Add regression test

Author: ahejlsberg

tests/cases/conformance/types/keyof/keyofAndIndexedAccess2.ts

@@ -127,3 +127,9 @@ function fn<T extends {elements: Array<string>} | {elements: Array<number>}>(par
 function fn2<T extends Array<string>>(param: T, cb: (element: T[number]) => void) {
     cb(param[0]);
 }
+
+// Repro from #31149
+
+function fn3<T extends ReadonlyArray<string>>(param: T, cb: (element: T[number]) => void) {
+    cb(param[0]); // Argument of type 'string' is not assignable to parameter of type 'T[number]'.
+}