include 'win32ax.inc' ; you can simply switch between win32ax, win32wx, win64ax and win64wx here
include 'macro\if.inc'
include 'macro\masm.inc'

.data

struct MY_STRUCT_1
  p1          db ?
  p2            dd ?
ends

struct MY_STRUCT_2
  p1          db ?
  union
    u1               db ?
    u2          dd ?
  ends
  p2          dd ?
ends

.code

start:

assume       eax: MY_STRUCT_1
assume  eax: MY_STRUCT_2

.end start
    

      match assumed, reg\#@assumed \\{ irp name, assumed \\\{ restore name \\\} \\}
;--- INCLUDE\MACRO\MASM.INC [29] Allow to use unions inside assumed structs
;     macro label . \\{ \\}
;+++
      macro label . \\{
        \\local .name
     match name rest, . \\\{ .name equ name \\\}
 match =.name, .name \\\{ .name equ . \\\}
   if ~ .name eqtype eax
         label .
   end if
      restore .name
      \\}
;===
      struc db [val] \\{ \common def@assumed reg,.,db val \\}