Suggest specifying generic as union if candidates are different

<!-- BUGS: Please use this template. --> <!-- QUESTIONS: This is not a general support forum! Ask Qs at http://stackoverflow.com/questions/tagged/typescript --> <!-- SUGGESTIONS: See https://github.com/Microsoft/TypeScript-wiki/blob/master/Writing-Good-Design-Proposals.md --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 2.6.1. **Code** > If a generic type can't be formed by picking one of the inference candidates, you'll get the error you posted. – https://stackoverflow.com/questions/39905523/why-isnt-the-type-argument-inferred-as-a-union-type/39905723#39905723 This makes sense, however I would like to question whether we can improve the user experience around this, so errors due to this constraint are easier to understand and fix. For example: ```ts { function compare<T>(x: T, y: T): number { return 1; } compare( 'oops', /* Argument of type '42' is not assignable to parameter of type 'string'. */ 42, ); } { function match<T>(cases: { foo: T; bar: T }): T { return cases.foo; } /* Argument of type '{ foo: number; bar: string; }' is not assignable to parameter of type '{ foo: number; bar: number; }'. Types of property 'bar' are incompatible. Type 'string' is not assignable to type 'number'. */ match({ foo: 1, bar: 'foo', }); } ``` As a TypeScript user, I have struggled with these errors many times, and I've only recently realised the specific constraint on the type system which is the root cause of these errors: generics are picked from the first candidate and are not widened to include all candidates. I have also seen other people struggle with this when learning TypeScript. We can fix this error by specifying the generic as a union: ``` ts match<string | number>({ foo: 1, bar: 'foo', }); ``` However, this fix is really not obvious from the error m…