[Issue 20941] New: __traits(getMember) should not allow safe code to access private fields

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20941

          Issue ID: 20941
           Summary: __traits(getMember) should not allow safe code to
                    access private fields
           Product: D
           Version: D2
          Hardware: x86
                OS: Mac OS X
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: andrei at erdani.com

This is taken from DIP 1035:

struct S {
    private union {
        int x;
        int* y;
    }
    private int tag; // x if 0, y if 1
    // public @trusted interface follows
    void set(int rhs) @trusted {
        if (tag == 0) x = rhs;
        else *y = rhs;
    }
    ....
}

Allowing safe code to modify tag from @safe code, followed by calling set(),
results in unsafe code from safe functions.

This could be relaxed to restrict access only for types that have @trusted
functions.

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