unexpected warning about unused variables from anonymous union with a template member

[Steve Ward <planet36@xxxxxxxxx>]

See the attached program.

Command:
g++ -Wunused anonymous_union.cpp

Output:
anonymous_union.cpp: In function 'void print_byte_array_2(T) [with T =
unsigned int]':
anonymous_union.cpp:95:   instantiated from here
anonymous_union.cpp:38: warning: unused variable 'y'
anonymous_union.cpp:38: warning: unused variable 'byte_array'

Expected output:
none


The four functions do the same thing.  Two use template parameters,
and the other two use non-template parameters.  Two use anonymous
unions, and the other two use named unions.

Warnings about unused variables are printed for the second function
(with a template parameter and anonymous union) but not for the other
three.

g++ --version
g++ (Ubuntu 4.4.1-4ubuntu8) 4.4.1


Steve

#include <stdint.h>

#include <iostream>

#include <iomanip>



template<typename T>
void print_byte_array_1(T x)
{
	union
	{
		T y;
		uint8_t byte_array[sizeof(T)];
	} u;

	u.y = x;

	for (unsigned int i = 0; i < sizeof(u.byte_array); ++i)
	{
		std::cout << std::hex << std::setfill('0') << std::setw(2) << static_cast<int>(u.byte_array[i]);
	}
	std::cout << '\n';
}



template<typename T>
void print_byte_array_2(T x)
{
	// anonymous union
	union
	{
		T y;
		uint8_t byte_array[sizeof(T)];
	};

	y = x;

	for (unsigned int i = 0; i < sizeof(byte_array); ++i)
	{
		std::cout << std::hex << std::setfill('0') << std::setw(2) << static_cast<int>(byte_array[i]);
	}
	std::cout << '\n';
}



void print_byte_array_3(uint32_t x)
{
	union
	{
		uint32_t y;
		uint8_t byte_array[sizeof(uint32_t)];
	} u;

	u.y = x;

	for (unsigned int i = 0; i < sizeof(u.byte_array); ++i)
	{
		std::cout << std::hex << std::setfill('0') << std::setw(2) << static_cast<int>(u.byte_array[i]);
	}
	std::cout << '\n';
}



void print_byte_array_4(uint32_t x)
{
	// anonymous union
	union
	{
		uint32_t y;
		uint8_t byte_array[sizeof(uint32_t)];
	};

	y = x;

	for (unsigned int i = 0; i < sizeof(byte_array); ++i)
	{
		std::cout << std::hex << std::setfill('0') << std::setw(2) << static_cast<int>(byte_array[i]);
	}
	std::cout << '\n';
}



int main()
{
	const uint32_t x = 0x11223344;

	print_byte_array_1(x);
	print_byte_array_2(x);
	print_byte_array_3(x);
	print_byte_array_4(x);

	return 0;
}

/*

g++ -Wunused anonymous_union.cpp
anonymous_union.cpp: In function 'void print_byte_array_2(T) [with T = unsigned int]':
anonymous_union.cpp:61:   instantiated from here
anonymous_union.cpp:38: warning: unused variable 'y'
anonymous_union.cpp:38: warning: unused variable 'byte_array'

g++ --version
g++ (Ubuntu 4.4.1-4ubuntu8) 4.4.1

*/