From 2ddeabceaf343e59ea24f052cb7df04f1e66547f Mon Sep 17 00:00:00 2001
From: Yuriy Glukhov <yglukhov@users.noreply.github.com>
Date: Fri, 6 Feb 2026 00:54:04 +0100
Subject: [PATCH] Fixes #25340 (#25389)

(cherry picked from commit 296b2789b52079d55f9f472336d122b197fa9f20)
---
 compiler/semtypes.nim    |  2 +-
 compiler/sigmatch.nim    | 50 ++++++++++++++++++++--------------------
 tests/typerel/t25340.nim | 16 +++++++++++++
 3 files changed, 42 insertions(+), 26 deletions(-)
 create mode 100644 tests/typerel/t25340.nim

diff --git a/compiler/semtypes.nim b/compiler/semtypes.nim
index add770e6d4..22d70fa64e 100644
--- a/compiler/semtypes.nim
+++ b/compiler/semtypes.nim
@@ -986,7 +986,7 @@ proc skipGenericInvocation(t: PType): PType {.inline.} =
 proc tryAddInheritedFields(c: PContext, check: var IntSet, pos: var int,
                         obj: PType, n: PNode, isPartial = false, innerObj: PType = nil): bool =
   if ((not isPartial) and (obj.kind notin {tyObject, tyGenericParam} or tfFinal in obj.flags)) or
-    (innerObj != nil and obj.sym.id == innerObj.sym.id):
+    (innerObj != nil and obj.id == innerObj.id):
     localError(c.config, n.info, "Cannot inherit from: '" & $obj & "'")
     result = false
   elif obj.kind == tyObject:
diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index 4ea4fe7c65..e213289bcb 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -1679,7 +1679,6 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
     elif a.kind == tyGenericInst:
       if roota.base == rootf.base:
         let nextFlags = flags + {trNoCovariance}
-        var hasCovariance = false
         # YYYY
         result = isEqual
 
@@ -1691,7 +1690,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
           if res notin {isEqual, isGeneric}:
             if trNoCovariance notin flags and ff.kind == aa.kind:
               let paramFlags = rootf.base[i-1].flags
-              hasCovariance =
+              let hasCovariance =
                 if tfCovariant in paramFlags:
                   if tfWeakCovariant in paramFlags:
                     isCovariantPtr(c, ff, aa)
@@ -1702,35 +1701,36 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
                     typeRel(c, aa, ff, flags) == isSubtype
               if hasCovariance:
                 continue
+            result = isNone
+            break
 
-            return isNone
-        if prev == nil: put(c, f, a)
-      else:
-        let fKind = rootf.last.kind
-        if fKind in {tyAnd, tyOr}:
-          result = typeRel(c, last(f), a, flags)
-          if result != isNone: put(c, f, a)
+        if result != isNone:
+          if prev == nil: put(c, f, a)
           return
 
-        var aAsObject = roota.last
+      let fKind = rootf.last.kind
+      if fKind in {tyAnd, tyOr}:
+        result = typeRel(c, last(f), a, flags)
+        if result != isNone: put(c, f, a)
+        return
 
-        if fKind in {tyRef, tyPtr}:
-          if aAsObject.kind == tyObject:
-            # bug #7600, tyObject cannot be passed
-            # as argument to tyRef/tyPtr
-            return isNone
-          elif aAsObject.kind == fKind:
-            aAsObject = aAsObject.base
+      var aAsObject = roota.last
 
-        if aAsObject.kind == tyObject and trIsOutParam notin flags:
-          let baseType = aAsObject.base
-          if baseType != nil:
-            if tfFinal notin aAsObject.flags:
-              inc c.inheritancePenalty, 1 + int(c.inheritancePenalty < 0)
-            let ret = typeRel(c, f, baseType, flags)
-            return if ret in {isEqual,isGeneric}: isSubtype else: ret
+      if fKind in {tyRef, tyPtr}:
+        if aAsObject.kind == tyObject:
+          # bug #7600, tyObject cannot be passed
+          # as argument to tyRef/tyPtr
+          return isNone
+        elif aAsObject.kind == fKind:
+          aAsObject = aAsObject.base
 
-        result = isNone
+      if aAsObject.kind == tyObject and trIsOutParam notin flags:
+        let baseType = aAsObject.base
+        if baseType != nil:
+          if tfFinal notin aAsObject.flags:
+            inc c.inheritancePenalty, 1 + int(c.inheritancePenalty < 0)
+          let ret = typeRel(c, f, baseType, flags)
+          return if ret in {isEqual,isGeneric}: isSubtype else: ret
     else:
       assert last(origF) != nil
       result = typeRel(c, last(origF), a, flags)
diff --git a/tests/typerel/t25340.nim b/tests/typerel/t25340.nim
new file mode 100644
index 0000000000..2ddcd4cbce
--- /dev/null
+++ b/tests/typerel/t25340.nim
@@ -0,0 +1,16 @@
+
+type
+  Foo[T] = object of T
+
+template inheritanceCheck(a, b: untyped) =
+  doAssert a is b
+  doAssert b isnot a
+
+inheritanceCheck Foo[RootObj], RootObj
+
+inheritanceCheck Foo[Foo[RootObj]], RootObj
+inheritanceCheck Foo[Foo[RootObj]], Foo[RootObj]
+
+inheritanceCheck Foo[Foo[Foo[RootObj]]], RootObj
+inheritanceCheck Foo[Foo[Foo[RootObj]]], Foo[RootObj]
+inheritanceCheck Foo[Foo[Foo[RootObj]]], Foo[Foo[RootObj]]