-O flag ; automatic cast in a bitshift

[Guillaume Lathoud via Digitalmars-d-learn]

Hello,

the code below behaves differently when compiled with or without 
the -O flag (both DMD and LDC2).

Two questions:

(1) does the D language explicitly specifies what the following 
expression should do? If yes, where?

 '<ulong> |= <ubyte> << <ubyte>'

In the example below, there seems to be a cast to 32 bits first, 
then a shift, then a cast to 64 bits.

(2) why the different result when putting the -O flag?

Best regards,
Guillaume Lathoud
.

import std.stdio;

void main()
{
  ubyte b = 84;
  ulong l = 0;
  ubyte shift = 50;

  l |= b << shift;

  writefln( "%064b", l );

  /*
    DMD64 D Compiler v2.080.1

    dmd cast_question.d ; cast_question
    
0000000000000000000000000000000000000001010100000000000000000000

    dmd -O cast_question.d ; cast_question
    
0000000000000000000000000000000000000000000000000000000000000000
  */

  /*
    LDC - the LLVM D compiler (1.10.0):
    based on DMD v2.080.1 and LLVM 6.0.0

    ldmd2 cast_question.d ; cast_question
    
0000000000000000000000000000000000000001010100000000000000000000

    ldmd2 -O cast_question.d ; cast_question
    
0000000000000000000000000000000000000000000000000000000000000000
   */
}