std.algorithm.remove and principle of least astonishment

[klickverbot]

Hello all,

I decided to have a go at solving some easy programming puzzles with 
D2/Phobos to see how Phobos, especially ranges and std.algorithm, work 
out in simple real-world use cases (the puzzle in question is from 
hacker.org, by the way).

The following code is a direct translation of a simple problem 
description to D (it is horrible from performance point of view, but 
that's certainly no issue here).

---
import std.algorithm;
import std.conv;
import std.stdio;

// The original input string is longer, but irrelevant to this post.
enum INPUT = "93752xxx746x27x1754xx90x93xxxxx238x44x75xx087509";

void main() {
    uint sum;

    auto tmp = INPUT.dup;
    size_t i;
    while ( i < tmp.length ) {
       char c = tmp[ i ];
       if ( c == 'x' ) {
          tmp = remove( tmp, i );
          i -= 2;
       } else {
          sum += to!uint( [ c ] );
          ++i;
       }
    }

    writeln( sum );
}
---

Quite contrary to what you would expect, the call to »remove« fails to 
compile with the following error messages: »std/algorithm.d(4287): 
Error: front(src) is not an lvalue« and »std/algorithm.d(4287): Error: 
front(tgt) is not an lvalue«.

I am intentionally posting this to this NG and not to d.…D.learn, since 
this is a quite gross violation of the principle of least surprise in my 
eyes.

If this isn't a bug, a better error message via a template constraint or 
a static assert would be something worth looking at in my opinion, since 
one would probably expect this to compile and not to fail within Phobos 
code.

David