partially fixes #20787 by having a char dummy member prepended to obj…

…s only containing an UncheckedArray (i.e. C FAM) (#21979)

partial fix for #20787

compiler/ccgtypes.nim

@@ -774,8 +774,15 @@ proc getRecordDesc(m: BModule; typ: PType, name: Rope,
  result = structOrUnion & " " & name
  result.add(getRecordDescAux(m, typ, name, baseType, check, hasField))
  let desc = getRecordFields(m, typ, check)
- if desc == "" and not hasField:
- result.addf("char dummy;$n", [])
+ if not hasField:
+ if desc == "":
+ result.add("\tchar dummy;\n")
+ elif typ.len == 1 and typ.n[0].kind == nkSym:
+ let field = typ.n[0].sym
+ let fieldType = field.typ.skipTypes(abstractInst)
+ if fieldType.kind == tyUncheckedArray:
+ result.add("\tchar dummy;\n")
+ result.add(desc)
  else:
  result.add(desc)
  result.add("};\L")

tests/ccgbugs/t20787.nim

@@ -0,0 +1,4 @@
+type
+ Obj = object
+ f: UncheckedArray[byte]
+let o = new Obj