ICE could not unify `!` with revealed type

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: --edition=2024
fn foo() -> impl ?Future<Output = impl Send> {
    ()
}

fn main() {}

original:

fn foo() -> impl ?Future<Output = impl Send> {
    //~^ ERROR the size for values of type `impl ?Sized` cannot be known at compilation time
    ()
}

fn main() {}

Version information

rustc 1.86.0-nightly (678e669cc 2025-01-19)
binary: rustc
commit-hash: 678e669cc4aab51e3c3fff0660d45362fb830ebd
commit-date: 2025-01-19
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc --edition=2024

Program output

warning: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
 --> /tmp/icemaker_global_tempdir.qzKHRUKhhL4b/rustc_testrunner_tmpdir_reporting.uYhpe96WGOAh/mvce.rs:1:18
  |
1 | fn foo() -> impl ?Future<Output = impl Send> {
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: function `foo` is never used
 --> /tmp/icemaker_global_tempdir.qzKHRUKhhL4b/rustc_testrunner_tmpdir_reporting.uYhpe96WGOAh/mvce.rs:1:4
  |
1 | fn foo() -> impl ?Future<Output = impl Send> {
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: 2 warnings emitted

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: could not unify `()` with revealed type:
                                expected opaque type, found `()`
 --> /tmp/icemaker_global_tempdir.qzKHRUKhhL4b/rustc_testrunner_tmpdir_reporting.uYhpe96WGOAh/mvce.rs:1:35
  |
1 | fn foo() -> impl ?Future<Output = impl Send> {
  |                                   ^^^^^^^^^
  |
note: delayed at compiler/rustc_hir_analysis/src/check/check.rs:396:34
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
         4: rustc_hir_analysis::check::check::check_item_type
         5: rustc_hir_analysis::check::wfcheck::check_well_formed
         6: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
         7: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 1]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
         8: rustc_query_impl::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
         9: rustc_middle::query::plumbing::query_ensure_error_guaranteed::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 1]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, ()>
        10: rustc_hir_analysis::check::wfcheck::check_mod_type_wf
        11: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
        12: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::LocalModDefId, rustc_middle::query::erase::Erased<[u8; 1]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        13: rustc_query_impl::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
        14: rustc_hir_analysis::check_crate
        15: rustc_interface::passes::run_required_analyses
        16: rustc_interface::passes::analysis
        17: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
        18: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        19: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
        20: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
        21: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        22: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
        23: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        24: std::sys::pal::unix::thread::Thread::new::thread_start
        25: <unknown>
        26: <unknown>
 --> /tmp/icemaker_global_tempdir.qzKHRUKhhL4b/rustc_testrunner_tmpdir_reporting.uYhpe96WGOAh/mvce.rs:1:35
  |
1 | fn foo() -> impl ?Future<Output = impl Send> {
  |                                   ^^^^^^^^^

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: rustc 1.86.0-nightly (678e669cc 2025-01-19) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z dump-mir-dir=dir

query stack during panic:
end of query stack

[matthiaskrgr]

use std::future::Future;
fn foo() -> impl ?Future<Output = impl Send> {
    ()
}

pub fn main() {}

I guess edition 2024 added Future to the prelude which is why the flag showed up as "required"

The above example compiles with nightly-2024-03-11 but ICEs with nightly-2024-03-12

[matthiaskrgr]

---

Regression in nightly-2024-03-12

---

fetching https://static.rust-lang.org/dist/2024-03-11/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2024-03-11: 40 B / 40 B [==============================================================================================] 100.00 % 1.21 MB/s converted 2024-03-11 to 3b1717c
fetching https://static.rust-lang.org/dist/2024-03-12/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2024-03-12: 40 B / 40 B [==============================================================================================] 100.00 % 1.20 MB/s converted 2024-03-12 to 4a0cc88
looking for regression commit between 2024-03-11 and 2024-03-12
fetching (via remote github) commits from max(3b1717c, 2024-03-09) to 4a0cc88
ending github query because we found starting sha: 3b1717c
get_commits_between returning commits, len: 12
commit[0] 2024-03-10: Auto merge of #122306 - workingjubilee:backtrace-0.3.70, r=workingjubilee
commit[1] 2024-03-10: Auto merge of #122312 - matthiaskrgr:rollup-0p8y7gg, r=matthiaskrgr
commit[2] 2024-03-11: Auto merge of #122132 - nnethercote:diag-renaming3, r=nnethercote
commit[3] 2024-03-11: Auto merge of #121752 - mu001999:dead_code/improve, r=pnkfelix
commit[4] 2024-03-11: Auto merge of #122050 - erikdesjardins:sret, r=nikic
commit[5] 2024-03-11: Auto merge of #117156 - jmillikin:os-unix-socket-ext, r=Amanieu,dtolnay
commit[6] 2024-03-11: Auto merge of #122331 - jhpratt:rollup-cbl8xsy, r=jhpratt
commit[7] 2024-03-11: Auto merge of #122305 - Nilstrieb:target-tiers, r=davidtwco
commit[8] 2024-03-11: Auto merge of #122140 - oli-obk:track_errors13, r=davidtwco
commit[9] 2024-03-11: Auto merge of #122338 - workingjubilee:rollup-xzpt4v4, r=workingjubilee
commit[10] 2024-03-11: Auto merge of #121796 - oli-obk:eager_opaque_checks3, r=lcnr
commit[11] 2024-03-11: Auto merge of #112267 - lqd:win-1cgu, r=Kobzol

[matthiaskrgr]

my guess would be #121796 🤔