[Concurrency] Look through caller-side default arguments for #isolation

when computing the isolated actor expression for a call.

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -3378,6 +3378,12 @@ namespace {
 
         // FIXME: CurrentContextIsolationExpr does not have its actor set
         // at this point.
+        if (auto *defaultArg = dyn_cast<DefaultArgumentExpr>(arg)) {
+          // Look through caller-side default arguments for #isolation.
+          if (defaultArg->isCallerSide()) {
+            arg = defaultArg->getCallerSideDefaultExpr();
+          }
+        }
         if (auto *macro = dyn_cast<MacroExpansionExpr>(arg)) {
           auto *expansion = macro->getRewritten();
           if (auto *isolation = dyn_cast<CurrentContextIsolationExpr>(expansion)) {

test/Concurrency/isolated_parameters.swift

@@ -477,3 +477,11 @@ nonisolated func fromNonisolated(ns: NotSendable) async -> NotSendable {
 func invalidIsolatedClosureParam<A: AnyActor> (
   _: (isolated A) async throws -> Void // expected-error {{'isolated' parameter type 'A' does not conform to 'Actor' or 'DistributedActor'}}
 ) {}
+
+public func useDefaultIsolation(
+  _ isolation: isolated (any Actor)? = #isolation
+) {}
+
+@MainActor func callUseDefaultIsolation() async {
+  useDefaultIsolation()
+}