mutable does not promote to shared

[dlangBugzillaToGithub]

Manu reported this on 2018-10-01T04:13:20Z

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

Description

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?

[dlangBugzillaToGithub]

turkeyman commented on 2018-10-01T05:32:29Z

Conversation: https://github.com/dlang/dmd/pull/8782

Reveals that `scope` is also necessary to guarantee that the promoted reference does not escape.
Promotion is safe so long as no promoted-reference outlives the call where the instance was promoted.

struct Bob
{
  void setThing() shared scope;
}

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

  b.setThing(); // this should work with `scope`
}