[Issue 19743] New: Foreach loops desugar into code that does not work with

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19743

          Issue ID: 19743
           Summary: Foreach loops desugar into code that does not work
                    with
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: monkeyworks12 at hotmail.com

I have the following code:

Needle* find(Haystack, Needle)(return scope Haystack haystack, scope Needle
needle) @safe
{
    foreach (ref val; haystack)
        if (val == needle)
            return &val; //Error: scope variable __r2 may not be returned

    return null;
}

void main() @safe
{
    int[9] haystack = [0, 10, 5, 6, 2, 3, 9, 4, 5];
    int* foundVal = haystack.find(3);
}

>From my understanding of `return scope` parameters, this should compile.
However, the foreach loop desugars into the following:

{
        int[] __r2 = haystack[];
        ulong __key3 = 0LU;
        for (; __key3 < __r2.length; __key3 += 1LU)
        {
                ref int val = __r2[__key3];
                if (val == needle)
                        return &val;
        }
}

The slicing of the static array obscures the knowledge that `haystack` is
`return scope` from the compiler. Furthermore, as `__r2` is inferred as
`scope`, the compiler thinks that I am escaping a reference to a local `scope`
variable, and does not allow this code to compile.

Unfortunately, due to https://issues.dlang.org/show_bug.cgi?id=19742, writing a
manually-desugared version does not quite work:

for (auto i = 0; i < haystack.length; i++)
    if (haystack[i] == needle)
        return &haystack[i]; //Error: returning &haystack[cast(ulong)i] escapes
a reference to parameter haystack, perhaps annotate with return

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