Provide additional debug info in loop decompositions

[nelsonje]

One of our collaborators tried writing code like this:

delegate( v, [g] (Vertex & v) {
  forall(adj(g, v), [] (Vertex & n) {
    doSomethingTo(n);
  }
}

and of course got the always-fun "Trying to context switch in no-switch region" message. The backtrace showed a big mess of templated loop decomposition stuff; it was not clear how to the error back to this line of code.

Can we plumb FILE, LINE, and/or func down the decomposition through the FORALL_OVERLOAD macro to make this sort of debugging easier?

Are there other places we could add additional debugging info in our templated code?

[bholt]

Good question. We're still not gonna escape the messy backtrace -- nesting lambdas is just terrible for naming. But it may be that we can surface the original source code line better in the error message.