Check that switch expressions with a natural type do not cause additional conversions #1739

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1. jskeet

   self-assigned this

   on Jul 22, 2026
2. jskeet

   added this to the C# 8.0 milestone on Jul 22, 2026
3. jskeet commented on Jul 22, 2026

   

   jskeet

   on Jul 22, 2026

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   (Shouldn't be too hard to check and/or fix, so marked as C# 8 - but if it becomes more fiddly, we may make it "pre-C# 9")
4. jskeet

   added

   meeting: discussThis issue should be discussed at the next TC49-TG2 meetingThis issue should be discussed at the next TC49-TG2 meeting

   meeting: proposalThere is an informal proposal in the issue, worth discussing in a meetingThere is an informal proposal in the issue, worth discussing in a meeting

   on Aug 26, 2026
5. jskeet commented on Aug 26, 2026

   

   jskeet

   on Aug 26, 2026

   ContributorAuthor

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   Okay, moving the first of Kalle's examples here, and adding a Main method:

   class B { }
   class C1 : B { }
   class C2 : B { }
   
   struct S
   {
       public static implicit operator S(B _) { Console.WriteLine("S(B)"); return default; }
       public static implicit operator S(C1 _) { Console.WriteLine("S(C1)"); return default; }
       public static implicit operator S(C2 _) { Console.WriteLine("S(C2)"); return default; }
   }
   
   class P
   {
       public static S M1(int i)
       {
           // First converts from C1, C2, or B to the switch common type B, then from B to S.
           return i switch { 1 => new C1(), 2 => new C2(), _ => new B() };
       }
   
       public static S M2(int i)
       {
           // Converts C1 or C2 directly to S.
           return i switch { 1 => new C1(), 2 => new C2(), _ => throw new System.ArgumentException() };
       }
   }
   
   class Program
   {
       static void Main()
       {
           P.M1(1);
           P.M1(2);
           P.M1(3);
           P.M2(1);
           P.M2(2);
       }
   }

   Observed behavior (admittedly with .NET 10) is:

   S(B)
   S(B)
   S(B)
   S(C1)
   S(C2)

   In other words, the "natural type" of the switch expression is used in the case of M1 before switch expression conversions.

   I don't think this is currently covered in the spec.

   We may be able to fix it by adding "which does not have a type " to 10.2.18:

   Before:

   There is an implicit conversion from a switch_expression (§12.11) to every type T for which there exists an implicit conversion from each switch_expression_arm’s switch_expression_arm_expression’s to T.

   After:

   There is an implicit conversion from a switch_expression (§12.11) which does not have a type to every type T for which there exists an implicit conversion from each switch_expression_arm’s switch_expression_arm_expression’s to T.

   Added "meeting: discuss" so we can check if that makes sense; it'll be easy to whip up a PR if so.
6. KalleOlaviNiemitalo commented on Aug 26, 2026

   

   KalleOlaviNiemitalo

   on Aug 26, 2026

   Contributor

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   Perhaps clearer with a comma in "does not have a type, to every type".

   👍React with 👍1Reacted by Jon Skeet
7. jskeet commented on Sep 9, 2026

   

   jskeet

   on Sep 9, 2026

   ContributorAuthor

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   Meeting resolution: proposal is fine. @jskeet to create a PR.
8. jskeet

   removed

   meeting: discussThis issue should be discussed at the next TC49-TG2 meetingThis issue should be discussed at the next TC49-TG2 meeting

   meeting: proposalThere is an informal proposal in the issue, worth discussing in a meetingThere is an informal proposal in the issue, worth discussing in a meeting

   on Sep 9, 2026
9. jskeet

   added a commit that references this issue on Sep 13, 2026

   Restrict switch_expression conversions

   ...

   This issue will close once commit 891944a is merged into the 'draft-v8' branch.

   891944a
10. BillWagner

    closed this as completedin #1806on Sep 14, 2026
11. pull

    added a commit that references this issue on Sep 15, 2026

    Restrict switch_expression conversions (dotnet#1806)

    ...

    This issue will close once commit df649b5 is merged into the 'draft-v8' branch.

    df649b5