Added LDM notes for January 11th, 2021.

meetings/2021/LDM-2021-01-11.md

@@ -0,0 +1,65 @@
+# C# Language Design Meeting for Jan. 11th, 2021
+
+## Agenda
+
+1. [Required properties simple form](#required-properties-simple-form)
+
+## Quote of the Day
+
+- "You didn't want to hear me say um anyway... So, um"
+
+## Discussion
+
+### Required properties simple form
+
+https://github.com/dotnet/csharplang/discussions/4209#discussioncomment-275575
+
+Today, we took a look at an extension to the existing required properties proposal, that proposed a syntax form
+that simplifies the base case of the proposal to remove complexity for what we believe is the 80% use case. This
+form adds a `require` modifier on a property definition. These requirements and then automatically added to the
+implicit parameterless constructor of a type, if present, and can be added to explicit constructors with
+`require default`, in the same place as the `init` lists of the last proposal.
+
+First, we discussed the syntax in the proposal, and potential alternatives. We like the move to put a modifier
+on properties and fields as it makes implicit constructor scenarios much simpler, but something still feels off
+about the full-blown form of this syntax, with `require { Property, List }`. We could draw on type parameter
+constraint clauses, and a rough first attempt looks promising:
+
+```cs
+public Person() require FirstName, LastName
+{
+}
+
+public Student() require ID
+    : base()
+{
+}
+```
+
+The proposed ability to assign to properties in the require list might be either odd or outright impossible here,
+depending on potential syntactic ambiguities we haven't thought of yet, but the syntax immediately feels more
+like C# than the previous curly-brace version.
+
+Where we spent the bulk of meeting, however, was on how implicit we can make the default parameter list. We
+had immediate pushback on `require default` even being necessary on constructors: why can't we just infer that
+for all constructors in a type, and then have a syntax for removing all requirements? There's a feeling that
+`require default` is just busywork, and the compiler should just infer the defaults from the properties and
+fields in the type that are marked `require`. Some proposals for the ability to remove all requirements are
+`require none` and `require default = _`. We also considered a version of the proposal that goes even further,
+that doesn't allow constructors to `require` additional items: you mark a property or field as required, then
+remove requirements in the constructor itself. In this model, constructors would be unable to add new requirements,
+which does remove some potential scenarios, but could simplify the feature significantly. Roughly speaking, the
+three versions of the proposal can be summarized as follows:
+
+1. Only implicit constructors get implicit require lists.
+2. All constructors get implicit require lists, and can add requirements of their own:
+    1. If the constructor calls base in some manner (including implicit calls to the `object` constructor), that
+    list is all the fields and properties in the type that are marked require.
+    2. If the constructor calls another constructor on `this`, then it simply advertises chaining to that
+    constructor, potentially removing some requirements if it takes care of them in its body.
+3. All constructors get implicit require lists, and cannot add to them. They can only remove them, and there
+is no `require` syntax. This version will need a new syntax for removing requirements, but that will likely
+be much simpler than the full `require` clause and need less user education.
+
+After a read of the room, we're interested in seeing where proposal 3 goes. We'll work on fleshing that out
+with examples and bring it back to LDM soon.

meetings/2021/README.md

@@ -4,7 +4,7 @@
 
 ## Schedule when convenient
 
-
+- IEnumerable and Length patterns - (Fred)
 
 ## Recurring topics
 
@@ -32,15 +32,16 @@
 - [#3428](https://github.com/dotnet/csharplang/issues/3428) Global usings for namespaces, types and aliases with command line switch (Mads/Aleksey)
 - [#137](https://github.com/dotnet/csharplang/issues/137) File scoped namespaces (Mads/Cyrus)
 
-## Jan 11, 2021
-
-- IEnumerable and Length patterns - (Fred)
-- Required properties simple form - (Fred)
-
 # C# Language Design Notes for 2021
 
 Overview of meetings and agendas for 2021
 
+## Jan 11, 2021
+
+[C# Language Design Notes for January 11th, 2021](https://github.com/dotnet/csharplang/blob/master/meetings/2021/LDM-2021-01-11.md)
+
+- Required properties simple form
+
 ## Jan 6, 2021
 
 [C# Language Design Notes for January 5th, 2021](https://github.com/dotnet/csharplang/blob/master/meetings/2021/LDM-2021-01-05.md)