Roslyn doesn't update variables' nullability states on user-defined conversions

[TessenR]

Version Used:

Branch master (31 Oct 2020)
Latest commit e48c865 by msftbot[bot]:
Merge pull request #49097 from CyrusNajmabadi/fixAltKey

Fix the chording for inline hints.

Steps to Reproduce:

Compile the following code:

#nullable enable
using System.Diagnostics.CodeAnalysis;

class C
{
  void M1(int? x)
  {
    C c = x; // CS8607
    Method(x); // CS8607
  }

  void M2(int? x)
  {
    Method(x); // CS8607
    C c = x; // fine
  }
  
  void Method([DisallowNull] int? t) { }
  
  public static extern implicit operator C([DisallowNull] int? s);
}

Expected Behavior:
A single CS8607 warning in M1

Actual Behavior:
Two warnings are reported for M1 but only one for M2

Notes
Roslyn currently updates variables' nullability states on passing them to non-nullable parameters but for some reason ignores non-nullable parameters of user-defined conversion operators.

[jaredpar]

@RikkiGibson seems like another possible corollary of #48605

[RikkiGibson]

might be due to #36164.