fixes #9940; genericAssign does not take care of the importC variables in refc [backport] (#23761)

fixes #9940

(cherry picked from commit 828cd58d8a)

compiler/ccgtypes.nim

@@ -1246,7 +1246,7 @@ proc genObjectInfo(m: BModule, typ, origType: PType, name: Rope; info: TLineInfo
   else:
     genTypeInfoAuxBase(m, typ, origType, name, rope("0"), info)
   var tmp = getNimNode(m)
-  if not isImportedType(typ):
+  if (not isImportedType(typ)) or tfCompleteStruct in typ.flags:
     genObjectFields(m, typ, origType, typ.n, tmp, info)
   m.s[cfsTypeInit3].addf("$1.node = &$2;$n", [tiNameForHcr(m, name), tmp])
   var t = typ[0]

tests/ccgbugs/tcgbug.nim

@@ -4,6 +4,7 @@ success
 M1 M2
 ok
 '''
+matrix: "--mm:refc;--mm:orc"
 """
 
 type
@@ -132,3 +133,30 @@ proc foo = # bug #23280
   doAssert L mod 6 == 0 
 
 foo()
+
+block: # bug #9940
+  {.emit:"""/*TYPESECTION*/
+typedef struct { int base; } S;
+""".}
+
+  type S {.importc: "S", completeStruct.} = object
+    base: cint
+  proc init(x:ptr S) =
+    x.base = 1
+
+  type
+    Foo = object
+      a: seq[float]
+      b: seq[float]
+      c: seq[float]
+      d: seq[float]
+      s: S
+
+  proc newT(): Foo =
+    var t: Foo
+    t.s.addr.init
+    doAssert t.s.base == 1
+    t
+
+  var t = newT()
+  doAssert t.s.base == 1