From bc1bd95e5a63643cfbbe1d84b2560117644bf8f4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Wed, 13 Nov 2019 14:28:15 -0800
Subject: [PATCH 1/2] Do not ICE on recovery from unmet associated type bound
 obligation

---
 src/librustc_typeck/check/mod.rs      | 11 ++++++++---
 src/test/ui/issues/issue-66353.rs     | 15 +++++++++++++++
 src/test/ui/issues/issue-66353.stderr | 18 ++++++++++++++++++
 3 files changed, 41 insertions(+), 3 deletions(-)
 create mode 100644 src/test/ui/issues/issue-66353.rs
 create mode 100644 src/test/ui/issues/issue-66353.stderr

diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 72b5018589cf8..be56157287c57 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -705,9 +705,14 @@ impl Inherited<'a, 'tcx> {
             span_bug!(obligation.cause.span, "escaping bound vars in predicate {:?}",
                       obligation);
         }
-        self.fulfillment_cx
-            .borrow_mut()
-            .register_predicate_obligation(self, obligation);
+        let _ = self.fulfillment_cx
+            .try_borrow_mut()
+            .map_err(|e| self.tcx.sess.delay_span_bug(obligation.cause.span, &format!(
+                "fullfillment context already borrowed: {:?} - {:?}",
+                e,
+                obligation,
+            )))
+            .map(|mut cx| cx.register_predicate_obligation(self, obligation));
     }
 
     fn register_predicates<I>(&self, obligations: I)
diff --git a/src/test/ui/issues/issue-66353.rs b/src/test/ui/issues/issue-66353.rs
new file mode 100644
index 0000000000000..d8abdd5206ef4
--- /dev/null
+++ b/src/test/ui/issues/issue-66353.rs
@@ -0,0 +1,15 @@
+// #66353: ICE when trying to recover from incorrect associated type
+
+trait _Func<T> {
+    fn func(_: Self);
+}
+
+trait _A {
+    type AssocT;
+}
+
+fn main() {
+    _Func::< <() as _A>::AssocT >::func(());
+    //~^ ERROR the trait bound `(): _A` is not satisfied
+    //~| ERROR the trait bound `(): _Func<_>` is not satisfied
+}
diff --git a/src/test/ui/issues/issue-66353.stderr b/src/test/ui/issues/issue-66353.stderr
new file mode 100644
index 0000000000000..8fd50300ca63e
--- /dev/null
+++ b/src/test/ui/issues/issue-66353.stderr
@@ -0,0 +1,18 @@
+error[E0277]: the trait bound `(): _A` is not satisfied
+  --> $DIR/issue-66353.rs:12:14
+   |
+LL |     _Func::< <() as _A>::AssocT >::func(());
+   |              ^^^^^^^^^^^^^^^^^^ the trait `_A` is not implemented for `()`
+
+error[E0277]: the trait bound `(): _Func<_>` is not satisfied
+  --> $DIR/issue-66353.rs:12:41
+   |
+LL |     fn func(_: Self);
+   |     ----------------- required by `_Func::func`
+...
+LL |     _Func::< <() as _A>::AssocT >::func(());
+   |                                         ^^ the trait `_Func<_>` is not implemented for `()`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

From b884205cf451463c9b0bda6e694831c1494e28d6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Thu, 14 Nov 2019 11:34:46 -0800
Subject: [PATCH 2/2] review comments

---
 src/librustc_typeck/check/mod.rs | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index be56157287c57..25cd6a74b6e60 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -705,14 +705,9 @@ impl Inherited<'a, 'tcx> {
             span_bug!(obligation.cause.span, "escaping bound vars in predicate {:?}",
                       obligation);
         }
-        let _ = self.fulfillment_cx
-            .try_borrow_mut()
-            .map_err(|e| self.tcx.sess.delay_span_bug(obligation.cause.span, &format!(
-                "fullfillment context already borrowed: {:?} - {:?}",
-                e,
-                obligation,
-            )))
-            .map(|mut cx| cx.register_predicate_obligation(self, obligation));
+        self.fulfillment_cx
+            .borrow_mut()
+            .register_predicate_obligation(self, obligation);
     }
 
     fn register_predicates<I>(&self, obligations: I)
@@ -3111,7 +3106,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         fallback_has_occurred: bool,
         mutate_fullfillment_errors: impl Fn(&mut Vec<traits::FulfillmentError<'tcx>>),
     ) {
-        if let Err(mut errors) = self.fulfillment_cx.borrow_mut().select_where_possible(self) {
+        let result = self.fulfillment_cx.borrow_mut().select_where_possible(self);
+        if let Err(mut errors) = result {
             mutate_fullfillment_errors(&mut errors);
             self.report_fulfillment_errors(&errors, self.inh.body_id, fallback_has_occurred);
         }