Add raw notes for 3/21/2018

meetings/2018/LDM-2018-03-21.md

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+# C# Language Design Notes for Mar 21, 2018
+
+***Warning: These are raw notes, and still need to be cleaned up. Read at your own peril!***
+
+## Agenda
+
+We discussed various open questions around nullable reference types
+
+
+# The null-forgiving operator
+
+## Name
+
+Null-acquiescing
+Null-forgiving
+Null-silencing
+Null-quieting
+Darnit
+
+## What does it do?
+
+Changes top-level nullability to not-null, as well as silencing warnings even on nested nullability
+
+``` c#
+void M(string? ns, List<string?>? ln)
+{
+    string s = ns!; // silence and change nullability
+    var s2 = ns!; // silence and produce string
+    ln = null;
+    List<string>? l = ln!; // silence and change
+    var l2 = ln!;
+    // Idea 1
+    l! // change nullability at the top level
+    l<!> // idea: change nullability at the nested level
+    // Idea 2
+    (List<string>?!)ln; // I do a cast that should warn but ! silences it
+    List<string>? l3 = (!)ln; // Shorthand
+}
+```
+
+Should `!` only change top-level nullability, and not suppress nested diagnostics? Or the other way around? Is it a problem that it does both and "suppresses too much".
+
+Since we made affordances for legacy code, the need for passing `null!` for instance is less: unannotated APIs already suppress warnings.
+
+Scenarios for `!` now that legacy APIs aren't a scenario:
+
+1. I'm in the middle of converting my own code; need them at the boundary
+2. The compiler can't figure it out, but I'm right
+
+Great example of the latter:
+
+``` c#
+List<string?> strings;
+var query = strings.Where(s => s != null).Select(s => s.Length); // What?? Why warning?
+```
+Here you can put the `!` in `s!.Length`, but what if you have
+
+``` c#
+void M(IEnumerable<string> source) { ... }
+List<string?> strings;
+M(strings.Where(s => s != null)); // What?? Why warning?
+```
+
+Here you need to suppress on the nested nullability, because you know better.
+
+Currently you can say `!` at the end of the argument to set it right. Should that be a different syntax?
+
+What's the problem with `!` doing both jobs?
+
+``` c#
+void M(string? ns, List<string?>? ln)
+{
+    // I happen to know that if ln is not null, it doesn't contain nulls
+    List<string>? l = ln!; 
+    _ = l.Count; // warning?
+}
+```
+
+Now `l.Length` is not warned on because `!` did "too much". We wanted it to silence warnings, but it also changed the null state.
+
+The simplest fix with current semantics would be to cast to a nullable type:
+
+``` c#
+    List<string>? l = (List<string>?)ln!;  // or
+    var l = (List<string>?)ln!;
+```
+
+Proposals:
+
+1. Change null state and suppress 4
+2. Two syntaxes 5
+3. Only suppress warnings 1
+4. Only change null-state 0
+
+Approachable and easy to use. They are all about telling the compiler to shut up when you know what you're doing.
+
+For 3, the things we would make people do to change the null state would also be a hurdle.
+
+If we separate it, one may be a temporary feature that will go away over time, once people have transitioned.
+
+We're not going to settle this today. The prototype does "1+" which is even more lenient than 1.
+
+
+
+# Special methods
+
+# Hidden diagnostic