Turn invalid_type_param_default into a lint again

src/librustc/lint/builtin.rs

@@ -130,6 +130,12 @@ declare_lint! {
     "detect private items in public interfaces not caught by the old implementation"
 }
 
+declare_lint! {
+    pub INVALID_TYPE_PARAM_DEFAULT,
+    Deny,
+    "type parameter default erroneously allowed in invalid location"
+}
+
 declare_lint! {
     pub RENAMED_AND_REMOVED_LINTS,
     Warn,
@@ -224,6 +230,7 @@ impl LintPass for HardwiredLints {
             TRIVIAL_CASTS,
             TRIVIAL_NUMERIC_CASTS,
             PRIVATE_IN_PUBLIC,
+            INVALID_TYPE_PARAM_DEFAULT,
             CONST_ERR,
             RENAMED_AND_REMOVED_LINTS,
             RESOLVE_TRAIT_ON_DEFAULTED_UNIT,

src/librustc_lint/lib.rs

@@ -196,6 +196,10 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(SAFE_EXTERN_STATICS),
             reference: "issue #36247 <https://github.com/rust-lang/rust/issues/36247>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(INVALID_TYPE_PARAM_DEFAULT),
+            reference: "issue #36887 <https://github.com/rust-lang/rust/issues/36887>",
+        },
         FutureIncompatibleInfo {
             id: LintId::of(EXTRA_REQUIREMENT_IN_IMPL),
             reference: "issue #37166 <https://github.com/rust-lang/rust/issues/37166>",
@@ -251,8 +255,6 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
         "converted into hard error, see https://github.com/rust-lang/rust/issues/33685");
     store.register_removed("inaccessible_extern_crate",
         "converted into hard error, see https://github.com/rust-lang/rust/issues/36886");
-    store.register_removed("invalid_type_param_default",
-        "converted into hard error, see https://github.com/rust-lang/rust/issues/36887");
     store.register_removed("super_or_self_in_global_path",
         "converted into hard error, see https://github.com/rust-lang/rust/issues/36888");
     store.register_removed("overlapping_inherent_impls",

src/librustc_typeck/collect.rs

@@ -54,6 +54,7 @@ There are some shortcomings in this design:
 */
 
 use astconv::{AstConv, Bounds};
+use lint;
 use constrained_type_params as ctp;
 use middle::lang_items::SizedTraitLangItem;
 use middle::const_val::ConstVal;
@@ -896,8 +897,12 @@ fn generics_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
         if !allow_defaults && p.default.is_some() {
             if !tcx.sess.features.borrow().default_type_parameter_fallback {
-                tcx.sess.span_err(p.span, "defaults for type parameters are only allowed in \
-                                           `struct`, `enum`, `type`, or `trait` definitions.");
+                tcx.sess.add_lint(
+                    lint::builtin::INVALID_TYPE_PARAM_DEFAULT,
+                    p.id,
+                    p.span,
+                    format!("defaults for type parameters are only allowed in `struct`, \
+                             `enum`, `type`, or `trait` definitions."));
             }
         }
 

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

@@ -10,11 +10,16 @@
 
 // gate-test-default_type_parameter_fallback
 
+#![deny(invalid_type_param_default)]
+#![allow(unused)]
+
 fn avg<T=i32>(_: T) {}
 //~^ ERROR defaults for type parameters are only allowed
+//~| WARN this was previously accepted
 
 struct S<T>(T);
 impl<T=i32> S<T> {}
 //~^ ERROR defaults for type parameters are only allowed
+//~| WARN this was previously accepted
 
 fn main() {}