From the spec:

Built-In Variables

When decorating with the Builtin decoration, only the following are valid, and these are further restricted as indicated by the table.

Builtin	Allowed types	Allowed execution models for the Input storage class	Allowed execution models for the Output storage class
Position	Vector of 4 32-bit floats		Vertex
VertexIndex	Scalar 32-bit integer	Vertex
InstanceIndex	Scalar 32-bit integer	Vertex
FrontFacing	Boolean	Fragment
FragCoord	Vector of 4 32-bit floats	Fragment
FragDepth	32-bit float		Fragment
NumWorkgroups	Vector of 3 32-bit ints	GLCompute
WorkgroupSize	Vector of 3 32-bit ints	GLCompute
LocalInvocationId	Vector of 3 32-bit ints	GLCompute
GlobalInvocationId	Vector of 3 32-bit ints	GLCompute
LocalInvocationIndex	32-bit int	GLCompute