Add notes for Feb. 20th, 2019

meetings/2019/LDM-2019-02-20.md

@@ -0,0 +1,117 @@
+# C# Language Design Notes for Feb 20th, 2019
+
+## Agenda
+
+- Nullable Reference Types: Open LDM Issues https://github.com/dotnet/csharplang/issues/2201
+
+## Discussion
+
+There are variants of this scenario with `string!` and `string~`. The question is whether we should learn in both branches, or whether we should treat one branch as unreachable.
+
+### 'Deliberate' Tests for Nullability
+
+This is a continuation of the discussion from the [last meeting](LDM-2019-02-13.md).
+
+Proposal: divide into "pure" and "not-pure" null checks. The "pure"
+null checks will affect both branches, but the "not-pure" will only
+affect one branch. Precisely: when a "pure" check is used, the
+nullability state is split for both branches, while a non-pure check
+does not have a "maybe-null" state after split.
+
+Proposed deliberate checks:
+
+    1. `x == null`
+
+    2. `x != null`
+
+    3. `(Type)x == null`
+
+    4. `(Type)x != null`
+
+    5. `x is null`
+
+    6. `s is string` (when the type of `s` is `string`)
+
+    7. `s is string s2` (when the type of `s` is `string`)
+
+    8. `s is string _` (when the type of `s` is `string`)
+
+
+Other checks:
+
+    1. `x is string`
+
+    2. `x is string s`
+
+    3. `x is C { Property = 3 }`
+
+    4. `TryGetValue` (`[NotNullWhenTrue]`)
+
+    5. `string.IsNullOrEmpty(s)` (`[NotNullWhenFalse]`)
+
+    6. `x?.ToString() != null`
+
+Follow up question: What about `?.` and related operators (e.g., `??`)?
+
+An example would be
+
+```C#
+void M(string s)
+{
+    if (s?.ToString() == null)
+    {
+        // is `s` maybe null?
+    }
+}
+```
+
+A corresponding rewrite would be:
+
+```C#
+void M(string s)
+{
+    if (s == null || s.ToString() == null)
+    {
+        // s would be maybe-null after this point
+    }
+}
+```
+
+**Conclusion**
+
+Pure:
+
+    1. `x == null`
+
+    2. `x != null`
+
+    3. `(Type)x == null`
+
+    4. `(Type)x != null`
+
+    5. `x is null`
+
+    6. `s is string` (when the type of `s` is `string`)
+
+
+All conditional access (`?.`, `??`) included
+
+Not-Pure
+
+    1. `x is string`
+
+    2. `x is string s`
+
+    3. `x is C { Property = 3 }`
+
+    4. `TryGetValue` (`[NotNullWhenTrue]`)
+
+    5. `string.IsNullOrEmpty(s)` (`[NotNullWhenFalse]`)
+
+    7. `s is string s2` (when the type of `s` is `string`)
+
+    8. `s is string _` (when the type of `s` is `string`)
+
+Switch statement/expression: switch statement treated as
+essentially the equivalent of an `if/else` rewrite. No
+decisions for the switch expression yet.

meetings/2019/README.md

@@ -44,14 +44,16 @@
     - Base calls
     - We currently have open issues around `protected`, `internal`, reabstraction, and `static` fields among others.
 
-## Feb 20, 2019
-
-- Nullable Reference Types: Open LDM Issues https://github.com/dotnet/csharplang/issues/2201
-
 # C# Language Design Notes for 2019
 
 Overview of meetings and agendas for 2019
 
+## Feb 20, 2019
+
+[C# Language Design Notes for Feb 20, 2019](LDM-2019-02-20.md)
+
+- Nullable Reference Types: Open LDM Issues https://github.com/dotnet/csharplang/issues/2201
+
 ## Feb 13, 2019
 
 [C# Language Design Notes for Feb 13, 2019](LDM-2019-02-13.md)