opDispatch cannot implement range primitives

[dlangBugzillaToGithub]

ryan reported this on 2015-10-30T01:38:28Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=15264

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Description

If you use opDispatch to implement range primitives, isInputRange passes yet it cannot be used in foreach loops:

---
import std.range;

struct R {
  auto vals = iota(0, 5);
  auto opDispatch(string s)() { return mixin("vals."~s); }

  // uncomment to pass:
  //auto front() { return vals.front; }
  //auto popFront() { return vals.popFront; }
  //auto empty() { return vals.empty; }
}

// R is an input range, but cannot be used in foreach:
static assert(isInputRange!R);                               // pass
static assert(__traits(compiles, { foreach(v ; R()) { } })); // fail
---

We either need to allow foreach via opDispatched range primitives or have is*Range return false in this case.

Personally, I would like to see foreach work in this case, though there may be concerns over 'accidental' range implementation:
http://forum.dlang.org/thread/tociqzmmryxyggzypmdk@forum.dlang.org