NR2: ICE in visit, at rust/typecheck/rust-tyty-variance-analysis-private.h:164

auto-reduced (treereduce-rust):

//@compile-flags: -frust-incomplete-and-experimental-compiler-do-not-use -frust-name-resolution-2.0
type FxpStorageHelper<const INT_BITS: u8> = PhantomU8;

// revisions: full min
#![cfg_attr(full, feature(const_generics))]
#![cfg_attr(full, allow(incomplete_features))]
#![cfg_attr(min, feature(min_const_generics))]

struct PhantomU8<const X: u8>;

trait FxpStorage {
    type SInt; // Add arithmetic traits as needed.
}

macro_rules! fxp_storage_impls {
    ($($($n:literal)|+ => $sint:ty),* $(,)?) => {
        $($(impl FxpStorage for PhantomU8<$n> {
            type SInt = $sint;
        })*)*
    }
}

fxp_storage_impls! {
    1 => i8,
    2 => i16,
    3 | 4 => i32,
    5 | 6 | 7 | 8 => i64,
    9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 => i128,
}

type FxpStorageHelper<const INT_BITS: u8, const FRAC_BITS: u8> =
    PhantomU8<{(FRAC_BITS + FRAC_BITS + 7) / 8}>;
    //[min]~^ ERROR generic parameters may not be used in const operations
    //[min]~| ERROR generic parameters may not be used in const operations

struct Fxp<const INT_BITS: u8, const FRAC_BITS: u8>
where
    FxpStorageHelper<INT_BITS, FRAC_BITS>: FxpStorage,
    //[full]~^ ERROR constant expression depends on a generic parameter
{
    storage: <i64 as FxpStorage>::SInt,
}

fn main() {
    Fxp::<1, 15> { storage: 0i16 };
    Fxp::<2, 15> { storage: 0i32 };
}

Version information:

765121736dfe3f5b319bbe9837880deda326394e

Command:
/home/matthias/vcs/github/gccrs/gccrs-build/gcc/crab1 -frust-incomplete-and-experimental-compiler-do-not-use -frust-name-resolution-2.0

/tmp/icemaker_global_tempdir.NKudfJBTYGQf/rustc_testrunner_tmpdir_reporting.Jhy7tsDB0QaY/mvce.rs:1:45: error: could not resolve type path ‘PhantomU8’
    1 | type FxpStorageHelper<const INT_BITS: u8> = PhantomU8;
      |                                             ^~~~~~~~~

Analyzing compilation unit

Time variable                                   usr           sys          wall           GGC
 TOTAL                              :   0.00          0.00          0.00          150k
Extra diagnostic checks enabled; compiler may run slowly.
Configure with --enable-checking=release to disable checks.

original stacktrace

crab1: internal compiler error: in visit, at rust/typecheck/rust-tyty-variance-analysis-private.h:164
0x88ff90 Rust::TyTy::VarianceAnalysis::VisitorBase<Rust::TyTy::VarianceAnalysis::Term>::visit(Rust::TyTy::PlaceholderType&)
	../../gcc/rust/typecheck/rust-tyty-variance-analysis-private.h:164
0xf54582 Rust::TyTy::VarianceAnalysis::GenericTyVisitorCtx::add_constraints_from_ty(Rust::TyTy::BaseType*, Rust::TyTy::VarianceAnalysis::Term)
	../../gcc/rust/typecheck/rust-tyty-variance-analysis.cc:400
0xf57d49 Rust::TyTy::VarianceAnalysis::GenericTyVisitorCtx::process_type(Rust::TyTy::ADTType&)
	../../gcc/rust/typecheck/rust-tyty-variance-analysis.cc:250
0xf57d49 Rust::TyTy::VarianceAnalysis::GenericTyPerCrateCtx::process_type(Rust::TyTy::ADTType&)
	../../gcc/rust/typecheck/rust-tyty-variance-analysis.cc:131
0xf57d49 Rust::TyTy::VarianceAnalysis::CrateCtx::add_type_constraints(Rust::TyTy::ADTType&)
	../../gcc/rust/typecheck/rust-tyty-variance-analysis.cc:26
0xf8bf4c Rust::Resolver::TypeCheckItem::visit(Rust::HIR::StructStruct&)
	../../gcc/rust/typecheck/rust-hir-type-check-item.cc:331
0xf7dbb7 Rust::Resolver::TypeCheckItem::Resolve(Rust::HIR::Item&)
	../../gcc/rust/typecheck/rust-hir-type-check-item.cc:59
0xf1518b Rust::Resolver::TypeResolution::Resolve(Rust::HIR::Crate&)
	../../gcc/rust/typecheck/rust-hir-type-check.cc:75
0xce8f73 Rust::Session::compile_crate(char const*)
	../../gcc/rust/rust-session-manager.cc:685
Please submit a full bug report, with preprocessed source (by using -freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

Reproduce the ICE with the reduced Rust case and the command shown in the issue. Start at rust/typecheck/rust-tyty-variance-analysis-private.h:164, then trace add_constraints_from_ty and process_type in rust/typecheck/rust-tyty-variance-analysis.cc. Done means the case no longer aborts in the compiler and reports a normal diagnostic.