Collection expression loophole: ability to invoke .Add() method with the parameter type not matching the collection expression element type

[controlflow]

Version Used: main branch

Steps to Reproduce:

using System.Collections;
using System.Collections.Generic;

BadCandidateForCollectionExpression xs = [123]; // no .Add(int)
BadCandidateForCollectionExpression ys = ["abc"]; // not int element type
BadCandidateForCollectionExpression zs = [default]; // ????

class BadCandidateForCollectionExpression : IEnumerable<int>
{
  public void Add(string s) { }

  IEnumerator<int> IEnumerable<int>.GetEnumerator() { yield break; }
  IEnumerator IEnumerable.GetEnumerator() { yield break; }
}

Expected Behavior:

Impossible to fill the collection expression, no Add method can accept the value of int type - the element type inferred as a collection expression element type.

Actual Behavior:

If the expression is both convertible to inferred element type and can be accepted by some non-int Add method overload - it compiles successfully. As the result you have a really weird language context - you can only pass expressions that are implicitly convertible to int, but the invoked method accepts string value.

I guess this is the poorly written check in frontend, I guess instead of checking the element expression to be convertible to int you probably must check the parameter type of invoked .Add() method to have type convertible to int.

[CyrusNajmabadi]

@cston @333fred to see if this is by design based on our last meetings

I don't imagine this is an interesting case to support given that it doesn't behave like any normal collection does.

[333fred]

This doesn't seem like it should compile. The type of default should be inferred to be the element type.

[jcouv]

I believe this is by-design from current spec. There exists a conversion from default to the iteration type int and default can also be used to invoke .Add. Those two checks are independent (the types don't have to match).

[RikkiGibson]

feels like a spec bug :)

[jaredpar]

Moving to csharplang to close the spec bug.

[Ai-N3rd]

This should not be supported unless we can find a reasonable use case. Otherwise, It shouldn't be supported for collection expressions.

[GeirGrusom]

A behavior I found a bit amusing here is that if you change the parameter into a float, and you try to pass a float to that method it will complain that it will only allow an int, not a float! If you give it an int however, it will silently pass a float instead.

[Ai-N3rd]

There should probably be a check at compile time. It would simply check if the .Add() method has the same type as the IEnumerable. If not, it can't be used in collections expressions.

[cston]

There should probably be a check at compile time.

Perhaps a diagnostic could be reported if the element as converted for the Add call is not implicitly convertible to the collection iteration type.

BadCandidateForCollectionExpression zs = [default]; // warning: converting 'default' to 'string' for Add(string)