jjl9807
jjl9807
opened on Apr 1, 2025

I tried this code:

trait Trait {}

struct W<T>(T);

impl<T, U> Trait for W<(<&Self as IntoIterator>, W<U>)>
where
    W<T>: Trait,
    W<U>: Trait,
{
}

fn impls<T: Trait>() {}

fn main() {
    impls::<W<_>>();
}

I expected to see this happen:

The compilation process should complete successfully, and any errors in the code should be reported with diagnostic messages.

Instead, this happened:

The compiler hangs indefinitely and appears to never terminate.

Meta

rustc --version --verbose:

rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-pc-windows-msvc
release: 1.85.1
LLVM version: 19.1.7
Backtrace 

error: expected `::`, found `,`
 --> .\temp.rs:7:48
  |
7 | impl<T, U> Trait for W<(<&Self as IntoIterator>, W<U>)>
  |                                                ^ expected `::`

error: expected `::`, found `,`
 --> .\temp.rs:7:48
  |
7 | impl<T, U> Trait for W<(<&Self as IntoIterator>, W<U>)>
  |                                                ^ expected `::`
  |
help: expressions must be enclosed in braces to be used as const generic arguments
  |
7 | impl<T, U> Trait for W<{ (<&Self as IntoIterator>, W<U>) }>
  |                        +                                 +

error[E0747]: constant provided when a type was expected
 --> .\temp.rs:7:24
  |
7 | impl<T, U> Trait for W<(<&Self as IntoIterator>, W<U>)>
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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    A-coherenceArea: CoherenceA-trait-systemArea: Trait systemC-bugCategory: This is a bug.I-hangIssue: The compiler never terminates, due to infinite loops, deadlock, livelock, etc.P-mediumMedium priorityS-has-bisectionStatus: A bisection has been found for this issueS-has-mcveStatus: A Minimal Complete and Verifiable Example has been found for this issueT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.T-typesRelevant to the types team, which will review and decide on the PR/issue.fixed-by-next-solverFixed by the next-generation trait solver, `-Znext-solver`.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.

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