Question on gl_VertexIndex and @builtin(vertex_index)

[Trap101]

I tried to write the equivalent version of the tutorial's shader which draws without a buffer in GLSL. as mentioned here
And I encountered the problem where gl_VertexIndex just don't seem to work at all compared to @builtin(vertex_index)

here is my simplest proof of concept I tried to recreate, it's the vertex shader and the pipeline is exactly the same as the tutorial (except for the part where I used GLSL) with 3 vertices or (0..3) range pretty much directly copied from here and the tutorial above
I know my pipeline should be fine given that when I pass in Vertex and Index buffers, it works just fine, is there anything I am doing terribly wrong then?

#version 450

layout(location = 0) out vec3 frag_color;
void main(){
    vec2 pos[3] = vec2[3]( vec2(-0.7, 0.7),
                           vec2(0.7, 0.7),
                           vec2(0.0, -0.7) );
    gl_Position = vec4( pos[uint(gl_VertexIndex)], 0.0, 1.0 );
    frag_color = vec3(1.0,1.0,0.0);
}