Compute: When can gl_GlobalInvocationID + gl_LocalInvocationIndex be used?

gl_GlobalInvocationID and gl_LocalInvocationIndex are both derived from, among others, gl_WorkGroupSize. There is language that says that

> It is a compile-time error to use _gl_WorkGroupSize_ in a shader that does not declare a fixed local group size, or before that shader has declared a fixed local group size, using **local_size_x**, **local_size_y**, and **local_size_z**.

So ... is it an error to use the derived values before a layout(local_size_*) is declared in that shader? The corollary to all this, is there some variant of

```
[shader 1]
#extension GL_ARB_compute_variable_group_size: enable;
layout(local_size_variable) in;

[shader 2]
void main() { use(gl_GlobalInvocationID); }
```
that is allowed when these are linked together into a single program?