[Issue 21583] New: `array` is not `@safe` for a certain combination of `iota` and data types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21583

          Issue ID: 21583
           Summary: `array` is not `@safe` for a certain combination of
                    `iota` and data types
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: industry, safe
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

I'm fairly certain the following should compile:
```
import std.range, std.array, std.algorithm;

@safe:

struct Block { int[] data; }

public struct Height
{
    ///
    public ulong value;

    /// Provides implicit conversion to `ulong`
    public alias value this;
}

void main () @safe
{
    auto x = getBlocksFrom(Height.init);
}

public auto getBlocksFrom (Height start_height) @safe nothrow
{
    auto r = iota(start_height, Height(42));
    auto r2 = r.map!(idx => const(Block).init);
    return r2.array;
}
```

Bug disappear if:
- The mapping is not to a `const(Block)`;
- I pass `ulong` or similar instead of `Height`;
- `Block` does not contain an array... Or if it's a `string`;

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