---

yaml
---
r: 150894
b: refs/heads/try2
c: f829d20
h: refs/heads/master
v: v3

Author: Ryman

[refs]

@@ -5,7 +5,7 @@ refs/heads/snap-stage3: 78a7676898d9f80ab540c6df5d4c9ce35bb50463
 refs/heads/try: 519addf6277dbafccbb4159db4b710c37eaa2ec5
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/ndm: f3868061cd7988080c30d6d5bf352a5a5fe2460b
-refs/heads/try2: 52a53e8ae78d77b3c3735fbaf00e53152402295e
+refs/heads/try2: f829d208a30a4a8880ffb07ce4582e30c8f8d57f
 refs/heads/dist-snap: ba4081a5a8573875fed17545846f6f6902c8ba8d
 refs/tags/release-0.2: c870d2dffb391e14efb05aa27898f1f6333a9596
 refs/tags/release-0.3: b5f0d0f648d9a6153664837026ba1be43d3e2503

branches/try2/src/librustc/middle/typeck/collect.rs

@@ -945,7 +945,24 @@ pub fn ty_generics(ccx: &CrateCtxt,
                 let param_ty = ty::param_ty {idx: base_index + offset,
                                              def_id: local_def(param.id)};
                 let bounds = @compute_bounds(ccx, param_ty, &param.bounds);
-                let default = param.default.map(|x| ast_ty_to_ty(ccx, &ExplicitRscope, x));
+                let default = param.default.map(|path| {
+                    let ty = ast_ty_to_ty(ccx, &ExplicitRscope, path);
+                    let cur_idx = param_ty.idx;
+
+                    ty::walk_ty(ty, |t| {
+                        match ty::get(t).sty {
+                            ty::ty_param(p) => if p.idx > cur_idx {
+                                ccx.tcx.sess.span_err(path.span,
+                                                        "type parameters with a default cannot use \
+                                                        forward declared identifiers")
+                            },
+                            _ => {}
+                        }
+                    });
+
+                    ty
+                });
+
                 let def = ty::TypeParameterDef {
                     ident: param.ident,
                     def_id: local_def(param.id),

branches/try2/src/test/compile-fail/generic-type-params-forward-mention.rs

@@ -12,8 +12,7 @@
 
 // Ensure that we get an error and not an ICE for this problematic case.
 struct Foo<T = Option<U>, U = bool>;
-
+//~^ ERROR type parameters with a default cannot use forward declared identifiers
 fn main() {
     let x: Foo;
-    //~^ ERROR missing type param `U` in the substitution of `std::option::Option<U>`
 }