Reproduction of an annoying "feature" of surface shaders

Readme.md

Reproduces a dumb situation with Unity surface shaders where you try to use _MainTex_ST and find it is undefined. Then when you try to define it, you get told that it's a duplicate definition.

This appears to be "intended behaviour".

The attempted workaround, however, doesn't work for me. It compiles fine, but I just get black for the UVs.

The actual, viable workaround is to name the variables so they don't start with uv_ (so that Unity doesn't stomp them for you), and then populate them from a custom vertex shader.

BrokenSurfaceShader.shader

Shader "Custom/BrokenSurfaceShader"
{
    Properties
    {
        _MainTex ("Albedo (RGB)", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 200

        CGPROGRAM
        #pragma surface surf Standard fullforwardshadows
        #pragma target 3.0

        sampler2D _MainTex;
        // This is commented out so you get "undeclared identifier '_MainTex'"
        // Uncomment this and you get "redefinition of '_MainTex'" instead.
        // float4 _MainTex_ST;

        struct Input
        {
            float2 uv_MainTex;
        };

        void surf (Input IN, inout SurfaceOutputStandard o)
        {
            float2 uv = IN.uv_MainTex * _MainTex_ST.xy;
            
            o.Albedo = float3(uv, 0.0);
            o.Metallic = 0.0;
            o.Smoothness = 1.0;
            o.Alpha = 1.0;
        }
        ENDCG
    }
    // FallBack "Diffuse"
}

BrokenSurfaceShaderAttemptedWorkaround.shader

Shader "Custom/BrokenSurfaceShaderAttemptedWorkaround"
{
    Properties
    {
        _MainTex ("Albedo (RGB)", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 200

        CGPROGRAM
        #pragma surface surf Standard fullforwardshadows
        #pragma target 3.0

        sampler2D _MainTex;
        float4 _MainTex_ST;

        struct Input
        {
            float2 uv_MainTexWorkaround;
        };

        void surf (Input IN, inout SurfaceOutputStandard o)
        {
            float2 uv = IN.uv_MainTexWorkaround * _MainTex_ST.xy;
            
            o.Albedo = float3(uv, 0.0);
            o.Metallic = 0.0;
            o.Smoothness = 1.0;
            o.Alpha = 1.0;
        }
        ENDCG
    }
    // FallBack "Diffuse"
}