In the sentence
What’s funny is Cat’s dry humour.
Wouldn’t it be better to put a comma after “funny”:
What’s funny, is Cat’s dry humour.
This question entails a (at the time of writing) heated discussion in the SO chat C++ lounge.
No, it would not be better.
The reason is that it's a single clause, with "What's funny" as the subject of the verb ("is") and "Cat's dry humour" as the complement.
Never separate the subject from its verb with a comma.
A comma isn't necessary there.
"What's funny" is a noun clause. It's similar to, for instance, "what you need."
It's the same thing inverted:
It was standard for much of Modern English history to use a comma to separate a complex subject (“What’s funny…”) from the rest of the sentence (“…is Cat’s dry humour”). This is in line with the purpose of punctuation in general—to indicate prosody. Many speakers actually do pause at that point in a sentence, so a comma seems only natural.
In the passages below, [,] denotes a comma which is present in the original text, but is widely (albeit arbitrarily) considered incorrect in contemporary English.
In short: it’s best avoided unless you’re intentionally writing in the style of the early 1800s.