Implement trait upcasting coercion type-checking.

src/test/ui/feature-gates/feature-gate-trait_upcasting.rs

@@ -9,5 +9,5 @@ impl Bar for () {}
 fn main() {
     let bar: &dyn Bar = &();
     let foo: &dyn Foo = bar;
-    //~^ ERROR trait upcasting is experimental [E0658]
+    //~^ ERROR trait upcasting coercion is experimental [E0658]
 }

src/test/ui/feature-gates/feature-gate-trait_upcasting.stderr

@@ -1,4 +1,4 @@
-error[E0658]: trait upcasting is experimental
+error[E0658]: trait upcasting coercion is experimental
   --> $DIR/feature-gate-trait_upcasting.rs:11:25
    |
 LL |     let foo: &dyn Foo = bar;

src/test/ui/issues/issue-11515.rs

@@ -1,10 +1,10 @@
 #![feature(box_syntax)]
 
 struct Test {
-    func: Box<dyn FnMut() + 'static>
+    func: Box<dyn FnMut() + 'static>,
 }
 
 fn main() {
     let closure: Box<dyn Fn() + 'static> = Box::new(|| ());
-    let test = box Test { func: closure }; //~ ERROR trait upcasting is experimental [E0658]
+    let test = box Test { func: closure }; //~ ERROR trait upcasting coercion is experimental [E0658]
 }

src/test/ui/issues/issue-11515.stderr

@@ -1,4 +1,4 @@
-error[E0658]: trait upcasting is experimental
+error[E0658]: trait upcasting coercion is experimental
   --> $DIR/issue-11515.rs:9:33
    |
 LL |     let test = box Test { func: closure };

src/test/ui/traits/trait-upcasting/issue-11515-upcast-fn_mut-fn.rs

@@ -0,0 +1,13 @@
+// run-pass
+#![feature(box_syntax, trait_upcasting)]
+#![allow(incomplete_features)]
+
+struct Test {
+    func: Box<dyn FnMut() + 'static>,
+}
+
+fn main() {
+    let closure: Box<dyn Fn() + 'static> = Box::new(|| ());
+    let mut test = box Test { func: closure };
+    (test.func)();
+}

src/test/ui/traits/trait-upcasting/multiple-occurence-ambiguousity.rs

@@ -0,0 +1,22 @@
+// check-fail
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Bar<T> {
+    fn bar(&self, _: T) {}
+}
+
+trait Foo : Bar<i32> + Bar<u32> {
+    fn foo(&self, _: ()) {}
+}
+
+struct S;
+
+impl Bar<i32> for S {}
+impl Bar<u32> for S {}
+impl Foo for S {}
+
+fn main() {
+    let s: &dyn Foo = &S;
+    let t: &dyn Bar<_> = s; //~ ERROR mismatched types
+}

src/test/ui/traits/trait-upcasting/multiple-occurence-ambiguousity.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/multiple-occurence-ambiguousity.rs:21:26
+   |
+LL |     let t: &dyn Bar<_> = s;
+   |            -----------   ^ expected trait `Bar`, found trait `Foo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Bar<_>`
+              found reference `&dyn Foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.