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.

compiler/ccgtypes.nim

@@ -247,10 +247,6 @@ proc hasNoInit(t: PType): bool =
 
 proc getTypeDescAux(m: BModule; origTyp: PType, check: var IntSet; kind: TypeDescKind): Rope
 
-proc isObjLackingTypeField(typ: PType): bool {.inline.} =
-  result = (typ.kind == tyObject) and ((tfFinal in typ.flags) and
-      (typ.baseClass == nil) or isPureObject(typ))
-
 proc isInvalidReturnType(conf: ConfigRef; typ: PType, isProc = true): bool =
   # Arrays and sets cannot be returned by a C procedure, because C is
   # such a poor programming language.

compiler/liftdestructors.nim

@@ -1197,7 +1197,7 @@ 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

@@ -1485,8 +1485,17 @@ proc commonSuperclass*(a, b: PType): PType =
     y = y.baseClass
 
 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
 
 proc computeSize*(conf: ConfigRef; typ: PType): BiggestInt =

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()