Lines_adjacency layout gl_in[3] value if draw array is 3 vertices long?

What should I expect the value of gl_in[3] to be if the drawing array only has three vertices to draw? Is there a default value chosen by OpenGL?

gl_in is sized by the layout declaration which declares the input primitive type. So if the input primitive type is triangles, gl_in will only have three elements and accessing gl_in[3] is invalid. If a geometry shader is active but the input primitive type doesn’t match the primitives which are actually generated, the drawing command will generate a GL_INVALID_OPERATION error.