only generate first field for default value of union (#24303)

fixes #20653

(cherry picked from commit 6df050d6d2)

compiler/ccgexprs.nim

@@ -3312,8 +3312,12 @@ proc getNullValueAux(p: BProc; t: PType; obj, constOrNil: PNode,
                      isConst: bool, info: TLineInfo) =
   case obj.kind
   of nkRecList:
+    let isUnion = tfUnion in t.flags
     for it in obj.sons:
       getNullValueAux(p, t, it, constOrNil, result, count, isConst, info)
+      if isUnion:
+        # generate only 1 field for default value of union
+        return
   of nkRecCase:
     getNullValueAux(p, t, obj[0], constOrNil, result, count, isConst, info)
     var res = ""

tests/ccgbugs/tunioninit.nim

@@ -0,0 +1,32 @@
+# issue #20653
+
+type
+  EmptySeq* {.bycopy.} = object
+
+  ChoiceWithEmptySeq_d* {.bycopy.} = object
+    a*: bool
+
+  INNER_C_UNION* {.bycopy, union.} = object
+    a*: char
+    b*: EmptySeq
+    c*: byte
+    d*: ChoiceWithEmptySeq_d
+
+  ChoiceWithEmptySeq_selection* = enum
+    ChoiceWithEmptySeq_NONE, 
+    ChoiceWithEmptySeq_a_PRESENT,
+    ChoiceWithEmptySeq_b_PRESENT,  
+    ChoiceWithEmptySeq_c_PRESENT,
+    ChoiceWithEmptySeq_d_PRESENT
+
+  ChoiceWithEmptySeq* {.bycopy.} = object
+    kind*: ChoiceWithEmptySeq_selection
+    u*: INNER_C_UNION
+
+  Og_Context* {.bycopy.} = object
+    state*: int
+    init_done*: bool
+    ch*: ChoiceWithEmptySeq
+    em*: EmptySeq
+
+var context* : Og_Context = Og_Context(init_done: false)