Unexpected error for throw null!;

[TessenR]

Version Used:

Branch C# 8.0: Nullable reference type (29 Oct 2018)
https://github.com/dotnet/csharplang/blob/master/proposals/nullable-reference-types.md
Latest commit 2f8fef by AlekseyTs:
Compare Nullable modifiers while comparing type symbols by default. (#30770)

Steps to Reproduce:

Compile the following code

class C
{
    void M1() => throw null;
    void M2() => throw null!;
}

https://sharplab.io/#v2:EYLgZgpghgLgrgJwgZwLQDk4BstWFiAJQkiQDsBjCAFQE8AHFAGhhCmRgB8ABAJgAIAwgFgAUAG8x/af24AWfgFkAjAAoAlPwC8APn4wAFggD2Ad35lsWANxSZ8pbw3a9hk+cs4AhLdEBfIA

Expected Behavior:

No errors or error with clear error message

Actual Behavior:

error CS0155: The type caught or thrown must be derived from System.Exception

[jaredpar]

Calling it out because it took me a second to see but it's the ! on the second throw null! that causes the issue here.

[jcouv]

When we fix this, we should also test that x == null ! does the right thing (null should be recognized, despite the !). There may be other scenarios where null (or default) is treated specially, which we should verify.

For instance: #31297 (escape rules fail to see through !)

[jcouv]

Here's third bug in this family: #31370 (delegate conversion doesn't recognize lambda with !).
Started a thread to discuss whether a general design could solve this.

[jcouv]

FYI @jaredpar I self-assigned this bug and moved to preview2, since this family of problems can manifest as a crash.

[chsienki]

Another from the same family: #31296