They're deprecated and should be replaced by normal functions. I don't think they are used very much at all in the wild, but they look like array-indexing syntax -- except that they can appear in the middle of variable declarations, where they trigger an error on the next line.
These look like:
Spec is:
15.6.4 Statement function
A statement function is a function defined by a single statement.
R1547 stmt-function-stmt is function-name ( [ dummy-arg-name-list ] ) = scalar-expr
They're deprecated and should be replaced by normal functions. I don't think they are used very much at all in the wild, but they look like array-indexing syntax -- except that they can appear in the middle of variable declarations, where they trigger an error on the next line.