To reproduce:

1. Download attached project and open it in Unity
2. Open "SampleScene" scene
3. In the Project window select "test.shader"
4. In Inspector window on "Compile and show code" button press dropdown button
5. In opened dropdown menu select only OpenGLES20 option
6. Press "Compile and show code" button

Expected results: Compilation succeeds and Shader Code is opened
Actual results: OpenGLES20 fails to generate the Shader and throws Error:

Input signature parameter SV_VertexID (1-based Entry 0) type must be a scalar uint. at line 19 (on gles)

Compiling Vertex program
Platform defines: UNITY_NO_DXT5nm UNITY_NO_RGBM UNITY_ENABLE_REFLECTION_BUFFERS UNITY_FRAMEBUFFER_FETCH_AVAILABLE UNITY_USE_DITHER_MASK_FOR_ALPHABLENDED_SHADOWS UNITY_PBS_USE_BRDF1 UNITY_SPECCUBE_BOX_PROJECTION UNITY_SPECCUBE_BLENDING UNITY_ENABLE_DETAIL_NORMALMAP SHADER_API_MOBILE UNITY_COLORSPACE_GAMMA UNITY_LIGHTMAP_FULL_HDR

Notes:
- This issue appears on both Windows and macOS
- On D3D11, OpenGLES30, Metal, OpenGLCore, and Vulkan everything works as Expected

Reproduced on Unity 2018.3.0a1, 2018.3.7f1, 2019.1.0b5 and 2019.2.0a6
Not reproduced on Unity 2018.2.20f1
Regression on Unity 2018.3.0a1