https://issues.dlang.org/show_bug.cgi?id=13006

          Issue ID: 13006
           Summary: Allow inout return type without inout parameters
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: DMD
          Assignee: nobody@puremagic.com
          Reporter: k.hara.pg@gmail.com

Currently unique expressions can be converted to arbitrary type qualifier.

            int * mp = new int(1);
  immutable(int)* ip = new int(1);

But, currently we cannot define a function that returns unique expression without pure attribute.

  int* foo() pure { return new int(1); }
            int * mp = foo();  // OK
  immutable(int)* ip = foo();  // OK

  int* bar() { return new int(1); }
            int * mp = bar();  // OK
  immutable(int)* ip = bar();  // NG!

I think it is possible by the subject proposal.

  inout(int)* bar1() { return new int(1); }
            int * mp = bar1();  // could be possible
  immutable(int)* ip = bar1();  // could be possible

  inout(int)* bar2() {
      inout(int)* p = new int(1);
      // <-- propagate expression uniqueness via inout qualifier
      return p;
  }
            int * mp = bar2();  // could be possible
  immutable(int)* ip = bar2();  // could be possible

In my thought, there's some assumptions as follows.

1. Unique expressions (eg. new int(1)) could be converted to arbitrary type
qualifier, so it should be convertible to inout type.

2. If inout type appears on return type without inout parameter, it would be converted to arbitrary type qualifier without restrictions from function argument types.

My proposal have some pros:

1. Some limitations about inout type qualifier could be lifted.
  For example, isInputRange definition would be simplified.
2. Unique expressions could be more useful in D.
  By using inout, we can propagate it in function body.

and cons:

1. We should have a way to copy inout objects - eg. qualified postblit feature
(DIP49) is necessary.

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