Only consider impliciy unboxed closure impl if the obligation is

actually for `Fn`, `FnMut`, or `FnOnce`.

Fixes #18019

Author: nikomatsakis

src/librustc/middle/traits/select.rs

@@ -608,6 +608,17 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
          * unified during the confirmation step.
          */
 
+        let tcx = self.tcx();
+        let kind = if Some(obligation.trait_ref.def_id) == tcx.lang_items.fn_trait() {
+            ty::FnUnboxedClosureKind
+        } else if Some(obligation.trait_ref.def_id) == tcx.lang_items.fn_mut_trait() {
+            ty::FnMutUnboxedClosureKind
+        } else if Some(obligation.trait_ref.def_id) == tcx.lang_items.fn_once_trait() {
+            ty::FnOnceUnboxedClosureKind
+        } else {
+            return Ok(()); // not a fn trait, ignore
+        };
+
         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
         let closure_def_id = match ty::get(self_ty).sty {
             ty::ty_unboxed_closure(id, _) => id,
@@ -622,37 +633,17 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                self_ty.repr(self.tcx()),
                obligation.repr(self.tcx()));
 
-        let tcx = self.tcx();
-        let fn_traits = [
-            (ty::FnUnboxedClosureKind, tcx.lang_items.fn_trait()),
-            (ty::FnMutUnboxedClosureKind, tcx.lang_items.fn_mut_trait()),
-            (ty::FnOnceUnboxedClosureKind, tcx.lang_items.fn_once_trait()),
-            ];
-        for tuple in fn_traits.iter() {
-            let kind = match tuple {
-                &(kind, Some(ref fn_trait))
-                    if *fn_trait == obligation.trait_ref.def_id =>
-                {
-                    kind
-                }
-                _ => continue,
-            };
-
-            // Check to see whether the argument and return types match.
-            let closure_kind = match self.typer.unboxed_closures().borrow().find(&closure_def_id) {
-                Some(closure) => closure.kind,
-                None => {
-                    self.tcx().sess.span_bug(
-                        obligation.cause.span,
-                        format!("No entry for unboxed closure: {}",
-                                closure_def_id.repr(self.tcx())).as_slice());
-                }
-            };
-
-            if closure_kind != kind {
-                continue;
+        let closure_kind = match self.typer.unboxed_closures().borrow().find(&closure_def_id) {
+            Some(closure) => closure.kind,
+            None => {
+                self.tcx().sess.span_bug(
+                    obligation.cause.span,
+                    format!("No entry for unboxed closure: {}",
+                            closure_def_id.repr(self.tcx())).as_slice());
             }
+        };
 
+        if closure_kind == kind {
             candidates.vec.push(UnboxedClosureCandidate(closure_def_id));
         }
 

src/test/run-pass/multidispatch-infer-from-single-impl.rs

@@ -0,0 +1,32 @@
+// 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.
+
+// Test that we correctly infer that `E` must be `()` here.  This is
+// known because there is just one impl that could apply where
+// `Self=()`.
+
+pub trait FromError<E> {
+    fn from_error(err: E) -> Self;
+}
+
+impl<E> FromError<E> for E {
+    fn from_error(err: E) -> E {
+        err
+    }
+}
+
+fn test() -> Result<(), ()> {
+    Err(FromError::from_error(()))
+}
+
+fn main() {
+    let result = (|| Err(FromError::from_error(())))();
+    let foo: () = result.unwrap_or(());
+}