don't just compare `ty::Const`

compiler/rustc_infer/src/infer/combine.rs

@@ -129,6 +129,8 @@ pub fn super_combine_consts<R>(
     where
         R: ConstEquateRelation<'tcx>,
     {
+        let a = self.tcx.expose_default_const_substs(a);
+        let b = self.tcx.expose_default_const_substs(b);
         debug!("{}.consts({:?}, {:?})", relation.tag(), a, b);
         if a == b {
             return Ok(a);

compiler/rustc_middle/src/ty/fold.rs

@@ -213,6 +213,10 @@ fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
         c.super_fold_with(self)
     }
 
+    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        p.super_fold_with(self)
+    }
+
     fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
         bug!("most type folders should not be folding MIR datastructures: {:?}", c)
     }
@@ -1205,6 +1209,42 @@ fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Sel
     }
 }
 
+impl<'tcx> TyCtxt<'tcx> {
+    /// This is a HACK(const_generics) and should probably not be needed.
+    /// Might however be perf relevant, so who knows.
+    ///
+    /// FIXME(@lcnr): explain this function a bit more
+    pub fn expose_default_const_substs<T: TypeFoldable<'tcx>>(self, v: T) -> T {
+        v.fold_with(&mut ExposeDefaultConstSubstsFolder { tcx: self })
+    }
+}
+
+struct ExposeDefaultConstSubstsFolder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> TypeFolder<'tcx> for ExposeDefaultConstSubstsFolder<'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        if ty.flags().intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
+            ty.super_fold_with(self)
+        } else {
+            ty
+        }
+    }
+
+    fn fold_predicate(&mut self, pred: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        if pred.inner.flags.intersects(TypeFlags::HAS_UNKNOWN_DEFAULT_CONST_SUBSTS) {
+            pred.super_fold_with(self)
+        } else {
+            pred
+        }
+    }
+}
+
 /// Collects all the late-bound regions at the innermost binding level
 /// into a hash set.
 struct LateBoundRegionsCollector<'tcx> {

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -974,6 +974,10 @@ fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::Br
 }
 
 impl<'tcx> TypeFoldable<'tcx> for ty::Predicate<'tcx> {
+    fn fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
+        folder.fold_predicate(self)
+    }
+
     fn super_fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
         let new = self.inner.kind.fold_with(folder);
         folder.tcx().reuse_or_mk_predicate(self, new)