Variable "tangent" domain conflicts with semantics "TANGENT"

Graphics Shaders & Materials
1

Here is the barebones shader I started with:

Shader "OMD/_Good"
{
	Properties 
	{
		_MainTexture("Main map", 2D) = "gray" {}	
	}
	
	SubShader 
	{
		Pass
		{
			Tags
			{
				"Queue"="Transparent"
				"IgnoreProjector"="True"
				"RenderType"="Transparent"
			}

			Blend SrcAlpha One
			AlphaTest Greater .01
			ColorMask RGB
			Cull Off
			Lighting Off
			ZWrite Off
			
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma fragmentoption ARB_precision_hint_fastest
			#pragma target 3.0
			#pragma multi_compile_particles
			#include "UnityCG.cginc"

			sampler2D _MainTexture;
			float4 _MainTexture_ST;

			struct vertexToFragment
			{
				float4 vertex : POSITION;
				float4 color : COLOR;
				float2 texcoord : TEXCOORD0;
			};

			vertexToFragment vert (vertexToFragment v)
			{
				vertexToFragment o;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				o.color = v.color;
				o.texcoord = TRANSFORM_TEX(v.texcoord,_MainTexture);
				return o;
			}

			fixed4 frag (vertexToFragment i) : COLOR
			{
				return float4(1.0,1.0,1.0,1.0);
			}
			ENDCG
		}
	}
	SubShader
	{
		Pass
		{
		}
	}
}

Then I realized that I’ll need normals which are apparently available in
that “appdata_full” thing. After looking at a few examples I tried to incorporate
it into the shader above:

			struct vertexToFragment
			{
				float3 worldNormal;
				float4 vertexColor;
				float3 worldPos;
			};

			void vert (inout appdata_full a, out vertexToFragment v)
			{
				v.worldNormal = mul( _Object2World, float4( a.normal, 0.0 ) ).xyz;
				v.vertexColor = a.color;
				v.worldPos = a.vertex.xyz;
			}

The result is a shader compile error:

What does this mean and how could I fix it?

2

This seems to be the solution:

			struct vertexToFragment
			{
				float3 worldNormal;
				float4 vertexColor : COLOR;
				float4 worldPosition : SV_POSITION;
			};

			vertexToFragment vert (appdata_full a)
			{
				vertexToFragment v;
				v.worldNormal = mul( _Object2World, float4( a.normal, 0.0 ) );
          		v.vertexColor = a.color;
          		v.worldPosition = a.vertex;
				return v;
			}