• via Digitalmars-d-bugs (59/59) Apr 21 2015 https://issues.dlang.org/show_bug.cgi?id=14478

via Digitalmars-d-bugs <digitalmars-d-bugs puremagic.com> writes:

https://issues.dlang.org/show_bug.cgi?id=14478

          Issue ID: 14478
           Summary: isInputRange failed to recognize some ranges
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: Phobos
          Assignee: nobody puremagic.com
          Reporter: ketmar ketmar.no-ip.org

assume we have this code:

import std.range;
import std.stdio;

struct S {
  int n;
   disable this (this);
}


void main () {
  S[2] s0, s1;
  foreach (ref el; chain(s0[], s1[])) writeln(el.n);
}


if we try to compile it, we got error:
(12): Error: template std.range.chain cannot deduce function from argument
types

yet if we will remove ` disable this (this);`, everything is working fine.

the bug is in `isInputRange` template:

template isInputRange(R)
{
    enum bool isInputRange = is(typeof(
    (inout int = 0)
    {
        R r = R.init;     // can define a range object
        if (r.empty) {}   // can test for empty
        r.popFront();     // can invoke popFront()
        auto h = r.front; // can get the front of the range (HERE!)
    }));
}

line `auto h = r.front;` causes the bug. althru `front` returning ref here, we
can't declare ref variables, so `h` looses `ref` and thus requires copying. yet
we explicitly disabled copying for our struct, so `isInputRange` is failing.

here is the proposed fix:

template isInputRange(R)
{
    enum bool isInputRange = is(typeof(
    (inout int = 0)
    {
        R r = R.init;     // can define a range object
        if (r.empty) {}   // can test for empty
        r.popFront();     // can invoke popFront()
        static void testfront(T) (auto ref T n) {} testfront(r.front);
    }));
}

here we using nested function with `auto ref` to not loose `ref` from `front`.

it would be good to check if some other templates requires such fix too.

p.s. with the proposed fix sample code works like a charm.

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