TypeScript 3.0 improved error UX meta-issue

[DanielRosenwasser]

In TypeScript 3.0, we're aiming for a better error message experience all around.

One of the related efforts we've been working on is related error spans: #25257.

However, in addition to that, we've received enough constructive feedback about error messages that we can act on.

TypeScript 3.0

• Better JSX attribute errors (Better JSX property error messages #23117)
• Better errors on object properties (Error spans are not close enough to missing properties on deeply nested object literals #22170, Related error spans for assignability checks #25030)
• Smarter elaborations when assigning Foo<T> to Foo<U> | Bar<V> (Use type references for better error messages when relating to a union type #24776)
• Smarter elaborations when assigning objects to Foo | Bar[] (When relating object literal types to unions, avoid elaborating on Array/ReadonlyArray #25750 )
• Better guidance for non-existent deep imports (Poor error guidance for deep imports of packages with @types #24781)
• Better errors on always true/false conditions (Confusing error message: Operator '===' cannot be applied to types '1' and '0'. #25245)
• Smarter elaborations against Foo | null | undefined (Skip ahead on error elaboration for source -> target | null | undefined when source is atomic #25086)
• Better truncation on big types (Better type truncation in error messages #24778)
• Better duplicate declaration errors (Limit duplicate declaration errors #25324)

TypeScript 3.x

These suggested improvements didn't make it in time for 3.0, but we'll hopefully continue working on them! Check out #26077 to keep track of them.

• Better hints for uncalled types ("Did you forget to call 'X'?" for assignability errors #25308)
• Better hints for 'cannot find name' errors (Better hints for 'cannot find name' errors #25309)
• Better errors on weak type intersections like IntrinsicAttributes (Simplify error messages against intersections of weak types #24146 🏃‍♂️)
• Better errors when matching implementation signatures (If call doesn’t resolve to an overload signature, issue a better error when the implementation would work #25488)

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• Collapsing identical errors (Consider collapsing multiple repeated type-related error messages when in --pretty #23393)
  • Subsumed by Limit duplicate declaration errors #25324.