Missing "CS8656: Call to non-readonly member from a 'readonly' member results in an implicit copy of 'this'" warning on inner field method calls

[controlflow]

Version Used:

master branch 06/26/2019

Steps to Reproduce:

Compile the following code:

struct A {
    int X;
    public void Incr() => X++;

    public readonly void Readonly() {
        Incr(); // CS8656: Call to non-readonly member 'I.Incr()' from a 'readonly' member results in an implicit copy of 'this'.
    }
}

struct B {
    A _inner;
    
    public readonly void Readonly() {
        _inner/*copy*/.Incr(); // Missing CS8656
    }
    
    public void Mutable() {
        _inner.Incr();
    }
}

sharplab

Expected Behavior:

Compiler warning

CS8656: Call to non-readonly member 'A.Incr()' from a 'readonly' member results in an implicit copy of 'this'.

Actual Behavior:

No warnings, field _inner being copied at runtime to prevent this mutation.

[jmarolf]

related to dotnet/runtime#64654

unfortunately this is by design. @RikkiGibson for more info....

[RikkiGibson]

key bit, which I really should place more prominently in that issue:

Note that we've decided to give a warning out of the box when a non-readonly member is called from a readonly member and the receiver is this. Implicit copy warnings on fields will only be handled by this warning wave, regardless of whether the copy is occurring due to the field being readonly or the containing method being readonly.

[RikkiGibson]

dotnet/runtime#64654