diff --git a/src/librustc/traits/fulfill.rs b/src/librustc/traits/fulfill.rs
index 1c091d68a2ef1..5ec2e32b6a4c1 100644
--- a/src/librustc/traits/fulfill.rs
+++ b/src/librustc/traits/fulfill.rs
@@ -112,7 +112,7 @@ impl<'a, 'gcx, 'tcx> FulfillmentContext<'tcx> {
                 selcx,
                 register_region_obligations: self.register_region_obligations
             });
-            debug!("select: outcome={:?}", outcome);
+            debug!("select: outcome={:#?}", outcome);
 
             // FIXME: if we kept the original cache key, we could mark projection
             // obligations as complete for the projection cache here.
diff --git a/src/librustc/traits/project.rs b/src/librustc/traits/project.rs
index 1c7942139e97b..0c35e20324c9c 100644
--- a/src/librustc/traits/project.rs
+++ b/src/librustc/traits/project.rs
@@ -196,7 +196,10 @@ pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
                 let span = obligation.cause.span;
                 match infcx.leak_check(false, span, &skol_map, snapshot) {
                     Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, result)),
-                    Err(e) => Err(MismatchedProjectionTypes { err: e }),
+                    Err(e) => {
+                        debug!("poly_project_and_unify_type: leak check encountered error {:?}", e);
+                        Err(MismatchedProjectionTypes { err: e })
+                    }
                 }
             }
             Err(e) => {
@@ -243,7 +246,10 @@ fn project_and_unify_type<'cx, 'gcx, 'tcx>(
             obligations.extend(inferred_obligations);
             Ok(Some(obligations))
         },
-        Err(err) => Err(MismatchedProjectionTypes { err: err }),
+        Err(err) => {
+            debug!("project_and_unify_type: equating types encountered error {:?}", err);
+            Err(MismatchedProjectionTypes { err: err })
+        }
     }
 }
 
diff --git a/src/librustc_mir/shim.rs b/src/librustc_mir/shim.rs
index af60a83a4a2e9..699a5b17435bd 100644
--- a/src/librustc_mir/shim.rs
+++ b/src/librustc_mir/shim.rs
@@ -210,7 +210,7 @@ fn build_drop_shim<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
     if let Some(..) = ty {
         let patch = {
-            let param_env = tcx.param_env(def_id);
+            let param_env = tcx.param_env(def_id).with_reveal_all();
             let mut elaborator = DropShimElaborator {
                 mir: &mir,
                 patch: MirPatch::new(&mir),
diff --git a/src/librustc_mir/transform/elaborate_drops.rs b/src/librustc_mir/transform/elaborate_drops.rs
index 2e8dd623d744d..f63a5ef301a68 100644
--- a/src/librustc_mir/transform/elaborate_drops.rs
+++ b/src/librustc_mir/transform/elaborate_drops.rs
@@ -50,7 +50,7 @@ impl MirPass for ElaborateDrops {
             (hir::BodyOwnerKind::Fn, None) => {},
             _ => return
         }
-        let param_env = tcx.param_env(src.def_id);
+        let param_env = tcx.param_env(src.def_id).with_reveal_all();
         let move_data = MoveData::gather_moves(mir, tcx).unwrap();
         let elaborate_patch = {
             let mir = &*mir;
diff --git a/src/librustc_mir/util/elaborate_drops.rs b/src/librustc_mir/util/elaborate_drops.rs
index 19f33ef5d45a8..e5719407da639 100644
--- a/src/librustc_mir/util/elaborate_drops.rs
+++ b/src/librustc_mir/util/elaborate_drops.rs
@@ -13,6 +13,7 @@ use rustc::hir;
 use rustc::mir::*;
 use rustc::middle::const_val::ConstVal;
 use rustc::middle::lang_items;
+use rustc::traits::Reveal;
 use rustc::ty::{self, Ty, TyCtxt};
 use rustc::ty::subst::{Kind, Substs};
 use rustc::ty::util::IntTypeExt;
@@ -206,6 +207,7 @@ impl<'l, 'b, 'tcx, D> DropCtxt<'l, 'b, 'tcx, D>
             let field = Field::new(i);
             let subpath = self.elaborator.field_subpath(variant_path, field);
 
+            assert_eq!(self.elaborator.param_env().reveal, Reveal::All);
             let field_ty = self.tcx().normalize_erasing_regions(
                 self.elaborator.param_env(),
                 f.ty(self.tcx(), substs),
diff --git a/src/librustc_traits/normalize_erasing_regions.rs b/src/librustc_traits/normalize_erasing_regions.rs
index 1857df5717bba..299433d479dc6 100644
--- a/src/librustc_traits/normalize_erasing_regions.rs
+++ b/src/librustc_traits/normalize_erasing_regions.rs
@@ -17,6 +17,8 @@ crate fn normalize_ty_after_erasing_regions<'tcx>(
     tcx: TyCtxt<'_, 'tcx, 'tcx>,
     goal: ParamEnvAnd<'tcx, Ty<'tcx>>,
 ) -> Ty<'tcx> {
+    debug!("normalize_ty_after_erasing_regions(goal={:#?})", goal);
+
     let ParamEnvAnd { param_env, value } = goal;
     tcx.sess.perf_stats.normalize_ty_after_erasing_regions.fetch_add(1, Ordering::Relaxed);
     tcx.infer_ctxt().enter(|infcx| {
diff --git a/src/test/run-pass/issue-49685.rs b/src/test/run-pass/issue-49685.rs
new file mode 100644
index 0000000000000..1e4e795532371
--- /dev/null
+++ b/src/test/run-pass/issue-49685.rs
@@ -0,0 +1,22 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #49685: drop elaboration was not revealing the
+// value of `impl Trait` returns, leading to an ICE.
+
+fn main() {
+    let _ = Some(())
+        .into_iter()
+        .flat_map(|_| Some(()).into_iter().flat_map(func));
+}
+
+fn func(_: ()) -> impl Iterator<Item = ()> {
+    Some(()).into_iter().flat_map(|_| vec![])
+}