[Issue 20183] New: Returning address of return ref parameter with temporary struct field

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20183

          Issue ID: 20183
           Summary: Returning address of return ref parameter with
                    temporary struct field
           Product: D
           Version: D2
          Hardware: x86
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: nick at geany.org

With dmd v2.088.0-rc.1 (and earlier):

@safe:

int* addr(return ref int b) {
    return &b;
}

struct S
{
    int i;
}

auto s() {return S();}

void main(){
    //auto p = addr(S().i); // cannot modify constant expression `S(0).i`
    auto p = addr(s().i);
    (*p)++;
}

Here the problem is that p outlives the int passed to addr. I think `return &b`
in addr should not be allowed, because the returned pointer can live longer
than the argument passed to the `ref int b` parameter.

Allowing &b to be returned also makes enhancing D to support @safe smart
containers harder than it needs to be, requiring flow analysis - any ref can be
turned into a scope pointer which lives longer.

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