convert from hard error to future-incompatible lint

src/librustc/lint/builtin.rs

@@ -127,6 +127,12 @@ declare_lint! {
     "detect private items in public interfaces not caught by the old implementation"
 }
 
+declare_lint! {
+    pub INVALID_TYPE_PARAM_DEFAULT,
+    Warn,
+    "type parameter default erroneously allowed in invalid location"
+}
+
 /// Does nothing as a lint pass, but registers some `Lint`s
 /// which are used by other parts of the compiler.
 #[derive(Copy, Clone)]
@@ -152,6 +158,7 @@ impl LintPass for HardwiredLints {
             TRIVIAL_CASTS,
             TRIVIAL_NUMERIC_CASTS,
             PRIVATE_IN_PUBLIC,
+            INVALID_TYPE_PARAM_DEFAULT,
             CONST_ERR
         )
     }

src/librustc_lint/lib.rs

@@ -143,8 +143,8 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                     UNUSED_MUT, UNREACHABLE_CODE, UNUSED_MUST_USE,
                     UNUSED_UNSAFE, PATH_STATEMENTS, UNUSED_ATTRIBUTES);
 
-    add_lint_group!(sess, "future_incompatible",
-                    PRIVATE_IN_PUBLIC);
+    add_lint_group!(sess, FUTURE_INCOMPATIBLE,
+                    PRIVATE_IN_PUBLIC, INVALID_TYPE_PARAM_DEFAULT);
 
     // We have one lint pass defined specially
     store.register_late_pass(sess, false, box lint::GatherNodeLevels);

src/librustc_typeck/collect.rs

@@ -65,6 +65,7 @@ There are some shortcomings in this design:
 */
 
 use astconv::{self, AstConv, ty_of_arg, ast_ty_to_ty, ast_region_to_region};
+use lint;
 use middle::def;
 use middle::def_id::DefId;
 use constrained_type_params as ctp;
@@ -92,7 +93,6 @@ use syntax::abi;
 use syntax::ast;
 use syntax::attr;
 use syntax::codemap::Span;
-use syntax::feature_gate::{GateIssue, emit_feature_err};
 use syntax::parse::token::special_idents;
 use syntax::ptr::P;
 use rustc_front::hir;
@@ -1936,13 +1936,12 @@ fn get_or_create_type_parameter_def<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
 
     if space != TypeSpace && default.is_some() {
         if !tcx.sess.features.borrow().default_type_parameter_fallback {
-            emit_feature_err(&tcx.sess.parse_sess.span_diagnostic,
-                             "default_type_parameter_fallback",
-                             param.span,
-                             GateIssue::Language,
-                             "other than on a `struct` or `enum` definition, \
-                              defaults for type parameters are experimental \
-                              and known to be buggy");
+            tcx.sess.add_lint(
+                lint::builtin::INVALID_TYPE_PARAM_DEFAULT,
+                param.id,
+                param.span,
+                format!("defaults for type parameters are only allowed \
+                         on `struct` or `enum` definitions (see issue #27336)"));
         }
     }
 

src/test/compile-fail/type-parameter-invalid-lint.rs

@@ -0,0 +1,17 @@
+// Copyright 2015 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.
+
+#![deny(future_incompatible)]
+#![allow(dead_code)]
+
+fn avg<T=i32>(_: T) {}
+//~^ ERROR defaults for type parameters are only allowed
+//~| NOTE HARD ERROR
+fn main() {}