From 797ba9c7cb177650be4a3bc6ac553e4f414a40c3 Mon Sep 17 00:00:00 2001
From: Chayim Refael Friedman <chayimfr@gmail.com>
Date: Fri, 3 Oct 2025 06:54:32 +0300
Subject: [PATCH] Fix erroneous diagnostic incorrect_generics_len when there
 are generics on enum variant used through type alias

---
 crates/hir-ty/src/infer/path.rs               | 25 +++++++++++++------
 .../src/handlers/incorrect_generics_len.rs    | 17 +++++++++++++
 2 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/crates/hir-ty/src/infer/path.rs b/crates/hir-ty/src/infer/path.rs
index 7517272362..e649e381dd 100644
--- a/crates/hir-ty/src/infer/path.rs
+++ b/crates/hir-ty/src/infer/path.rs
@@ -95,14 +95,23 @@ impl<'db> InferenceContext<'_, 'db> {
             return Some(ValuePathResolution::NonGeneric(ty));
         };
 
-        let substs = self.with_body_ty_lowering(|ctx| {
-            let mut path_ctx = ctx.at_path(path, id);
-            let last_segment = path.segments().len().checked_sub(1);
-            if let Some(last_segment) = last_segment {
-                path_ctx.set_current_segment(last_segment)
-            }
-            path_ctx.substs_from_path(value_def, true, false)
-        });
+        let substs = if self_subst.is_some_and(|it| !it.is_empty())
+            && matches!(value_def, ValueTyDefId::EnumVariantId(_))
+        {
+            // This is something like `TypeAlias::<Args>::EnumVariant`. Do not call `substs_from_path()`,
+            // as it'll try to re-lower the previous segment assuming it refers to the enum, but it refers
+            // to the type alias and they may have different generics.
+            self.types.empty_args
+        } else {
+            self.with_body_ty_lowering(|ctx| {
+                let mut path_ctx = ctx.at_path(path, id);
+                let last_segment = path.segments().len().checked_sub(1);
+                if let Some(last_segment) = last_segment {
+                    path_ctx.set_current_segment(last_segment)
+                }
+                path_ctx.substs_from_path(value_def, true, false)
+            })
+        };
 
         let parent_substs_len = self_subst.map_or(0, |it| it.len());
         let substs = GenericArgs::fill_rest(
diff --git a/crates/ide-diagnostics/src/handlers/incorrect_generics_len.rs b/crates/ide-diagnostics/src/handlers/incorrect_generics_len.rs
index 7402133f74..894e044642 100644
--- a/crates/ide-diagnostics/src/handlers/incorrect_generics_len.rs
+++ b/crates/ide-diagnostics/src/handlers/incorrect_generics_len.rs
@@ -203,6 +203,23 @@ impl WithSignals for Player {
     fn __signals_from_external(&self) -> Self::SignalCollection<'_, Self> {
         Self::SignalCollection { _v: self }
     }
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn enum_type_alias_default_param() {
+        check_diagnostics(
+            r#"
+//- minicore: result
+
+struct Error;
+
+type Result<T, E = Error> = core::result::Result<T, E>;
+
+fn main() {
+    let _ = Result::<()>::Ok(());
 }
         "#,
         );