CS8620 when using compound overloaded operator with nullable value type known as non-null

[drewnoakes]

Version Used: 16.5.0 Preview 2.0 [29614.18.master]

Steps to Reproduce:

#nullable enable

struct S
{
    public static S operator |(S s, int _) => s;
}

class Program
{
    void M()
    {
        S? s = null;

        s ??= new S();

        s.ToString(); // No warning

        s |= 321; // CS8620 
    }
}

Argument of type 'S?' cannot be used for parameter 's' of type 'S' in 'S S.operator |(S s, int _)' due to differences in the nullability of reference types.

Expected Behavior:

No warning.

Actual Behavior:

Warning.

The problem does not exist if you use the longer form:

s = s | 312;

[jnm2]

Is this the same as what I'm hitting (16.8.3 and 16.9.P2)?

#nullable enable

struct S
{
    void M(S? left, S? right)
    {
        // ⚠ CS8620 Argument of type 'S?' cannot be used for parameter 'left' of type 'S' in 'S S.operator +(S left, S
        // right)' due to differences in the nullability of reference types.
        // ⚠ CS8620 Argument of type 'S?' cannot be used for parameter 'right' of type 'S' in 'S S.operator +(S left, S
        /* right)' due to differences in the nullability of reference types.
        ↓↓↓↓    ↓↓↓↓↓ */
        left += right;
        
        // No warnings here
        left = left + right;
    }

    public static S operator +(S left, S right) => throw new System.NotImplementedException();
}