Avoid extra error for type mismatches in patterns

When a type error has already occurred, don't call ty::subst,
which may ICE due to the mismatch in the number of type params
involved.

I'm deeming this too small to review.

Closes #3680

Author: catamorphism

src/librustc/middle/typeck/check/alt.rs

@@ -95,7 +95,16 @@ fn check_pat_variant(pcx: pat_ctxt, pat: @ast::pat, path: @ast::path,
                 let vinfo =
                     ty::enum_variant_with_id(
                         tcx, v_def_ids.enm, v_def_ids.var);
-                vinfo.args.map(|t| { ty::subst(tcx, expected_substs, *t) })
+                let var_tpt = ty::lookup_item_type(tcx, v_def_ids.var);
+                vinfo.args.map(|t| {
+                    if var_tpt.bounds.len() == expected_substs.tps.len() {
+                        ty::subst(tcx, expected_substs, *t)
+                    }
+                    else {
+                        *t // In this case, an error was already signaled
+                           // anyway
+                    }
+                })
             };
 
             kind_name = "variant";

src/test/compile-fail/issue-3680.rs

@@ -8,8 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// xfail-test
-fn f() {
+fn main() {
     match None {
             Err(_) => () //~ ERROR expected `core::result
     }