Could Postgres warn about incorrect volatility class?

Hi all,
I recently discovered that a custom function that I thought was being inlined was not being inlined because I had declared it IMMUTABLE, but the function body cast an enum value to text which is a STABLE operator. Once I corrected my function's definition to declare it STABLE, Postgres inlined it.

Since Postgres can apparently determine during query parsing that my function has a volatility class mismatch, is there a reason that Postgres can't make the same determination and inform me about it when I define the function? In this case a helpful message would have been "I sure hope you know what you're doing..." :-)

Thanks
Philip