[SILGen]: fix a bug in isolated default argument handling

Changes in https://github.com/swiftlang/swift/pull/81370 addressed a
number of issues with isolated default argument handling. However, there
seemed to still be a problem when dealing with isolated default
arguments of instance/static methods under the following conditions:

- The method contained an isolated default parameter
- The method contained a defaulted parameter that expanded to 0 or >1
  values
- Said parameter was followed by another defaulted parameter

The attempted fix here was to adjust the isolated default argument
iteration bookkeeping.

lib/SILGen/SILGenApply.cpp

@@ -3202,6 +3202,7 @@ done:
       // Otherwise, reset the current index adjustment.
       } else {
         indexAdjustment = 0;
+        indexAdjustmentSite = site;
       }
 
       assert(!siteArgs[argIndex] &&

test/SILGen/isolated_default_arguments.swift

@@ -161,3 +161,66 @@ func useCallerIsolatedDefaultArg() async {
 // CHECK:         // function_ref default argument 0 of
 // CHECK-NEXT:    [[GEN:%.*]] = function_ref @$s4test24callerIsolatedDefaultArg1xySi_tYaFfA_ :
 // CHECK-NEXT:    [[ARG:%.*]] = apply [[GEN]]()
+
+// The following tests failed due to a book-keeping error when processing delayed
+// isolated arguments in methods with:
+//
+//  - a `self` parameter
+//  - delayed arguments that expanded to zero or greater than one value
+//  - a subsequent delayed argument
+//
+// https://github.com/swiftlang/swift/issues/84647
+
+@MainActor
+enum E {
+  static func tupleIsolatedDefaultArgStatic(
+    _: Int,
+    x: Int = main_actor_int_x(),
+    // need not be isolated defaults
+    tup: (Int, Int) = (1, 2),
+    i: Int = 0
+  ) {}
+
+  static func voidIsolatedDefaultArgStatic(
+    _: Int,
+    v: Void = main_actor_void(),
+    // need not be isolated defaults
+    tup: (Int, Int) = (1, 2),
+    i: Int = 0
+  ) {}
+}
+
+func testTupleIsolatedDefaultArgStatic() async {
+  await E.tupleIsolatedDefaultArgStatic(0)
+}
+
+func testVoidIsolatedDefaultArgStatic() async {
+  await E.voidIsolatedDefaultArgStatic(0)
+}
+
+@MainActor
+final class Klazz {
+  func tupleIsolatedDefaultArgInstanceMethod(
+    _: Int,
+    x: Int = main_actor_int_x(),
+    // need not be isolated defaults
+    tup: (Int, Int) = (1, 2),
+    i: Int = 0
+  ) {}
+
+  func voidIsolatedDefaultArgInstanceMethod(
+    _: Int,
+    v: Void = main_actor_void(),
+    // need not be isolated defaults
+    tup: (Int, Int) = (1, 2),
+    i: Int = 0
+  ) {}
+}
+
+func testTupleIsolatedDefaultArgInstanceMethod(_ klazz: Klazz) async {
+  await klazz.tupleIsolatedDefaultArgInstanceMethod(0)
+}
+
+func testVoidIsolatedDefaultArgInstanceMethod(_ klazz: Klazz) async {
+  await klazz.voidIsolatedDefaultArgInstanceMethod(0)
+}