fixes #25346; compare the last sons of tyGenericInst

compiler/types.nim

@@ -1284,7 +1284,20 @@ proc sameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
     withoutShallowFlags:
       for ff, aa in underspecifiedPairs(rhs, lhs, 1, -1):
         if not sameTypeAux(ff, aa, c): return false
-    return true
+    if lhs.base.last.kind == tyOr:
+      # issue #25346
+      # In following code, both types of parameters of instanciated `foo` are `Foo[char]`,
+      # but they are different types.
+      #
+      # type
+      #   Foo[T] = seq[T] or set[T]
+      #
+      # proc foo[T](x: Foo[T]) = discard
+      # foo(@['1', '2'])
+      # foo({'1', '2'})
+      return sameTypeAux(lhs.skipModifier, rhs.skipModifier, c)
+    else:
+      return true
 
   case a.kind
   of tyEmpty, tyChar, tyBool, tyNil, tyPointer, tyString, tyCstring,

tests/generics/t25346.nim

@@ -0,0 +1,16 @@
+# issue #25346
+
+type
+  Foo[T] = seq[T] or set[T]
+
+proc foo[T](x: Foo[T]) = discard
+foo(@['a', 'b'])
+foo({'a', 'b'})
+
+type
+  Bar[T] = seq[T] or ptr UncheckedArray[T]
+
+proc bar[T](x: Bar[T]) = discard
+var x = @[1, 2]
+bar(x)
+bar(cast[ptr UncheckedArray[int]](addr x[0]))