[Issue 24563] New: Make the compiler provide a reason for why it has disabled a function

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24563

          Issue ID: 24563
           Summary: Make the compiler provide a reason for why it has
                    disabled a function
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: issues.dlang at jmdavisProg.com

Currently, whenever the compiler is unable to generate a function that it would
normally generate for a type (e.g. a copy constructor, because it has a member
variable with a copy constructor, or an opAssign, because it has a member
variable with an overloaded opAssign), it seems to just mark it as @disable as
if the user had explicitly written the function prototype and then marked it
with @disable. The result of this is that when you try to use such an operation
on a type, you get an extremely uninformative error message - e.g. from
https://issues.dlang.org/show_bug.cgi?id=24562:

---
q.d(36): Error: generated function `q.main.S2.opAssign` cannot be used because
it is annotated with `@disable`
---

It _is_ fortunately smart enough to say that it's a generated function, but it
says nothing about why opAssign has been disabled. This can make it extremely
difficult to figure out what the actual problem is, and I'm unaware of any way
to get that information short of running the compiler through a debugger.

So, please fix it so that when the compiler says that a generated function was
disabled, it explains _why_ it was disabled, so that we have some hope of
actually figuring out what the problem is and how to fix it. In some cases, it
might be a simple fix on the programmer's part, whereas in others, it could be
a compiler bug. Either way, there's no hint as to what the problem could be.
The compiler has basically just said that it failed without giving an actual
reason.

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