context pointer inside structs constness problems

[dlangBugzillaToGithub]

Shachar Shemesh (@Shachar) reported this on 2018-03-06T14:47:23Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=18563

CC List

• Ketmar Dark
• Timoses

Description

The following program does not compile:
void main() {
    struct S {
        uint value;

        ~this() {
        }
    }

    const S a = S(12);
    S b = a;
}

test.d(10): Error: cannot implicitly convert expression a of type const(S) to S 

The reason is that the context pointer stored in a is const, and thus implies that the context it points to is also const. This cannot be copied to the non-const context pointer in b.

This context pointer is not, however, actually treated as const. The following code compiles and passes:

unittest {
    int i = 0;
    struct S {
        int n;
        void fun() const {
            i++;
        }
    }
    const S s;
    assert(i == 0);
    s.fun();
    assert(i == 1);
} 

Full discussion thread at https://forum.dlang.org/thread/p7lp2b$1jod$1@digitalmars.com

I would argue that the correct solution is to allow the assignment.

[dlangBugzillaToGithub]

timosesu commented on 2018-07-19T08:43:00Z

Related: https://forum.dlang.org/post/rxolioihwfrljpfjfmxw@forum.dlang.org

// dmd v2.081.1
unittest {
    struct S {
        uint value;
        ~this() pure {
        }
    }
    S makeS() pure
    {
     	S s = S();
        return s;
    }
    // Error: cannot implicitly convert expression makeS() of type S to immutable(S)
    immutable S s = makeS();
}

Without the destructor compiles fine.

Perhaps a better error message could be found.

Further related issue: https://issues.dlang.org/show_bug.cgi?id=18567