Collection Initializer cannot handle Extensions akin to IEnumerable<T>.Add(IEnumerable<T>) ( IDE0028 vs CS0029)

[LukasGelke]

Version Used:
Visual Studio: Version 17.9.2
dotnet: 6.0.419

Steps to Reproduce:

1. have legacy Extension-Code like this

public class Class1
{
  public void Issue()
  {
    int[] numbers = [1, 2,3];
    Dictionary<string, int> dict = new() { numbers.Select(i => new KeyValuePair<string, int>(i.ToString(), i)) };
  }

  public void Issue2()
  {
    Dictionary<string, int> dict = new() { GetPairs() };
  }

  private static ICollection<KeyValuePair<string, int>> GetPairs() => throw new NotImplementedException();
}


public static class MyExtensions2
{
  public static void Add<T, TEnumElement>(this ICollection<T> collection,
    IEnumerable<TEnumElement>? elements)
    where TEnumElement : T
  {
    if (elements is null)
      return;
    ArgumentNullException.ThrowIfNull(collection);
    if (collection is List<T> list)
      list.AddRange((IEnumerable<T>)elements);
    else
      foreach (T element in elements)
        collection.Add(element);
  }
}

2. invoke CodeFix
   

3. get compiler error
   

A minimal repro, with source-code provided, is ideal. Using sharplab is preferred for compiler/language issues whenever possible.

Diagnostic Id:

• IDE0028 Collection initialization can be simplified
• CS0029 Cannot implicitly convert type 'System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<string, int>>' to 'System.Collections.Generic.KeyValuePair<string, int>' ClassLibrary2 D:\repos\WinFormsApp1\ClassLibrary2\Class1.cs

Expected Behavior:
something like

Dictionary<string, int> dict = [.. GetPairs()];

Actual Behavior:

• IDE0028 is very eager to find "...Add(...)" Methods, however does not care if it produces valid code.
• Collection Initalizer cannot handle Dictionaries/KeyValuePairs very well (see just above)

Notes:

we've experienced the same Problem with Lists/Collections (ie Generics with Arity=1), however I'm unable to get a working minimal example:
(in this case ExecuteSync() returns an ICollection<string?>, but we know it's exactly one element so capacity=2)

corrected (and working):

[sharwell]

This is also covered in #71240. Collection expressions do not behave the same way as collection initializers (they are significantly restricted relative to them).