The problem is that, for reasons unknown to me, it's valid to wrap parameter names into parenthesis in prototypes. So
Foo f(int(x));
can be interpreted as
Foo f(int x);
that is considered as
Foo f(int);
The real issue is however that C++ authors, also for reasons unknown to me, decided that it was cool to have two different syntax forms for the almost very same semantic (instance initialization).
This introduces an syntax ambiguity that is "resolved" by saying that "if something can be both a declaration and a definition, then it's a declaration", triggering the trap.
Because of that a C++ parser therefore must be able to parse an arbitrarily large number of tokens before being able to decide what is the semantic meaning of the very first of them.
This apparently wouldn't have been too much of an issue except for compiler writers, but however it means that also who reads C++ code to understand it must be able to do the same, and for we humans this is harder. From that the "most vexing".
