MSL: using the variable identifier as an argument to refract built-ins fails on macOS 10.14

[jiradeto]

On MacOS 10.14, I tried to convert SPIR-V to MSL and found an issue that when using a float variable identifier as the argument of a refract function, the MSL compiler gives the following error:

result.msl:15:15: error: no matching function for call to 'refract'
    float b = refract(1.0, 1.0, a);
              ^~~~~~~

The GLSL fragment shader:

void main()
{
    float a = 1.0;
    float b = refract(1.0, 1.0, a);
    _GLF_color = vec4(10.0);
}

The MSL shader obtained by SPIRV-Cross:

fragment main0_out main0()
{
    main0_out out = {};
    float a = 1.0;
    float b = refract(1.0, 1.0, a);
    out._GLF_color = float4(10.0);
    return out;
}

Versions:

• SPIRV-Cross: git-41399fc899b9b941bc17810a72d9aa8750bb18e9 (Most recent version of this writing)
• glslangValidator: Glslang Version: 7.12.3226

Steps to reproduce:

• Convert the GLSL shader to SPIR-V using glslangValidator with the command glslangValidator -V sample.frag -o sample.spv
• Convert the SPIR-V to MSL using SPIRV-Cross with the command spirv-cross --msl sample.spv --output result.msl
• Build a Metal library with the command xcrun --sdk macosx10.14 metal -x metal -std=macos-metal1.2 result.msl

Note that before running SPIRV-Cross, I also used spirv-val to check that the SPIR-V produced by glslangValidator is valid.

This Archive.zip contains the original fragment shader, the associated SPIR-V, and MSL shader.

Issue found using GraphicsFuzz.

[cdavis5e]

That is because MSL lacks overloads of refract() for scalars. Most of the functions in <metal_geometric>, like the length() function (as you discovered in #1047), lack scalar overloads.

[HansKristian-Work]

Odd that this is even allowed in SPIR-V, another easy fix I guess ...