fix when statements in inheritable generic objects (#13667) [backport]

compiler/semtypes.nim

@@ -804,7 +804,7 @@ proc addInheritedFieldsAux(c: PContext, check: var IntSet, pos: var int,
         addInheritedFieldsAux(c, check, pos, lastSon(n[i]))
       else: internalError(c.config, n.info, "addInheritedFieldsAux(record case branch)")
   of nkRecList, nkRecWhen, nkElifBranch, nkElse:
-    for i in 0..<n.len:
+    for i in int(n.kind == nkElifBranch)..<n.len:
       addInheritedFieldsAux(c, check, pos, n[i])
   of nkSym:
     incl(check, n.sym.name.id)

tests/objects/tobject3.nim

@@ -82,3 +82,14 @@ proc newArrayAnimationSampler*[T](): ArrayAnimationSampler[T] =
 
 discard newArrayAnimationSampler[Foo6]()
 discard newArrayAnimationSampler[AnotherFoo]()
+
+type
+  DefaultIsNone* = pointer | ptr | ref | proc {.nimcall.} | cstring | cstringArray
+  OptionKind* {.pure.} = enum None, Some
+  OptionA* [T] = object of RootObj
+    when T is DefaultIsNone:
+      value: T
+    else:
+      value: T
+      kind: OptionKind
+  SomeA* [T] = object of OptionA[T]