resolve '_ in dyn Trait just like ordinary elision

cc #48468

src/librustc/middle/resolve_lifetime.rs

@@ -19,6 +19,7 @@ use hir::map::Map;
 use hir::def::Def;
 use hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
 use hir::ItemLocalId;
+use hir::LifetimeName;
 use ty::{self, TyCtxt};
 
 use std::cell::Cell;
@@ -569,10 +570,26 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                 for bound in bounds {
                     self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
                 }
-                if lifetime.is_elided() {
-                    self.resolve_object_lifetime_default(lifetime)
-                } else {
-                    self.visit_lifetime(lifetime);
+                match lifetime.name {
+                    LifetimeName::Implicit => {
+                        // If the user does not write *anything*, we
+                        // use the object lifetime defaulting
+                        // rules. So e.g. `Box<dyn Debug>` becomes
+                        // `Box<dyn Debug + 'static>`.
+                        self.resolve_object_lifetime_default(lifetime)
+                    }
+                    LifetimeName::Underscore => {
+                        // If the user writes `'_`, we use the *ordinary* elision
+                        // rules. So the `'_` in e.g. `Box<dyn Debug + '_>` will be
+                        // resolved the same as the `'_` in `&'_ Foo`.
+                        //
+                        // cc #48468
+                        self.resolve_elided_lifetimes(slice::from_ref(lifetime), false)
+                    }
+                    LifetimeName::Static | LifetimeName::Name(_) => {
+                        // If the user wrote an explicit name, use that.
+                        self.visit_lifetime(lifetime);
+                    }
                 }
             }
             hir::TyRptr(ref lifetime_ref, ref mt) => {

src/test/ui/underscore-lifetime/dyn-trait-underscore.rs

@@ -0,0 +1,32 @@
+// Copyright 2015 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.
+
+// Check that the `'_` in `dyn Trait + '_` acts like ordinary elision,
+// and not like an object lifetime default.
+//
+// cc #48468
+
+#![feature(dyn_trait)]
+#![feature(underscore_lifetimes)]
+
+fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
+    //                      ^^^^^^^^^^^^^^^^^^^^^ bound *here* defaults to `'static`
+    Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
+}
+
+fn b<T>(items: &[T]) -> Box<dyn Iterator<Item=&T> + '_> {
+    Box::new(items.iter()) // OK, equivalent to c
+}
+
+fn c<'a, T>(items: &'a [T]) -> Box<dyn Iterator<Item=&'a T> + 'a> {
+    Box::new(items.iter()) // OK, equivalent to b
+}
+
+fn main() { }

src/test/ui/underscore-lifetime/dyn-trait-underscore.stderr

@@ -0,0 +1,27 @@
+error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
+  --> $DIR/dyn-trait-underscore.rs:21:20
+   |
+LL |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
+   |                    ^^^^
+   |
+note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the function body at 19:1...
+  --> $DIR/dyn-trait-underscore.rs:19:1
+   |
+LL | / fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
+LL | |     //                      ^^^^^^^^^^^^^^^^^^^^^ bound *here* defaults to `'static`
+LL | |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
+LL | | }
+   | |_^
+note: ...so that reference does not outlive borrowed content
+  --> $DIR/dyn-trait-underscore.rs:21:14
+   |
+LL |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
+   |              ^^^^^
+   = note: but, the lifetime must be valid for the static lifetime...
+   = note: ...so that the expression is assignable:
+           expected std::boxed::Box<std::iter::Iterator<Item=&T> + 'static>
+              found std::boxed::Box<std::iter::Iterator<Item=&T>>
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0495`.