It should be possible to use consts as external placeholder variables.

[ami.ganguli@…]

The right-and-side of placeholder let-expressions are not declared as const, so common use-cases like:

placeholder<int> _i;
smatch what;
what.let(_i = 1);

or:

placeholder<MyClass *> _p;
smatch what;
what.let(_p = this);

fail to compile. It would be nice to have some way to specify a placeholder for non-mutable data. Perhaps:

placeholder<int const> _i;

(suggested by Eric Niebler)