Type 'string' cannot be used to index type 'T' - after upgrade from 3.4 to 3.5

[FrancescoBorzi]

TypeScript Version: 3.6.0-dev.20190804 as well as 3.5.3. It was fine in version 3.4.5

Code

Here's a minimal reproduction tested using tsc:

class ParentType {
  [key: string]: string|number;
}

class ChildType extends ParentType {
  someKey: number;
}

class MyClass<T extends ParentType> {
  field: T;

  doStuff() {
    this.field['someKey'] = 123;
  }
}

error TS2536: Type 'string' cannot be used to index type 'T'.

Expected behavior:

There shouldn't be breaking changes from one minor version to another

Actual behavior:

Upgrading my app from 3.4.5 to 3.5.3 caused a build error

Real code: azerothcore/Keira3#12

[jack-williams]

Changed by #30769. Specifically see:

Given a type variable T with a constraint C, when an indexed access T[K] occurs on the target side of a type relationship, index signatures in C are now ignored. This is because a type argument for T isn't actually required to have an index signature, it is just required to have properties with matching types.

[FrancescoBorzi]

then shouldn't the major version be updated? e.g. 4.0

[jack-williams]

#14116

[FrancescoBorzi]

if this is an intended change, how am I supposed to change my code to comply with the new version?

also @RyanCavanaugh what is the duplicating issue?

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.