2 files changed, 69 insertions, 1 deletions

diff --git a/crates/ra_assists/src/ast_transform.rs b/crates/ra_assists/src/ast_transform.rs
index 9ac65ab39..d81218bc8 100644
--- a/crates/ra_assists/src/ast_transform.rs
+++ b/crates/ra_assists/src/ast_transform.rs
@@ -2,6 +2,7 @@
 use rustc_hash::FxHashMap;
 
 use hir::{PathResolution, SemanticsScope};
+use hir_def::type_ref::TypeRef;
 use ra_ide_db::RootDatabase;
 use ra_syntax::{
     algo::SyntaxRewriter,
@@ -51,7 +52,24 @@ impl<'a> SubstituteTypeParams<'a> {
             .into_iter()
             // this is a trait impl, so we need to skip the first type parameter -- this is a bit hacky
             .skip(1)
-            .zip(substs.into_iter())
+            // The actual list of trait type parameters may be longer than the one
+            // used in the `impl` block due to trailing default type parametrs.
+            // For that case we extend the `substs` with an empty iterator so we
+            // can still hit those trailing values and check if they actually have
+            // a default type. If they do, go for that type from `hir` to `ast` so
+            // the resulting change can be applied correctly.
+            .zip(substs.into_iter().map(Some).chain(std::iter::repeat(None)))
+            .filter_map(|(k, v)| match v {
+                Some(v) => Some((k, v)),
+                None => match k.default(source_scope.db)? {
+                    TypeRef::Path(path) => Some((
+                        k,
+                        ast::make::type_arg(&format!("{}", path.mod_path().as_ident()?))
+                            .type_ref()?,
+                    )),
+                    _ => None,
+                },
+            })
             .collect();
         return SubstituteTypeParams {
             source_scope,
diff --git a/crates/ra_assists/src/handlers/add_missing_impl_members.rs b/crates/ra_assists/src/handlers/add_missing_impl_members.rs
index c1ce87914..22e1156d2 100644
--- a/crates/ra_assists/src/handlers/add_missing_impl_members.rs
+++ b/crates/ra_assists/src/handlers/add_missing_impl_members.rs
@@ -618,4 +618,54 @@ impl Foo for S {
 }"#,
         )
     }
+
+    #[test]
+    fn test_generic_single_default_parameter() {
+        check_assist(
+            add_missing_impl_members,
+            r#"
+trait Foo<T = Self> {
+    fn bar(&self, other: &T);
+}
+
+struct S;
+impl Foo for S { <|> }"#,
+            r#"
+trait Foo<T = Self> {
+    fn bar(&self, other: &T);
+}
+
+struct S;
+impl Foo for S {
+    <|>fn bar(&self, other: &Self) {
+        todo!()
+    }
+}"#,
+        )
+    }
+
+    #[test]
+    fn test_generic_default_parameter_is_second() {
+        check_assist(
+            add_missing_impl_members,
+            r#"
+trait Foo<T1, T2 = Self> {
+    fn bar(&self, this: &T1, that: &T2);
+}
+
+struct S<T>;
+impl Foo<T> for S<T> { <|> }"#,
+            r#"
+trait Foo<T1, T2 = Self> {
+    fn bar(&self, this: &T1, that: &T2);
+}
+
+struct S<T>;
+impl Foo<T> for S<T> {
+    <|>fn bar(&self, this: &T, that: &Self) {
+        todo!()
+    }
+}"#,
+        )
+    }
 }