Why doesn't this enum convert to int?

Question

Why does the following code not compile under g++ (C++14), MSVC (C++14), or ARM (C++03)?

The named Error instance calls the integer constructor, but the anonymous Er

Answer 1

This comes from the same place as "The Most Vexing Parse" - the rule that if it can be a declaration, it is a declaration.
And surprisingly, you're allowed to put parentheses around the identifier in a variable declaration.
(I have no idea why, but I'm guessing that it simplified C's parser back in the day.)

The following are all valid declarations of int variables:

int (foo);
int (bar) = 0;
int (baz)(3);
int (twaddle)(baz);