fixes #24725; Invalid =sink generated for pure inheritable object (#24726)

fixes #24725

`lacksMTypeField` doesn't take the base types into consideration. And
for ` {.inheritable, pure.}`, it shouldn't generate a `m_type` field.

(cherry picked from commit e449813c61)

compiler/liftdestructors.nim

@@ -1188,7 +1188,7 @@ proc produceSym(g: ModuleGraph; c: PContext; typ: PType; kind: TTypeAttachedOp;
       fillStrOp(a, typ, result.ast[bodyPos], d, src)
     else:
       fillBody(a, typ, result.ast[bodyPos], d, src)
-      if tk == tyObject and a.kind in {attachedAsgn, attachedSink, attachedDeepCopy, attachedDup} and not lacksMTypeField(typ):
+      if tk == tyObject and a.kind in {attachedAsgn, attachedSink, attachedDeepCopy, attachedDup} and not isObjLackingTypeField(typ):
         # bug #19205: Do not forget to also copy the hidden type field:
         genTypeFieldCopy(a, typ, result.ast[bodyPos], d, src)
 

compiler/types.nim

@@ -1425,6 +1425,17 @@ proc matchType*(a: PType, pattern: openArray[tuple[k:TTypeKind, i:int]],
     a = a[i]
   result = a.kind == last
 
+proc lacksMTypeField*(typ: PType): bool {.inline.} =
+  ## Returns true if the type is an object that lacks a m_type field.
+  ## It doesn't check base classes.
+  (typ.sym != nil and sfPure in typ.sym.flags) or tfFinal in typ.flags
+
+proc isObjLackingTypeField*(typ: PType): bool {.inline.} =
+  ## Returns true if the type is an object that lacks a type field.
+  ## Object types that store type headers are not final or pure and
+  ## have inheritable root types, which are not pure, neither.
+  result = (typ.kind == tyObject) and ((tfFinal in typ.flags) and
+      (typ.baseClass == nil) or isPureObject(typ))
 
 include sizealignoffsetimpl
 
@@ -1867,6 +1878,3 @@ proc isCharArrayPtr*(t: PType; allowPointerToChar: bool): bool =
       result = allowPointerToChar
     else:
       discard
-
-proc lacksMTypeField*(typ: PType): bool {.inline.} =
-  (typ.sym != nil and sfPure in typ.sym.flags) or tfFinal in typ.flags

tests/ccgbugs/tcgbug.nim

@@ -161,3 +161,21 @@ typedef struct { int base; } S;
 
   var t = newT()
   doAssert t.s.base == 1
+
+type QObject* {.inheritable, pure.} = object
+  h*: pointer
+
+proc `=destroy`(self: var QObject) =discard
+
+proc `=copy`(dest: var QObject, source: QObject) {.error.}
+
+type QAbstractItemModel* = object of QObject
+
+type VTable = ref object
+  inst:  QAbstractItemModel
+
+proc g() =
+  var x: VTable = VTable()
+  x.inst = QAbstractItemModel()
+
+g()