AVR Freaks

Maybe your compiler doesn't like unnamed unions... I never remember if that's valid or not, I thought it would be ok since you have an unnamed struct inside the union. So maybe this will work:

  typedef union Control_b_t {
    struct   {
      Register attach                     : 1;  // ATTACH
      Register global_NACK                : 1;  // GNACK
      Register remote_wake_up             : 1;  // RWAKEUP
      Register reserved_3                 : 1;  // Reserved
      Register pullup_held_during_reset   : 1;  // PULLRST
      Register reserved_5_7               : 3;  // Reserved
    } bits;
    Register byte;
  } Control_b; 

I think there is some other error in the source file.  Has the type Register been defined?

$ cat zz.c
#include <avr/io.h>

typedef uint8_t Register;

typedef union  {
    struct   {
      Register attach                     : 1;  // ATTACH
      Register global_NACK                : 1;  // GNACK
      Register remote_wake_up             : 1;  // RWAKEUP
      Register reserved_3                 : 1;  // Reserved
      Register pullup_held_during_reset   : 1;  // PULLRST
      Register reserved_5_7               : 3;  // Reserved
    } bits;
    Register byte;
  } Control_b; 

volatile Control_b a, b, c;
$ /usr/bin/avr-gcc -mmcu=atmega328p -c zz.c 
(no errors)

>The use of the _t though seems stupid to me

The _t is telling you its a type that was defined somewhere, and is not native to the language. It also avoids name collision, unless you have a habit of naming non-type things that end in _t. If you are creating types, then _t is a good way to see that it is a type and not some other thing.

>Do you use int_t, char_t and float_t?

A char/int/float is native to the language so no need to redefine it by making it another type. 

>It is an i/o device register.  I believe Atmel does this also. 

there are probably many variations they do, but this is one-

typedef volatile uint8_t register8_t;

then they make the struct members volatile and create a define for access (and they typically do not provide bit access for some reason in avr/arm, but do in the pics)

typedef struct SOME_struct {

    register8_t CTRLA;

    register8_t CTRLB;

} SOME_t;

#define SOME  (*(SOME_t *) 0x0000)

but they could just as well do the following, where the volatile is moved to the access point.

typedef struct SOME_struct {

    uint8_t CTRLA;

    unsigned char CTRLB;

} SOME_t;

#define SOME  (*(volatile SOME_t *) 0x0000)

I would do something like-

https://godbolt.org/z/ME4eEj

where the anonymous structs make access nicer

>I have C++ code I wish I could make work with a C compiler by making some simple changes.

You will get better help if you describe what compiler you are dealing with. Sounds like an old compiler.

The actual error is elsewhere. Believe it or not, we cannot see the rest of your code from here.

And half a dozen posts later, we still can't see enough of the code to see what's wrong.   :-(

Before anonymous structs, we would have done something like:

typedef struct mystruct_ {
    int blah;
    char bar;
} mystruct_t;