Returned Enum value has its type needlessly widened #20254

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1. ahejlsberg commented on Nov 25, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on Nov 25, 2017

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   This is working as intended. See #10676 (comment).
2. Anders Hejlsberg (ahejlsberg)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Nov 25, 2017
3. OliverJAsh commented on Nov 25, 2017

   

   Oliver Joseph Ash (OliverJAsh)

   on Nov 25, 2017

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   Anders Hejlsberg (@ahejlsberg)

   Interesting, thanks for the linking to that explanation!

   Widening occurs only if the inferred return type is a singleton literal type.

   #10676 (comment)

   Here is an example where the return type is not a singleton literal type but a union of objects containing literal types, and the compiler still appears to widen both literal types to their enum union type:

   {
       enum Item {
           potato = 'potato',
           cucumber = 'cucumber',
       }
   
       {
           const get = () => true ? { item: Item.potato, otherVar: true } : { item: Item.cucumber };
           /* Unexpected error:
           Type '{ item: Item; otherVar: boolean; } | { item: Item; }' is not assignable to type '{ item: Item.potato; otherVar: boolean; } | { item: Item.cucumber; }'.
               Type '{ item: Item; otherVar: boolean; }' is not assignable to type '{ item: Item.potato; otherVar: boolean; } | { item: Item.cucumber; }'.
                   Type '{ item: Item; otherVar: boolean; }' is not assignable to type '{ item: Item.cucumber; }'.
                   Types of property 'item' are incompatible.
                       Type 'Item' is not assignable to type 'Item.cucumber'.
           */
           const result: { item: Item.potato; otherVar: boolean } | { item: Item.cucumber } = get();
           result;
       }
   }

   Is this also expected behaviour, or a separate issue?
4. ahejlsberg commented on Nov 26, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on Nov 26, 2017

   Last edited by ahejlsberg

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   Oliver Joseph Ash (@OliverJAsh) My comment above was regarding literal types as the actual return type. In your example the return type is a union of object types, and the literal types are widened because they are inferred for a mutable location (i.e. a property in an object literal). That is the expected behavior.
5. OliverJAsh commented on Nov 26, 2017

   

   Oliver Joseph Ash (OliverJAsh)

   on Nov 26, 2017

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   Aha, good to know, thanks. Here's an example for anyone else who might be interested:

   {
       enum Item {
           potato = 'potato',
           cucumber = 'cucumber',
       }
   
       {
           const x = { item: Item.potato }
           /* Error:
           Type '{ item: Item; }' is not assignable to type '{ item: Item.potato; }'.
               Types of property 'item' are incompatible.
                   Type 'Item' is not assignable to type 'Item.potato'. */
           const result: { item: Item.potato; } = x;
           result;
       }
       // workaround 1: type assertion on property
       {
           const x = { item: Item.potato as Item.potato }
           const result: { item: Item.potato; } = x;
           result;
       }
       // workaround 2: avoid inference
       {
           const x: { item: Item.potato; } = { item: Item.potato }
           const result: { item: Item.potato; } = x;
           result;
       }
   }
6. OliverJAsh commented on Nov 26, 2017

   

   Oliver Joseph Ash (OliverJAsh)

   on Nov 26, 2017

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   Also for anyone else who is interested, here is some discussion about widening in object literals and a proposal to change this #20195
7. typescript-bot commented on Dec 11, 2017

   

   TypeScript Bot (typescript-bot)

   on Dec 11, 2017

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   Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.
8. TypeScript Bot (typescript-bot)

   closed this as completedon Dec 11, 2017
9. microsoft

   locked and limited conversation to collaborators on Jun 14, 2018