Error. Please correct it.

On the page: https://lesscss.org/features/#mixins-feature-guarded-namespaces

Invalid code:

#sp_1 when (default()) {
#sp_2 when (default()) {
.mixin() when not(default()) { /* */ }
}
}

Namespaces in LESS cannot be "default" in the same sense as mixins. The default() function in LESS only works with mixins and their parameters, not with namespaces.

Namespaces (#sp_1, #sp_2) are not mixins. They are used to group mixins and variables, but do not participate in the default() logic.
when (default()) only applies to mixins. It checks whether the mixin was called with default parameters, or was not called at all.
There is no call to the mixin in your code. The only mixin .mixin() is inside namespaces, and it is never called.
Namespaces are logical groups, not executable code.

Open the guarded namespaces example at https://lesscss.org/features/#mixins-feature-guarded-namespaces and compare the shown default() usage with the surrounding explanation. Correct the example or its explanatory text so it does not present namespaces as participating in default() logic, then verify the rendered section no longer contains the invalid claim.