Inconsistent nullable diagnostics for constructors

[TessenR]

Version Used:

Branch master (27 Oct 2020)
Latest commit 576eb30 by Sam Harwell:
Merge pull request #48934 from sharwell/no-api-telemetry

Disable ApiUsageIncrementalAnalyzerProvider for production scenarios

Steps to Reproduce:

#nullable enable
using System.Diagnostics.CodeAnalysis;

class C<T>
{
  [NotNull] public T Prop { get; set; }
  
  public C(T t) { Prop = t; } // no warnings
  public C(T t, int x) { Prop = t; Prop.ToString(); } // CS8602
  
  [MemberNotNull("Prop")] public void Init(T t) 
  {
    Prop = t;
  } // CS8774
}

Expected Behavior:
CS8618 is reported for the first constructor

Actual Behavior:
No warnings for the first constructor

Notes
Note that if you add a dereference after the assignment e.g. Prop.ToString(); the compiler correctly emits CS8602 for that. So the compiler recognizes that the property might still be nullable contrary to its annotation yet the assignment suppresses the initialization warning in the constructor. I believe this behavior contradicts the following proposal: https://github.com/dotnet/csharplang/blob/master/proposals/nullable-constructor-analysis.md /cc @RikkiGibson

There's a similar problem in the following code (where annotations makes more sense). The compiler understands that the value of Prop might be nullable after the assignment so it emits CS8602 if I dereference the property in both the constructor and Init method but only warns about leaving the instance in invalid state in Init but not in the constructor

#nullable enable
using System.Diagnostics.CodeAnalysis;

class C<T>
{
  [NotNull, DisallowNull] public T Prop { get; set; }
  
  public C(T t) {
    Prop = t; // CS8607
    if ("".Length > 0) { Prop.ToString(); } // CS8602
  } // no warnings
  
  [MemberNotNull("Prop")] public void Init(T t) 
  {
    Prop = t; // CS8607
    if ("".Length > 0) { Prop.ToString(); } // CS8602
  } // CS8774
}

[RikkiGibson]

Thanks, my best guess is we are not folding the flow analysis annotations when determining the required state for the field when exiting the constructor.