Unexpected copy constructor behavior

[Steven Schveighoffer]

On 7/9/20 6:08 PM, psycha0s wrote:
> import std.stdio;
> 
> struct Foo {
>      int value;
> 
>      this(int n)
>      {
>          value = n;
>          writeln("constuctor ", &this);
>      }
> 
>      ~this()
>      {
>          writeln("destuctor ", &this);
>      }
> 
>      this(ref return scope Foo other)
>      {
>          value = other.value;
>          writeln("copy constuctor ", &this);
>      }
> }
> 
> void main()
> {
>      writeln("begin");
>      auto foo1 = Foo(1);
>      auto foo2 = foo1;
>      writeln("---");
>      foo2 = foo1;
>      writeln("===");
>      writeln("end");
> }

Looking at the generated AST, it's because the compiler is adding an 
auto-generated opAssign, which accepts a Foo by value. It is that object 
that is being created and destroyed. Your objects aren't moving.

Here is what AST looks like for main:

void main()
{
	writeln("begin");
	Foo foo1 = foo1 = 0 , foo1.this(1);
	try
	{
		Foo foo2 = foo2 = 0 , foo2.this(foo1);
		try
		{
			writeln("---");
			foo2.opAssign(((Foo __copytmp434 = __copytmp434 = 0 , 
__copytmp434.this(foo1);) , __copytmp434));
			writeln("===");
			writeln("end");
		}
		finally
			foo2.~this();
	}
	finally
		foo1.~this();
	return 0;
}

-Steve