[Issue 19279] New: mutable does not promote to shared

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19279

          Issue ID: 19279
           Summary: mutable does not promote to shared
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: turkeyman at gmail.com

struct Bob
{
  void setThing() shared;
}

As I understand, `shared` attribution intends to guarantee that I dun
synchronisation internally.
This method is declared shared, so if I have shared instances, I can
call it... because it must handle thread-safety internally.

void f(ref shared Bob a, ref Bob b)
{
  a.setThing(); // I have a shared object, can call shared method

  b.setThing(); // ERROR
}

The method is shared, which suggests that it must handle thread-safety. My
instance `b` is NOT shared, that is, it is thread-local.
A method that handles thread-safety doesn't not work when it's only accessed
from a single thread.

mutable -> shared should work the same as mutable -> const... because surely
that's safe?

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