Preserve error elaboration for indexed access type assignments

When assigning to an indexed access type like this["faa"], the checker
resolves the constraint and performs a structural comparison that
generates specific error messages (e.g., missing properties or type
mismatches). However, reportRelationError unconditionally clears
errorInfo before reporting the generic "could be instantiated with an
arbitrary type" message, discarding the useful elaboration.

This change preserves errorInfo when the target is an IndexedAccess
type, so users see both the generic message and the specific details
about what went wrong (e.g., "Property 'key' is missing" or "Types of
property 'key' are incompatible").

Fixes #63206

Author: WhiteMinds

src/compiler/checker.ts

@@ -22656,7 +22656,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     );
                 }
                 else {
-                    errorInfo = undefined;
+                    // When the target is an indexed access type (e.g. this["faa"]), preserve any
+                    // specific error elaboration from the constraint comparison (e.g., missing
+                    // property or type mismatch errors) rather than discarding them. For plain
+                    // type parameters, clear the error info as the constraint-based errors may
+                    // be misleading.
+                    if (!(target.flags & TypeFlags.IndexedAccess)) {
+                        errorInfo = undefined;
+                    }
                     reportError(
                         Diagnostics._0_could_be_instantiated_with_an_arbitrary_type_which_could_be_unrelated_to_1,
                         targetType,

tests/baselines/reference/mappedTypeRelationships.errors.txt

@@ -10,15 +10,23 @@ mappedTypeRelationships.ts(25,5): error TS2536: Type 'K' cannot be used to index
 mappedTypeRelationships.ts(26,12): error TS2536: Type 'K' cannot be used to index type 'T'.
 mappedTypeRelationships.ts(30,5): error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
   'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'T[keyof T] | undefined'.
+    Type 'undefined' is not assignable to type 'T[keyof T]'.
+      'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
 mappedTypeRelationships.ts(35,5): error TS2322: Type 'T[K] | undefined' is not assignable to type 'T[K]'.
   'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'T[K] | undefined'.
+    Type 'undefined' is not assignable to type 'T[K]'.
+      'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
 mappedTypeRelationships.ts(40,5): error TS2322: Type 'U[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
   'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'U[keyof T] | undefined'.
+    Type 'undefined' is not assignable to type 'T[keyof T]'.
+      'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
 mappedTypeRelationships.ts(41,5): error TS2322: Type 'T[keyof T]' is not assignable to type 'U[keyof T] | undefined'.
   Type 'T[string] | T[number] | T[symbol]' is not assignable to type 'U[keyof T] | undefined'.
     Type 'T[string]' is not assignable to type 'U[keyof T] | undefined'.
 mappedTypeRelationships.ts(45,5): error TS2322: Type 'U[K] | undefined' is not assignable to type 'T[K]'.
   'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'U[K] | undefined'.
+    Type 'undefined' is not assignable to type 'T[K]'.
+      'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
 mappedTypeRelationships.ts(46,5): error TS2322: Type 'T[K]' is not assignable to type 'U[K] | undefined'.
   Type 'T[keyof T]' is not assignable to type 'U[K] | undefined'.
     Type 'T[string] | T[number] | T[symbol]' is not assignable to type 'U[K] | undefined'.
@@ -125,6 +133,8 @@ mappedTypeRelationships.ts(168,5): error TS2322: Type '{ [P in K]: T[P]; }' is n
         ~~~~
 !!! error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
 !!! error TS2322:   'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'T[keyof T] | undefined'.
+!!! error TS2322:     Type 'undefined' is not assignable to type 'T[keyof T]'.
+!!! error TS2322:       'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
         y[k] = x[k];
     }
 
@@ -133,6 +143,8 @@ mappedTypeRelationships.ts(168,5): error TS2322: Type '{ [P in K]: T[P]; }' is n
         ~~~~
 !!! error TS2322: Type 'T[K] | undefined' is not assignable to type 'T[K]'.
 !!! error TS2322:   'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'T[K] | undefined'.
+!!! error TS2322:     Type 'undefined' is not assignable to type 'T[K]'.
+!!! error TS2322:       'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
         y[k] = x[k];
     }
 
@@ -141,6 +153,8 @@ mappedTypeRelationships.ts(168,5): error TS2322: Type '{ [P in K]: T[P]; }' is n
         ~~~~
 !!! error TS2322: Type 'U[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
 !!! error TS2322:   'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'U[keyof T] | undefined'.
+!!! error TS2322:     Type 'undefined' is not assignable to type 'T[keyof T]'.
+!!! error TS2322:       'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
         y[k] = x[k];  // Error
         ~~~~
 !!! error TS2322: Type 'T[keyof T]' is not assignable to type 'U[keyof T] | undefined'.
@@ -153,6 +167,8 @@ mappedTypeRelationships.ts(168,5): error TS2322: Type '{ [P in K]: T[P]; }' is n
         ~~~~
 !!! error TS2322: Type 'U[K] | undefined' is not assignable to type 'T[K]'.
 !!! error TS2322:   'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'U[K] | undefined'.
+!!! error TS2322:     Type 'undefined' is not assignable to type 'T[K]'.
+!!! error TS2322:       'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
         y[k] = x[k];  // Error
         ~~~~
 !!! error TS2322: Type 'T[K]' is not assignable to type 'U[K] | undefined'.

tests/baselines/reference/nonPrimitiveConstraintOfIndexAccessType.errors.txt

@@ -8,6 +8,7 @@ nonPrimitiveConstraintOfIndexAccessType.ts(12,5): error TS2322: Type 'string' is
   'T[P]' could be instantiated with an arbitrary type which could be unrelated to 'string'.
 nonPrimitiveConstraintOfIndexAccessType.ts(15,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
   'T[P]' could be instantiated with an arbitrary type which could be unrelated to 'string'.
+    Type 'string' is not assignable to type 'never'.
 nonPrimitiveConstraintOfIndexAccessType.ts(18,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
   'T[P]' could be instantiated with an arbitrary type which could be unrelated to 'string'.
 nonPrimitiveConstraintOfIndexAccessType.ts(21,5): error TS2322: Type 'string' is not assignable to type 'T[P]'.
@@ -51,6 +52,7 @@ nonPrimitiveConstraintOfIndexAccessType.ts(30,5): error TS2322: Type 'string' is
         ~~
 !!! error TS2322: Type 'string' is not assignable to type 'T[P]'.
 !!! error TS2322:   'T[P]' could be instantiated with an arbitrary type which could be unrelated to 'string'.
+!!! error TS2322:     Type 'string' is not assignable to type 'never'.
     }
     function k<T extends number, P extends keyof T>(s: string, tp: T[P]): void {
         tp = s;