rustc_typeck: unify expected return types with formal return types to propagate coercions through calls of generic functions.

src/test/run-pass/coerce-expect-unsized.rs

@@ -30,4 +30,7 @@ pub fn main() {
     let _: &Fn(int) -> _ = &{ |x| (x as u8) };
     let _: &Show = &if true { false } else { true };
     let _: &Show = &match true { true => 'a', false => 'b' };
+
+    let _: Box<[int]> = Box::new([1, 2, 3]);
+    let _: Box<Fn(int) -> _> = Box::new(|x| (x as u8));
 }

src/test/run-pass/coerce-unify-return.rs

@@ -0,0 +1,26 @@
+// 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.
+
+// Check that coercions unify the expected return type of a polymorphic
+// function call, instead of leaving the type variables as they were.
+
+struct Foo;
+impl Foo {
+    fn foo<T>(self, x: T) -> Option<T> { Some(x) }
+}
+
+pub fn main() {
+    let _: Option<fn()> = Some(main);
+    let _: Option<fn()> = Foo.foo(main);
+
+    // The same two cases, with implicit type variables made explicit.
+    let _: Option<fn()> = Some::<_>(main);
+    let _: Option<fn()> = Foo.foo::<_>(main);
+}