[Issue 23987] New: Copy construction should not disable implicit conversion

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23987

          Issue ID: 23987
           Summary: Copy construction should not disable implicit
                    conversion
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: timon.gehr at gmx.ch

DMD 2.104.0:

```d
struct T {
    // no copy constructor
}

struct S {
    this(const ref S _){
        // dummy copy constructor
    }
}

void main(){
    void foo()inout{
        immutable(T) t1;
        auto t2=t1; // ok, can convert from mutable to immutable
        immutable(S) s1;
        auto s2=s1; // error, copy constructor returns mutable, but immutable
is required
    }
}
```

The code should compile, it can just create a mutable object and convert it to
immutable, as in the first case.

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