Idempotent modifiers in C#

It’s a bug in the compiler – at least in Roslyn version 1.0.0.50618. From section 8.5.2 of the C# 5 specification:

A local-constant-declaration declares one or more local constants.

local-constant-declaration:
   const type constant-declarators

constant-declarators:
  constant-declarator
  constant-declarators , constant-declarator

constant-declarator:
  identifier = constant-expression

As you can see, that grammar doesn’t allow for const const const bool flag = true;.

I’ve filed a bug against Roslyn so that it can get fixed.