Possibly a unification bug in the type checker

[Walter Bright]

I haven't looked deeply into this yet, but the pattern matching design and 
implementation of IsExpressions was done for D1. D1 did not have const, shared, 
etc. The behavior of the pattern matcher probably remained the same through the 
transition to const, etc.

There's been so much water under the bridge since it would be very risky to 
change its behavior now.