defn(`a b'): is it standard or extension? Is it reliable?

Hey,

The following evaluates to "Hello, world!":

define(`a b',`Hello, world!')defn(`a b')

Is this standard? Or is it a GNU extension? Can I rely on this
behavior? Sounds useful to fill the macro namespace without worrying.

$ m4 --version | head -1 | xclip -se c
m4 (GNU M4) 1.4.20

Best regards,
Bento