DIP 1016 should use expression lowering, not statement lowering

[Nicholas Wilson]

On Tuesday, 29 January 2019 at 11:52:40 UTC, Andrei Alexandrescu 
wrote:
> While writing this example:
>
> int[] a = cast(int[]) alloc.allocate(100 * int.sizeof);
> if (alloc.reallocate(a, 200 * int.sizeof))
> {
>     assert(a.length == 200);
> }
>
> =====>
>
> int[] a = cast(int[]) alloc.allocate(100 * int.sizeof);
> void[] __temp0 = a;
> if (alloc.reallocate(__temp0, 200 * int.sizeof)
> {
>     assert(a.length == 200);
> }
>
> I noticed a problem - the lowering as informally described in 
> DIP 1016 makes it difficult to figure how function calls 
> present in control statements like if, while, etc. should 
> behave. Where should the temporary go? An expression-based 
> lowering clarifies everything. A statement-based lowering would 
> need to work on a case basis for all statements involving 
> expressions.

On the contrary, an expression lowering cannot inject temporary 
declarations and is impossible.

The correct lowering in the case for `if` & friends follows the 
form of C++ initialiser conditions(?) i.e:

  if (auto val = expr(); val) { ... },

  or the slightly more ugly valid D:

  if ((){return expr(); }()) { ... }

this lambdification will work for just about anything: if, while, 
assert...