I recently came across a sentence of a poem from Victor Hugo,

> 
> On a beau tout rêver, tu dépasses le rêve.
> 
> 
> 

([see here](https://fr.wikisource.org/wiki/L%E2%80%99Amour_%28Hugo%29#IV._En_Gr.C3.A8ce_.21), twelveth-to-last line in the penultimate paragraph).

I'd like to understand how this *avoir + infinitif* construction works grammatically.
Are there any restrictions on the kind of adjectives one can use? A French-speaker I asked said he would only use it with "beau".

He translated (e.g.) "On a beau dire,..." as "Even if we say,...". Is this the only interpretation? How would this construction be interpreted, resp. translated (as well as possible) in general?

Also, I'd like to make sure I understand the meaning of the above quote as well as possible, as my French is a bit rusty after a long time without use)...

From what I've understood so far (in the context of a poem about love), I'd translate it (very loosely) as:

> 
> No matter what I dream(ed) about, you surpass my dream(s).
> 
> 
> 

I don't expect this to be a very good try, so I'd welcome any feedback.