meson html:alias vs. html:custom

When I run

meson compile -C build html

I get this error message:

INFO: autodetecting backend as ninja

ERROR: Can't invoke target `html`: ambiguous name. Add target type 
and/or path:
- ./doc/src/sgml/html:custom
- ./doc/src/sgml/html:alias

If I follow that advice and run meson compile -C build html:alias or 
html:custom, then it works.

The setup in doc/src/sgml/meson.build is

html = custom_target('html', ...)

alias_target('html', html)

If I remove the alias_target, then my original command works.  What is 
the purpose of this alias?

(The equivalent problem exists for "man".)