RSRP-469111: NUnit TestDelegate that returns a Span/ReadOnlySpan causes "Cannot use 'System.ReadOnlySpan'" error

Using NUnit (3.7.10) in combination with Span<T> / ReadOnlySpan<T>, Resharper is showing an error on "Assert.That(() => SomethingThatReturnsSpan, ....)", while the project builds and the test runs fine.

In the following sample, the first 2 methods have "That" that is underlined in red, and only the third one is ok.

Also, regarding the 2nd one: "TestDelegate" in NUnit is defined as "public delegate void TestDelegate()", and so I don't understand why R# still thinks that that "() => _ = ..." returns a Span while it should return void here?

(VS2017 15.6.4, R# 2018.1 EAP4)

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[Test, SuppressMessage("ReSharper", "ExpressionIsAlwaysNull")]
public void Test_Span()
{
string s = null;

//ERROR: "Cannot use 'System.ReadOnlySpan<char>' as a type argument"
Assert.That(() => s.AsReadOnlySpan(), Throws.Exception);

//ERROR: "Cannot use 'System.ReadOnlySpan<char>' as a type argument"
Assert.That(() => _ = s.AsReadOnlySpan(), Throws.Exception);

// OK
Assert.That(() => { _ = s.AsReadOnlySpan(); }, Throws.Exception);
}
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Output:
1 passed, 0 failed, 0 skipped, took 1,52 seconds (NUnit 3.7.1).
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