Semantics of scope(exit/success) modifying return value

[dlangBugzillaToGithub]

johanengelen reported this on 2018-04-23T20:22:39Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=18793

CC List

• RazvanN
• Simen Kjaeraas

Description

When the return value is modified inside a scope(exit/success) block, the spec does not say what the semantics are.

A test program that demonstrates that the implementation exhibits different semantics depending on function call ABI (returned large structs are passed by ref):
```
import std.stdio;

struct Big {
    ulong a;
    ulong[4] b;
}

struct Small {
    ulong a;
    ulong b;
}

auto quirk(T)() {
    T result;

    scope(success)
    {
        result.a = 10;
    }
    
    return result;
}


void main() {
    auto small = quirk!Small();
    writeln(small);

    auto big = quirk!Big();
    writeln(big);
}
```


The issue is further complicated if the return expression also modifies the return value; i.e. we also need semantics for this testcase:
```
import std.stdio;

struct Big {
    ulong a;
    ulong[4] b;
}

struct Small {
    ulong a;
    ulong b;
}

auto ref addOne(T)(ref T t)
{
 	t.a += 1;
    return t;
}

auto quirk(T)() {
    T result;

    scope(exit)
    {
        result.a = 10;
    }
    
    return addOne(result);
}


void main() {
    auto small = quirk!Small();
    writeln(small);

    auto big = quirk!Big();
    writeln(big);
}
```

[dlangBugzillaToGithub]

simen.kjaras commented on 2018-04-24T08:52:07Z

Worth noting: On win32, the Small struct used above is not small enough to be passed by value. The intended behavior shows up when Small.sizeof <= 8.