[Issue 23295] New: [dip1000] explain why scope inference failed

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23295

          Issue ID: 23295
           Summary: [dip1000] explain why scope inference failed
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

This comes up a lot in Phobos when using range functions (e.g. `reduce` or
`format`) that have a lot of overloads and design by introspection complexity,
making the source of scope errors really hard to track down.

Minimized example:
```
@safe:
void main() 
{
    scope int* x;
    foo(x, null);
}

auto foo(int* y, int** w)
{
    fooImpl(y, null);
}

auto fooImpl(int* z, int** w)
{
    // <tons of code>
    auto f = &z;
    // <tons of code>
}
```

The error is:

> Error: scope variable `x` assigned to non-scope parameter `y` calling onlineapp.foo

It doesn't tell you anything more than that, I would like the error to tell me:

> Error: scope variable `x` assigned to non-scope parameter `y`
>        which is assigned to non-scope parameter `z`
>        which is not inferred `scope` because `&z`

--