How to write dual source blend output in Metal

[TellowKrinkle]

The second blend color for dual source blending is accessed in Metal by writing a variable tagged [[color(0), index(1)]].
Attempting the HLSL way of SV_Target1 unsurprisingly results in a [[color(1)]] variable
So I tried using the spir-v syntax:

Texture2D<float4> tex0;
Texture2D<float4> tex1;
SamplerState samp;

struct Output {
	[[vk::location(0), vk::index(0)]] float4 col0;
	[[vk::location(0), vk::index(1)]] float4 col1;
};

[[shader("fragment")]]
Output main(float2 coord : TEXCOORD0) {
	return { tex0.Sample(samp, coord), tex1.Sample(samp, coord) };
}

The docs say "The [vk::location(N)] attributes on stage input/output parameters are respected", but it seems neither is picked up?

#include <metal_stdlib>
#include <metal_math>
#include <metal_texture>
using namespace metal;
struct Output_0
{
    float4 col0_0 [[user(_SLANG_ATTR)]];
    float4 col1_0 [[user(_SLANG_ATTR)]];
};

Output_0 Output_x24init_0(float4 col0_1, float4 col1_1)
{
    thread Output_0 _S1;
    (&_S1)->col0_0 = col0_1;
    (&_S1)->col1_0 = col1_1;
    return _S1;
}

struct pixelInput_0
{
    float2 coord_0 [[user(TEXCOORD)]];
};

struct KernelContext_0
{
    texture2d<float, access::sample> tex0_0;
    sampler samp_0;
    texture2d<float, access::sample> tex1_0;
};

[[fragment]] Output_0 main_0(pixelInput_0 _S2 [[stage_in]], texture2d<float, access::sample> tex0_1 [[texture(0)]], sampler samp_1 [[sampler(0)]], texture2d<float, access::sample> tex1_1 [[texture(1)]])
{
    KernelContext_0 kernelContext_0;
    (&kernelContext_0)->tex0_0 = tex0_1;
    (&kernelContext_0)->samp_0 = samp_1;
    (&kernelContext_0)->tex1_0 = tex1_1;
    return Output_x24init_0((((&kernelContext_0)->tex0_0).sample(((&kernelContext_0)->samp_0), (_S2.coord_0))), ((tex1_1).sample(((&kernelContext_0)->samp_0), (_S2.coord_0))));
}

Both outputs are marked with [[user(_SLANG_ATTR)]], rather than color(0).

[jkwak-work]

I guess this is related to
https://registry.khronos.org/OpenGL/extensions/ARB/ARB_blend_func_extended.txt

I don't think Slang supports it yet.
Please create a new "issue" as a new feature request.

[TellowKrinkle]

Filed as #8003