New upstream version 1.23.0+dfsg1

[rustc.git] / src / librustc / infer / glb.rs

diff --git a/src/librustc/infer/glb.rs b/src/librustc/infer/glb.rs
index d7afeba7dc96b430531a5697cd7ba019b4e8908f..fd14e0e40e234985629e587307fa0083faa4b86d 100644 (file)
--- a/src/librustc/infer/glb.rs
+++ b/src/librustc/infer/glb.rs
@@ -15,6 +15,7 @@ use super::Subtype;
 
 use traits::ObligationCause;
 use ty::{self, Ty, TyCtxt};
+use ty::error::TypeError;
 use ty::relate::{Relate, RelateResult, TypeRelation};
 
 /// "Greatest lower bound" (common subtype)
@@ -67,14 +68,39 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
                b);
 
         let origin = Subtype(self.fields.trace.clone());
-        Ok(self.fields.infcx.region_vars.glb_regions(origin, a, b))
+        Ok(self.fields.infcx.borrow_region_constraints().glb_regions(self.tcx(), origin, a, b))
     }
 
     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
                   -> RelateResult<'tcx, ty::Binder<T>>
         where T: Relate<'tcx>
     {
-        self.fields.higher_ranked_glb(a, b, self.a_is_expected)
+        debug!("binders(a={:?}, b={:?})", a, b);
+        let was_error = self.infcx().probe(|_snapshot| {
+            // Subtle: use a fresh combine-fields here because we recover
+            // from Err. Doing otherwise could propagate obligations out
+            // through our `self.obligations` field.
+            self.infcx()
+                .combine_fields(self.fields.trace.clone(), self.fields.param_env)
+                .higher_ranked_glb(a, b, self.a_is_expected)
+                .is_err()
+        });
+        debug!("binders: was_error={:?}", was_error);
+
+        // When higher-ranked types are involved, computing the LUB is
+        // very challenging, switch to invariance. This is obviously
+        // overly conservative but works ok in practice.
+        match self.relate_with_variance(ty::Variance::Invariant, a, b) {
+            Ok(_) => Ok(a.clone()),
+            Err(err) => {
+                debug!("binders: error occurred, was_error={:?}", was_error);
+                if !was_error {
+                    Err(TypeError::OldStyleLUB(Box::new(err)))
+                } else {
+                    Err(err)
+                }
+            }
+        }
     }
 }