loosen number index check, fixes #15768

src/compiler/checker.ts

@@ -18758,13 +18758,10 @@ namespace ts {
             if (isTypeAssignableTo(indexType, getIndexType(objectType))) {
                 return type;
             }
-            // Check if we're indexing with a numeric type and the object type is a generic
-            // type with a constraint that has a numeric index signature.
-            if (maybeTypeOfKind(objectType, TypeFlags.TypeVariable) && isTypeAssignableToKind(indexType, TypeFlags.NumberLike)) {
-                const constraint = getBaseConstraintOfType(objectType);
-                if (constraint && getIndexInfoOfType(constraint, IndexKind.Number)) {
-                    return type;
-                }
+            // Check if we're indexing with a numeric type and if either object or index types
+            // is a generic type with a constraint that has a numeric index signature.
+            if (getIndexInfoOfType(getApparentType(objectType), IndexKind.Number) && isTypeAssignableToKind(indexType, TypeFlags.NumberLike)) {
+                return type;
             }
             error(accessNode, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(objectType));
             return type;

tests/baselines/reference/genericNumberIndex.js

@@ -0,0 +1,5 @@
+//// [genericNumberIndex.ts]
+type X<I extends number> = ['a'][I];
+
+
+//// [genericNumberIndex.js]

tests/baselines/reference/genericNumberIndex.symbols

@@ -0,0 +1,6 @@
+=== tests/cases/compiler/genericNumberIndex.ts ===
+type X<I extends number> = ['a'][I];
+>X : Symbol(X, Decl(genericNumberIndex.ts, 0, 0))
+>I : Symbol(I, Decl(genericNumberIndex.ts, 0, 7))
+>I : Symbol(I, Decl(genericNumberIndex.ts, 0, 7))
+

tests/baselines/reference/genericNumberIndex.types

@@ -0,0 +1,6 @@
+=== tests/cases/compiler/genericNumberIndex.ts ===
+type X<I extends number> = ['a'][I];
+>X : ["a"][I]
+>I : I
+>I : I
+

tests/cases/compiler/genericNumberIndex.ts

@@ -0,0 +1 @@
+type X<I extends number> = ['a'][I];