Normalize type parameters in create_mono_items_for_default_impls.

src/librustc_mir/monomorphize/collector.rs

@@ -1166,8 +1166,13 @@ fn create_mono_items_for_default_impls<'tcx>(
                         }
                     });
 
+                    let param_env = ty::ParamEnv::reveal_all();
+                    let substs = tcx.normalize_erasing_regions(
+                        param_env,
+                        substs,
+                    );
                     let instance = ty::Instance::resolve(tcx,
-                                                         ty::ParamEnv::reveal_all(),
+                                                         param_env,
                                                          method.def_id,
                                                          substs).unwrap();
 

src/test/run-pass/issue-58375-monomorphize-default-impls.rs

@@ -0,0 +1,23 @@
+// Make sure that the mono-item collector does not crash when trying to
+// instantiate a default impl for DecodeUtf16<<u8 as A>::Item>
+// See https://github.com/rust-lang/rust/issues/58375
+// compile-flags:-C link-dead-code
+
+#![crate_type = "rlib"]
+
+pub struct DecodeUtf16<I>(I);
+
+pub trait Arbitrary {
+    fn arbitrary() {}
+}
+
+pub trait A {
+    type Item;
+}
+
+impl A for u8 {
+    type Item = char;
+}
+
+impl Arbitrary for DecodeUtf16<<u8 as A>::Item>  {
+}