Investigation into #74634

src/librustc_middle/infer/canonical.rs

@@ -327,12 +327,14 @@ impl<'tcx> CanonicalVarValues<'tcx> {
                     GenericArgKind::Lifetime(..) => tcx
                         .mk_region(ty::ReLateBound(ty::INNERMOST, ty::BoundRegion::BrAnon(i)))
                         .into(),
-                    GenericArgKind::Const(ct) => tcx
-                        .mk_const(ty::Const {
+                    GenericArgKind::Const(ct) => {
+                        assert!(!tcx.sess.has_errors());
+                        tcx.mk_const(ty::Const {
                             ty: ct.ty,
                             val: ty::ConstKind::Bound(ty::INNERMOST, ty::BoundVar::from_u32(i)),
                         })
-                        .into(),
+                        .into()
+                    }
                 })
                 .collect(),
         }

src/librustc_middle/ty/fold.rs

@@ -556,6 +556,7 @@ impl<'tcx> TyCtxt<'tcx> {
         // identity for bound types and consts
         let fld_t = |bound_ty| self.mk_ty(ty::Bound(ty::INNERMOST, bound_ty));
         let fld_c = |bound_ct, ty| {
+            assert!(!self.sess.has_errors());
             self.mk_const(ty::Const { val: ty::ConstKind::Bound(ty::INNERMOST, bound_ct), ty })
         };
         self.replace_escaping_bound_vars(value.as_ref().skip_binder(), fld_r, fld_t, fld_c)
@@ -803,6 +804,7 @@ impl TypeFolder<'tcx> for Shifter<'tcx> {
                         debruijn.shifted_out(self.amount)
                     }
                 };
+                assert!(!self.tcx.sess.has_errors());
                 self.tcx.mk_const(ty::Const { val: ty::ConstKind::Bound(debruijn, bound_ct), ty })
             }
         } else {

src/librustc_traits/chalk/db.rs

@@ -560,12 +560,14 @@ fn bound_vars_for_item(tcx: TyCtxt<'tcx>, def_id: DefId) -> SubstsRef<'tcx> {
             ))
             .into(),
 
-        ty::GenericParamDefKind::Const => tcx
-            .mk_const(ty::Const {
+        ty::GenericParamDefKind::Const => {
+            assert!(!tcx.sess.has_errors());
+            tcx.mk_const(ty::Const {
                 val: ty::ConstKind::Bound(ty::INNERMOST, ty::BoundVar::from(param.index)),
                 ty: tcx.type_of(param.def_id),
             })
-            .into(),
+            .into()
+        }
     })
 }
 

src/test/ui/const-generics/issues/issue-74634.rs

@@ -0,0 +1,27 @@
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+trait If<const COND: bool> {}
+impl If<true> for () {}
+
+trait IsZero<const N: u8> {
+    type Answer;
+}
+
+struct True;
+struct False;
+
+impl<const N: u8> IsZero<N> for ()
+where (): If<{N == 0}> {
+    type Answer = True;
+}
+
+trait Foobar<const N: u8> {}
+
+impl<const N: u8> Foobar<N> for ()
+where (): IsZero<N, Answer = True> {}
+
+impl<const N: u8> Foobar<N> for ()
+where (): IsZero<N, Answer = False> {}
+
+fn main() {}