[Proposal]: A checked arithmetic expression on ushort/ubyte/byte can be lifted or inferred to return ushort/ubyte/byte in a checked context.

[otac0n]

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: Not Started

Summary

Checked expressions can use type inference to avoid compile-time errors.

Motivation

Subtracting from unsigned or small integer types currently requires a cast. Checked expressions already provide guarantees that the type system needs.

ushort x = 3;
x = checked(x - 1);

Currently this fails with a CS0266 error. Allowing such expressions would thus be enabling a previously illegal statement, which is non-breaking from a language-only perspective.

Design Meetings

None yet

[otac0n]

This is similar and particularly frustrating:

ushort x = 3;
x = x switch
{
    > 0 => checked(x - 1),
    _ => x,
};

[colejohnson66]

IIRC, this has to do with how the CIL encodes arithmetic operations (everything is upcast to an int or larger), not overflow support.

[otac0n]

You are basically correct, buy I'm proposing a change in semantics that can add the casts for you.
Lowering x = checked(x - 1); to x = checked((ushort)(x - 1)); in the case of what would today be an error.
It would require a change in semantics for arithmetic operations, but can be made backwards compatible with existing code by only applying in today's error case.

[huoyaoyuan]

checked(ushort.MaxValue + 1) is valid and will return 65536, without throwing. checked(short.MinValue - 1) will also return an int32.

[otac0n]

Yes, it would be checked((ushort)(ushort.MaxValue + 1)). Although you will probably hit a compile-time constant folding error rather than an AritmeticOverflow.

[otac0n]

If, in a checked context, you add a checked() wrapper around an arithmetic expression that would be forced to int, can we instead lift the expression to the lowest inferred type?