RSRP-469098: C#7 - Erroneous "access to modified closure", with no mutation, on local vars during local-function recursion

The problem manifests only inside local functions, functions defined inside of functions in C#7+

"Access to modified closure" appears when closing over a local variable binding (input formal parameter local var bindings), inside a local function, when there is a recursive call to itself (in tail position or otherwise, provided it's not provably branched away from). Note - this is in code where no mutation is taking place.

static void Main(string[] args)
{
    void Thing(int val)
    {
        var foo = new Func<int>(() => val); // val gets the 'access to modified closure' warning
        if (val <= 0)
        {
            return;
        }
        Thing(foo() - 1);
    }

    Thing(10);
    Console.WriteLine("Done");
}

If the recursive call is removed, no more warning

    void Thing(int val)
    {
        var foo = new Func<int>(() => val); // no warning here now.
        if (val <= 0)
        {
            return;
        }
        // Thing(foo() - 1);  // Take out the recursion, no more warning on the 'val'
    }

Theoretically, derefencing the call back, alone, doesn't trigger the warning,

Expected: No "access to modified closure" warning on the 'val' variable when there is recursion.

Resharper Ultimate 2017.3.5
Devstudio 2017 v 15.6.2, .Net Framework 4.7.02556 (Using 4.7.1)