Steps to reproduce:
1) Open the attached project (1065883_repro.zip).
2) In PlayerSettings, set the Graphics API to Metal.

An error is thrown in the Console:
Shader error in 'EdgeLengthTessellationIssue': 'vertexDataFunc': output parameter 'v' not completely initialized at line 28 (on metal)
Compiling Vertex program with UNITY_PASS_SHADOWCASTER SHADOWS_DEPTH INSTANCING_ON
Platform defines: UNITY_ENABLE_REFLECTION_BUFFERS UNITY_USE_DITHER_MASK_FOR_ALPHABLENDED_SHADOWS UNITY_PBS_USE_BRDF1 UNITY_SPECCUBE_BOX_PROJECTION UNITY_SPECCUBE_BLENDING UNITY_ENABLE_DETAIL_NORMALMAP SHADER_API_DESKTOP UNITY_COLORSPACE_GAMMA UNITY_LIGHT_PROBE_PROXY_VOLUME UNITY_LIGHTMAP_RGBM_ENCODING

The shader won't compile with Metal. This problem does not occur with OpenGL.

Reproduced on:
2018.3.0a8, 2017.1.4p1