rustc: don't trip an assertion for enums with present but uninhabited variants.

src/librustc/ty/layout.rs

@@ -888,6 +888,11 @@ impl<'a, 'tcx> LayoutCx<'tcx, TyCtxt<'a, 'tcx, 'tcx>> {
                     if x < min { min = x; }
                     if x > max { max = x; }
                 }
+                // We might have no inhabited variants, so pretend there's at least one.
+                if (min, max) == (i128::max_value(), i128::min_value()) {
+                    min = 0;
+                    max = 0;
+                }
                 assert!(min <= max, "discriminant range is {}...{}", min, max);
                 let (min_ity, signed) = Integer::repr_discr(tcx, ty, &def.repr, min, max);
 

src/test/run-pass/issue-50731.rs

@@ -0,0 +1,15 @@
+// Copyright 2018 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.
+
+enum Void {}
+fn foo(_: Result<(Void, u32), (Void, String)>) {}
+fn main() {
+    let _: fn(_) = foo;
+}