Add design notes for July 10, 2019

meetings/2019/LDM-2019-07-10.md

@@ -0,0 +1,87 @@
+
+# C# LDM Notes
+
+## Agenda
+
+1. Empty switch statement
+1. `[DoesNotReturn]` attribute
+1. Revisiting the `param!` null-checking feature
+
+## Discussion
+
+### Empty switch expression
+
+Should the empty switch expression (`expr switch { }`) be an error?
+
+**Conclusion**
+
+We can't really find a reason to disallow this.
+
+### DoesNotReturn
+
+The current design for the `DoesNotReturnAttribute` indicates that the method annotated with it
+is unreachable after a call to it for the purpose of flow analysis.
+
+The questions are:
+
+  1. Confirm that it only implies unreachability for nullability
+  2. Confirm the name, which doesn't mention nullability
+
+**Conclusion**
+
+Yes to both questions. If we want to add general-case unreachability later we will do it through
+a different mechanism. That would require altered code generation (to ensure both verifiability
+and that the code after is truly unreachable) and is out of scope for the current feature set.
+
+### `param!`
+
+We previously discussed the `param!` feature, which is meant to insert a `throw`
+if the parameter is null.
+
+We'd like to confirm some of the details.
+
+Illegal placement:
+
+We think there should be diagnostics for the follow constructs:
+
+- Warning for default parameter value being null (e.g. `void M(string p! = null)`)
+- Error on an out parameter
+- Error on methods without bodies
+    - Including the declaration part of a partial
+- Warning if the type of the parameter is a nullable reference type or nullable value type
+- No place to put `!` on the property setter
+
+Semantics:
+
+- Should work with `notnull` constraint
+- Confirmed that the null checks should be the first possible code
+    - Including before field initializers, iterator kick-off, constructor chaining
+- Do we want to use a well-known helper instead of `throw`?
+    - CoreFX uses throw helpers, so it wouldn't be able to use the feature
+
+Syntax:
+
+It was previously mentioned that the syntax could be confusing because `!` has different meanings
+in expressions or parameter names. Do we want to change the syntax?
+
+Here are some syntax options we discussed:
+
+1. `T p!`
+2. `T p!!`
+3. `T! p`
+4. `checked T p`
+6. `nonnull T p`
+
+We can't decide on a syntax right now. There are concerns that some of the
+proposed syntaxes are too verbose and remove the purpose of the feature. There's
+also a possible follow-on proposal: right now the `!` syntax already has meaning
+in an expression context, but if we used a different syntax, like `!!`, we could
+also add a "null-checked expression" which does the same thing for expressions
+that it does for parameters. These ideas are interesting enough that we think
+we should hold the feature for the next C# version and discuss these possibilities
+more.
+
+**Conclusion**
+
+Some revisions to the feature and some open questions. We want the feature in
+some form and are committed to resolving these questions for the next release.

meetings/2019/README.md

@@ -43,12 +43,6 @@
     - Triage championed features https://github.com/dotnet/csharplang/projects/4#column-4649189
     - Triage milestones
 
-## Jul 10, 2019
-
-- Review auto-null-checked parameters
-- Confirm whether `a switch { }` should be a warning or an error (@333fred, @gafter)
-- Confirm other decisions as time allows
-
 ## May 22, 2019
 
 - Confirm whether we want `abstract` modifier to be implied for interface implementations inside derived interfaces unless there is an implementation.
@@ -62,6 +56,14 @@
 
 Overview of meetings and agendas for 2019
 
+## Jul 10, 2019
+
+[C# Language Design Notes for July 10, 2019](LDM-2019-07-10.md)
+
+1. Empty switch statement
+1. `[DoesNotReturn]` attribute
+1. Revisiting the `param!` null-checking feature
+
 ## May 15, 2019
 [C# Language Design Notes for May 15, 2019](LDM-2019-05-15.md)