ICE from self region with generic impl using default method (not cross-crate)

[thestinger]

error: internal compiler error: ty::Region#subst(): Reference to self region when given substs with no self region: substs(self_r=None, self_ty=None, tps=[char])

The code is part of #7334 but it will likely be merged soon.

cc @msullivan

[thestinger]

Nominating for the feature complete milestone.

[msullivan]

I have a pending fix for this.