Kani panics on a SIMD intrinsic instantiated with a scalar type #4921

I tried this code:

#![feature(core_intrinsics)]
#![allow(internal_features)]

pub unsafe fn my_imin<T: Copy>(a: T, b: T) -> T {
    let mask: T = unsafe { std::intrinsics::simd::simd_lt(a, b) };
    unsafe { std::intrinsics::simd::simd_select(mask, a, b) }
}

#[kani::proof]
fn check_imin_scalar() {
    let _ = unsafe { my_imin::<i32>(kani::any(), kani::any()) };
}

using the following command line invocation:

cargo kani

with Kani version: main at 16722a9

I expected to see this happen: simd_lt on i32 reported as an unsupported construct. rustc rejects the program with error[E0511]: invalid monomorphization of simd_lt intrinsic: expected SIMD input type, found non-SIMD i32, but that check is in its LLVM backend.

Instead, this happened:

thread 'rustc' panicked at kani-compiler/src/codegen_cprover_gotoc/codegen/intrinsic.rs:1548:29:
called `Option::unwrap()` on a `None` value

cargo kani autoharness -Z autoharness panics the same way on the function alone, since generic instantiation (#4679) picks i32 for T. In the standard library this is stdarch's simd_imin and simd_imax, so core does not compile under kani autoharness --std on the nightly-2026-09-23 library. The closest earlier report is #2692.