Type inference not producing error

[Jojoshua]

TypeScript Version: 3.3.3

type inference
optional properties

Code

interface User{
    editorParams?: SelectParams | AutoCompleteParams;
}

let user: User;

let autoComplete1: AutoCompleteParams = {
    searchFunc: (term, values) => {
      //search for exact matches
      var matches: string[] = [];
      //return matches;
    },
    listItemFormatter: function(value, title) {
      //prefix all titles with the work "Mr"
      return "Mr " + title;
    },
  };
  user.editorParams = autoComplete1;
  
  user.editorParams = {
    searchFunc: (term,values) => {
      //search for exact matches
      var matches: string[] = [];
      //return matches;
    },
    listItemFormatter: function(value, title) {
      //prefix all titles with the work "Mr"
      return "Mr " + title;
    },
  };

  interface SelectParams {
    listItemFormatter?: (value: string, text: string) => string;
  }

  interface AutoCompleteParams {
    listItemFormatter?: (value: string, text: string) => string;
    searchFunc: (term: string, values: string[]) => string[];
  }

Expected behavior:
I expected when commenting out the return statement in the inferred usage of searchFunc that a compiler error would occur. The annotated method above does give the compiler error. The playground link shows this as well.

Actual behavior:
No compiler error for the inferred usage. Hovering the inferred searchFunc in VSCode shows it as AutoCompleteParams as well.

Playground Link:
https://www.typescriptlang.org/play/#src=interface%20User%7B%0D%0A%20%20%20%20editorParams%3F%3A%20SelectParams%20%7C%20AutoCompleteParams%3B%0D%0A%7D%0D%0A%0D%0Alet%20user%3A%20User%3B%0D%0A%0D%0Alet%20autoComplete1%3A%20AutoCompleteParams%20%3D%20%7B%0D%0A%20%20%20%20searchFunc%3A%20(term%2C%20values)%20%3D%3E%20%7B%0D%0A%20%20%20%20%20%20%2F%2Fsearch%20for%20exact%20matches%0D%0A%20%20%20%20%20%20var%20matches%3A%20string%5B%5D%20%3D%20%5B%5D%3B%0D%0A%20%20%20%20%20%20%2F%2Freturn%20matches%3B%0D%0A%20%20%20%20%7D%2C%0D%0A%20%20%20%20listItemFormatter%3A%20function(value%2C%20title)%20%7B%0D%0A%20%20%20%20%20%20%2F%2Fprefix%20all%20titles%20with%20the%20work%20%22Mr%22%0D%0A%20%20%20%20%20%20return%20%22Mr%20%22%20%2B%20title%3B%0D%0A%20%20%20%20%7D%2C%0D%0A%20%20%7D%3B%0D%0A%20%20user.editorParams%20%3D%20autoComplete1%3B%0D%0A%20%20%0D%0A%20%20user.editorParams%20%3D%20%7B%0D%0A%20%20%20%20searchFunc%3A%20(term%2Cvalues)%20%3D%3E%20%7B%0D%0A%20%20%20%20%20%20%2F%2Fsearch%20for%20exact%20matches%0D%0A%20%20%20%20%20%20var%20matches%3A%20string%5B%5D%20%3D%20%5B%5D%3B%0D%0A%20%20%20%20%20%20%2F%2Freturn%20matches%3B%0D%0A%20%20%20%20%7D%2C%0D%0A%20%20%20%20listItemFormatter%3A%20function(value%2C%20title)%20%7B%0D%0A%20%20%20%20%20%20%2F%2Fprefix%20all%20titles%20with%20the%20work%20%22Mr%22%0D%0A%20%20%20%20%20%20return%20%22Mr%20%22%20%2B%20title%3B%0D%0A%20%20%20%20%7D%2C%0D%0A%20%20%7D%3B%0D%0A%0D%0A%20%20interface%20SelectParams%20%7B%0D%0A%20%20%20%20listItemFormatter%3F%3A%20(value%3A%20string%2C%20text%3A%20string)%20%3D%3E%20string%3B%0D%0A%20%20%7D%0D%0A%0D%0A%20%20interface%20AutoCompleteParams%20%7B%0D%0A%20%20%20%20listItemFormatter%3F%3A%20(value%3A%20string%2C%20text%3A%20string)%20%3D%3E%20string%3B%0D%0A%20%20%20%20searchFunc%3A%20(term%3A%20string%2C%20values%3A%20string%5B%5D)%20%3D%3E%20string%5B%5D%3B%0D%0A%20%20%7D

Related Issues:
Possibly
#27704

[j-oliveras]

Workaround:

  interface SelectParams {
    listItemFormatter?: (value: string, text: string) => string;
    searchFunc?: undefined; // Added this
  }

[RyanCavanaugh]

From the checker's perspective, you've created a valid SelectParams object which happens to have an extraneous searchFunc, which is in turn not flagged as an excess property because it's a valid property name in AutoCompleteParams. The combination of the last two things is unfortunate but each of those behaviors is a local maximum.

In general you should try hard to not have a union type T | U where U is a subtype of T because it tends to produce odd behavior like that. @j-oliveras 's suggestion is a good one.

[Jojoshua]

@RyanCavanaugh How did the mouse hover in VSCode get it right? It knew which interface to pull details for.