From 497e1e04bb5f83272213cba1a0a44a416b939249 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Fri, 22 May 2026 23:41:21 +0800
Subject: [PATCH] fix: improve generic subtype handling in typeRel function

---
 compiler/sigmatch.nim | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index 994a2a6bc2..0d6b0b9ab2 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -1754,22 +1754,21 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
     considerPreviousT:
       if a == f or a.kind == tyGenericInst and a.skipGenericAlias[0] == f:
         bindingRet isGeneric
-      var depth = -1
-      # Generic constraints like `F: Future` reach sigmatch as a `tyGenericBody`.
-      # Accept descendants of the generic family here as well, not just direct
-      # `Future[...]` instantiations.
-      if isGenericSubtype(c, a, f, depth, f):
-        if depth > 0:
+      let ff = last(f)
+      if ff != nil:
+        result = typeRel(c, ff, a, flags)
+      if result == isNone and a.kind == tyGenericInst:
+        var depth = -1
+        # Generic constraints like `F: Future` can miss in `last(f)` when the
+        # actual type inherits from a concrete generic instantiation.
+        if isGenericSubtype(c, a, f, depth, f) and depth > 0:
           var askip = skippedNone
           let aobj = a.skipToObject(askip)
           if aobj != nil and tfFinal notin aobj.flags:
             # Keep overload ranking consistent with other inheritance-based
             # matches: deeper descendants are slightly worse candidates.
             inc c.inheritancePenalty, depth + int(c.inheritancePenalty < 0)
-        bindingRet(if depth == 0: isGeneric else: isSubtype)
-      let ff = last(f)
-      if ff != nil:
-        result = typeRel(c, ff, a, flags)
+          result = isGeneric
   of tyGenericInvocation:
     var x = a.skipGenericAlias
     if x.kind == tyGenericParam and x.len > 0: