Inaccurate error message when assigning to ref readonly variable

[svick]

Version Used: SharpLab using Roslyn master from 14 March: 644e1e0

Steps to Reproduce:

Compile the following code:

class C
{
    void M(int[] a)
    {
        ref readonly int i = ref a[0];
        i = 42;
    }
}

Expected Behavior:

Accurate error message, possibly:

error CS8331: Cannot assign to variable 'in int' because it is a readonly variable

Actual Behavior:

error CS0131: The left-hand side of an assignment must be a variable, property or indexer

This error message is not accurate, because i is a variable.

[jcouv]

Tagging @VSadov

[verelpode]

Likewise the problem also occurs when accessing struct members via a ref readonly local variable:

struct STestStruct
{
    public int A;
    public int B;
}

class MyTestRefReturns
{
    STestStruct fTestStructField;

    ref readonly STestStruct GetFieldRef()
    {
        return ref fTestStructField;
    }

    void Test()
    {
        GetFieldRef().A = 123;  // Error CS8332 (makes sense).
        ref readonly STestStruct r = ref GetFieldRef();
        r.A = 123;  // Error CS0131 (does not make sense).
    }
}

Error CS8332 Cannot assign to a member of method 'MyTestRefReturns.GetFieldRef()' because it is a readonly variable
Error CS0131 The left-hand side of an assignment must be a variable, property or indexer

The incorrect error CS0131 occurs when compiling with VS 2019 16.0.3.