[HLSL 2021] Overloaded array operator error about constness

[tomjohnstone]

The following snippet illustrates an error I get when trying to overload the array operator. The error output is:

error: no viable overloaded operator[] for type 'const BindlessTexture2D'
note: candidate function not viable: 'this' argument has type 'const BindlessTexture2D', but method is not marked const

Regards,
Tom

Texture2D Textures2D[] : register(t0, space10);

struct BindlessTexture2D
{
    uint Idx;

    float4 operator[](in uint2 pos)
    {
        return Textures2D[Idx][pos];
    }
};

struct MyConstantBuffer
{
    BindlessTexture2D MyTexture2D;
};

ConstantBuffer<MyConstantBuffer> CB : register(b1);

[numthreads(1, 1, 1)]
void CSMain()
{
    float4 value = CB.MyTexture2D[uint2(0, 1)];
}

[tex3d]

It would appear we support a normal named method this way without const, which HLSL does not allow on methods. Seems like a bug we should fix one way or another.

Incidentally, if you try to write to Idx in the method, you'll get another bug:

Internal Compiler error: llvm::cast<X>() argument of incompatible type!

[llvm-beanz]

#5018 should address the initial usability issue of the subscript operator on const variables by the same mechanism HLSL supports other methods on const instances (by ignoring the const-ness, which is gross).

We also have a proposal to add const instance methods to HLSL which we are tracking here:

https://github.com/microsoft/hlsl-specs/blob/main/proposals/0007-const-instance-methods.md

And a PR on that proposal I wrote yesterday extends it to more broadly adopt const-correctness for HLSL data types:
microsoft/hlsl-specs#34