https://issues.dlang.org/show_bug.cgi?id=13231

          Issue ID: 13231
           Summary: Safe left shift for checkedint
           Product: D
           Version: D2
          Hardware: x86
                OS: Windows
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody@puremagic.com
          Reporter: bearophile_hugs@eml.cc

The purpose of checkedint functions/intrinsics is to detect overflows and bugs.

When you have a number x of type int/uint/long/ulong where one or more of the most n significant bits is set to 1, and you shift x on left by n, you lose some information, so you have an overflow.

So I suggest to add to the checkedint.d module the leftShift functions that perform a checked left shifting. They should also warn against too much large shifts:

leftShift(in uint x, in uint n, ref bool overflow)
leftShift(in ulong x, in uint n, ref bool overflow)

They check that:
1) n is in the appropriate range [0, sizeof(x) * 8]. Otherwise they set
overflow to true.
2) None of the n most significant bits of x is 1 before the shifting.

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