Preserve error elaboration for indexed access type assignments

[WhiteMinds]

Fixes microsoft/TypeScript#63206

Note: I understand the current contribution scope is limited to 6.0/7.0 differences and crashes. This is a diagnostic quality improvement rather than a behavioral change — no type-checking logic is altered, only the error message detail level. Happy to hold this until 7.0 if preferred.

Problem

When assigning to an indexed access type (e.g. this["faa"]), reportRelationError unconditionally clears errorChain before reporting the generic "could be instantiated with an arbitrary type" message, discarding the specific elaboration already computed from the constraint comparison (missing properties, type mismatches, etc.).

Fix

Preserve errorChain when the target is an IndexedAccess type, so users see both the generic message and the specific details.

Before

this.foo = {}
// Type '{}' is not assignable to type 'this["faa"]'.
//   'this["faa"]' could be instantiated with an arbitrary type which could be unrelated to '{}'.

After

this.foo = {}
// Type '{}' is not assignable to type 'this["faa"]'.
//   'this["faa"]' could be instantiated with an arbitrary type which could be unrelated to '{}'.
//     Property 'key' is missing in type '{}' but required in type '{ key: number | null; }'.

this.foo = {key: "str"}
// Type '{ key: string; }' is not assignable to type 'this["faa"]'.
//   'this["faa"]' could be instantiated with an arbitrary type which could be unrelated to '{ key: string; }'.
//     Type '{ key: string; }' is not assignable to type '{ key: number | null; }'.
//       Types of property 'key' are incompatible.
//         Type 'string' is not assignable to type 'number'.

Two existing baselines updated to reflect the additional elaboration (no errors removed, only more specific detail added).

[jakebailey]

Maybe I don't have context here, but, I don't find the new messages more helpful than the existing one...

[gary-donut]

Maybe I don't have context here, but, I don't find the new messages more helpful than the existing one...

Consider this example:

class A {
    declare foo: this["faa"];
    declare faa: {key: number|null}

    fuu() {
        this.foo = {}           // vague error
        this.foo = {key: "str"} // vague error
    }
}

let foo: {key: number|null};
foo = {}           // precise error
foo = {key: "str"} // precise error

Both assign to the same underlying type {key: number|null}.
Direct assignment gives clear errors — "Property 'key' is missing"* or *"Type 'string' is not assignable to type 'number'".
But via this["faa"], you only get "could be instantiated with an arbitrary type which could be unrelated to '{}'".

[DanielRosenwasser]

Looking at the error in general, it looks pretty confusing to have this message anywhere other than at the bottom of an elaboration.

Maybe guard all of this in an if r.errorChain != nil

[WhiteMinds]

Do you mean skipping the entire "could be instantiated..." elaboration when errorChain already has specific diagnostics? Something like adding r.errorChain == nil to the outer guard:

if targetFlags&TypeFlagsTypeParameter != 0 && target != r.c.markerSuperTypeForCheck && target != r.c.markerSubTypeForCheck && r.errorChain == nil {

So when the constraint comparison has already produced precise errors (e.g., missing property or type mismatch), we skip the generic message entirely instead of prepending it.

Before (current):

Type '{}' is not assignable to type 'this["faa"]'.
  'this["faa"]' could be instantiated with an arbitrary type which could be unrelated to '{}'.

After:

Type '{}' is not assignable to type 'this["faa"]'.
  Property 'key' is missing in type '{}' but required in type '{ key: number | null; }'.

Type '{ key: string; }' is not assignable to type 'this["faa"]'.
  Type '{ key: string; }' is not assignable to type '{ key: number | null; }'.
    Types of property 'key' are incompatible.
      Type 'string' is not assignable to type 'number'.