Erroneous compile errors when returning impl trait

[chlobes]

Code

trait Foo {}
fn a(x: impl Foo) -> impl Foo {
    if true { x } else { a(a(x)) }
}

more examples on the playground

Meta

Triggers ICE on stable, beta, and nightly 1.87.
On nightly 1.88 no ICE but a strange compile error instead

rustc --version --verbose:

rustc 1.87.0-nightly (1aeb99d24 2025-03-19)
binary: rustc
commit-hash: 1aeb99d248e1b0069110cb03c6f1dcc7b36fd7f3
commit-date: 2025-03-19
host: aarch64-apple-darwin
release: 1.87.0-nightly
LLVM version: 20.1.0

Error output

nightly:

29 | fn a(x: impl Foo) -> impl Foo {
   |         -------- this generic parameter must be used with a generic type parameter
30 |     if true { x } else { a(a(x)) }
   |                          ^^^^^^^

error: type parameter `impl Foo` is part of concrete type but not used in parameter list for the `impl Trait` type alias
  --> src/lib.rs:30:26
   |
30 |     if true { x } else { a(a(x)) }
   |                          ^^^^^^^

Backtrace

  |
  = note: delayed at compiler/rustc_middle/src/ty/opaque_types.rs:191:25
             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, &str>
             4: <rustc_middle::ty::opaque_types::ReverseMapper as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
             5: <rustc_middle::ty::OpaqueHiddenType>::remap_generic_params_to_declaration_params
             6: rustc_hir_analysis::collect::type_of::type_of_opaque
             7: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::type_of_opaque::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
             8: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
             9: rustc_query_impl::query_impl::type_of_opaque::get_query_non_incr::__rust_end_short_backtrace
            10: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>>
            11: rustc_hir_analysis::collect::type_of::type_of
            12: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
            13: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            14: rustc_query_impl::query_impl::type_of::get_query_non_incr::__rust_end_short_backtrace
            15: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>>
            16: rustc_hir_analysis::check::check::check_item_type
            17: rustc_hir_analysis::check::wfcheck::check_well_formed
            18: 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]>>
            19: 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>
            20: rustc_query_impl::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
            21: 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>, ()>
            22: rustc_hir_analysis::check::wfcheck::check_mod_type_wf
            23: 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]>>
            24: 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>
            25: rustc_query_impl::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
            26: rustc_hir_analysis::check_crate
            27: rustc_interface::passes::run_required_analyses
            28: rustc_interface::passes::analysis
            29: 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]>>
            30: 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>
            31: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
            32: 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}
            33: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
            34: 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}, ()>
            35: <<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}
            36: std::sys::pal::unix::thread::Thread::new::thread_start
            37: <unknown>
            38: clone

[theemathas]

Related to #139406

cc @lcnr

[theemathas]

The poor error message in nightly is a variant of #139350.

[moxian]

This was (accidentally?) fixed in #138785, but without new regression test for this case, AFAICT

Originally regressed in nightly-2024-02-06, likely #120556 ?

full PR list for 2024-02-06

 - 2024-02-04: #120624 (Rollup of 8 pull requests) by matthiaskrgr
    - #120484 (Avoid ICE when is_val_statically_known is not of a supported type)
    - #120516 (pattern_analysis: cleanup manual impls)
    - #120517 (never patterns: It is correct to lower `!` to `_`.)
    - #120523 (Improve `io::Read::read_buf_exact` error case)
    - #120528 (Store SHOULD_CAPTURE as AtomicU8)
    - #120529 (Update data layouts in custom target tests for LLVM 18)
    - #120531 (Remove a bunch of `has_errors` checks that have no meaningful or the wrong effect)
    - #120533 (Correct paths for hexagon-unknown-none-elf platform doc)
 - 2024-02-04: #120649 (Rollup of 8 pull requests) by matthiaskrgr
    - #119759 (Add FileCheck annotations to dataflow-const-prop tests)
    - #120323 (On E0277 be clearer about implicit `Sized` bounds on type params and assoc types)
    - #120473 (Only suggest removal of `as_*` and `to_` conversion methods on E0308)
    - #120540 (add test for try-block-in-match-arm)
    - #120547 (`#![feature(inline_const_pat)]` is no longer incomplete)
    - #120552 (Correctly check `never_type` feature gating)
    - #120555 (put pnkfelix (me) back on the review queue.)
    - #120556 (Improve the diagnostics for unused generic parameters)
 - 2024-02-05: #120497 (Move predicate, region, and const stuff into their own modules in middle) by compiler-errors
 - 2024-02-05: #120508 (Update cargo) by weihanglo
 - 2024-02-05: #120660 (Rollup of 9 pull requests) by matthiaskrgr
    - #119481 (Clarify ambiguity in select_nth_unstable docs)
    - #119600 (Remove outdated references to librustc_middle)
    - #120458 (Document `&CStr` to `CString` conversion)
    - #120569 (coverage: Improve handling of function/closure spans)
    - #120572 (Update libc to 0.2.153)
    - #120587 (miri: normalize struct tail in ABI compat check)
    - #120607 (fix #120603 by adding a check in default_read_buf)
    - #120636 (Subtree update of `rust-analyzer`)
    - #120641 (rustdoc: trait.impl, type.impl: sort impls to make it not depend on serialization order)
 - 2024-02-05: #120671 (Rollup of 8 pull requests) by matthiaskrgr
    - #113833 (`std::error::Error` -> Trait Implementations: lifetimes consistency improvement)
    - #115386 (PartialEq, PartialOrd: update and synchronize handling of transitive chains)
    - #116284 (make matching on NaN a hard error, and remove the rest of illegal_floating_point_literal_pattern)
    - #118960 (Add LocalWaker and ContextBuilder types to core, and LocalWake trait to alloc.)
    - #120384 (Use `<T, U>` for array/slice equality `impl`s)
    - #120518 (riscv only supports split_debuginfo=off for now)
    - #120657 (Remove unused struct)
    - #120661 (target: default to the medium code model on LoongArch targets)
 - 2024-02-05: #117372 (Update stdarch submodule) by Amanieu
 - 2024-02-05: #120313 (pattern_analysis: Gracefully abort on type incompatibility) by Nadrieril

@rustbot label: +E-needs-test -E-needs-bisection -needs-triage