How to initialize immutable variables with an expression that throws an exception to catch?

[Steven Schveighoffer]

On 4/3/20 2:56 AM, FeepingCreature wrote:
> Consider the following code:
> 
> struct S { }
> ....
>    S s = void;
>    try
>      s = fun();
>    catch (Exception)
>      return;
>    call(s);
> 
> Now you change S to be immutable.
> 
> How are you supposed to initialize s? You can't assign to it anymore.
> 
> Assume that we explicitly don't want to pull the call into the try body, 
> for instance because we want exceptions from call to not be caught.
> 
> The only way I've found is to make fun() return Algebraic!(S, Exception) 
> but that's kind of ugly and bypasses a basic language feature.
> 
> Maybe D could allow to initialize an immutable variable from the try{} 
> body if the catch{} body is statically known to exit by return/throw?

Do it the old-fashioned way -- use casting ;)

S s_val = void;
try
   s_val = fun();
catch (Exception)
   return;

immutable s = s_val.assumeUnique;
call(s);

It's not as pretty and has a "code-by-assumption" issue. I wish you 
could just lambda-initialize this, but the return statement throws a 
wrench into that.

-Steve