Traits do allow TypeParameters with defaults

Core traits such as the arithmetic operations have generic arguments such
as: pub trait Add<Rhs = Self>

Addresses #249

Author: philberty

gcc/rust/hir/rust-ast-lower-item.h

@@ -613,29 +613,6 @@ class ASTLoweringItem : public ASTLoweringBase
     if (trait.has_generics ())
       {
 	generic_params = lower_generic_params (trait.get_generic_params ());
-
-	for (auto &generic_param : generic_params)
-	  {
-	    switch (generic_param->get_kind ())
-	      {
-		case HIR::GenericParam::GenericKind::TYPE: {
-		  const HIR::TypeParam &t
-		    = static_cast<const HIR::TypeParam &> (*generic_param);
-
-		  if (t.has_type ())
-		    {
-		      // see https://github.com/rust-lang/rust/issues/36887
-		      rust_error_at (
-			t.get_locus (),
-			"defaults for type parameters are not allowed here");
-		    }
-		}
-		break;
-
-	      default:
-		break;
-	      }
-	  }
       }
 
     std::vector<std::unique_ptr<HIR::TypeParamBound>> type_param_bounds;