[Sema]: ban @isolated(any) conversions to synchronous function types

per SE-0431, function conversions from an @isolated(any) function to a synchronous,
non-@isolated(any) function type should not be allowed. updates the logic in the
constraint solver to enforce this.

Author: jamieQ

lib/Sema/CSSimplify.cpp

@@ -2943,6 +2943,11 @@ ConstraintSystem::matchFunctionIsolations(FunctionType *func1,
                                           ConstraintLocatorBuilder locator) {
   auto isolation1 = func1->getIsolation(), isolation2 = func2->getIsolation();
 
+  // An @isolated(any) function cannot be converted to a non-@isolated(any)
+  // synchronous function regardless of whether or not it is itself async.
+  if (isolation1.isErased() && !isolation2.isErased() && !func2->isAsync())
+    return false;
+
   // If we have a difference in isolation kind, we need a conversion.
   // Make sure that we're looking for a conversion, and increase the
   // function-conversion score to make sure this solution is worse than

test/Concurrency/isolated_any.swift

@@ -64,11 +64,17 @@ func testEraseFromIsolatedArgument() {
   requireIsolatedAnyWithActorArgument(hasIsolatedArgument)
 }
 
-func requireSendableNonIsolated(_ fn: @Sendable () -> ()) {}
+func requireSendableNonIsolated(_ fn: @Sendable () async -> ()) {}
 func testConvertIsolatedAnyToNonIsolated(fn: @Sendable @isolated(any) () -> ()) {
   requireSendableNonIsolated(fn)
 }
 
+func requireSendableNonIsolated_sync(_ fn: @Sendable () -> ()) {}
+func testConvertIsolatedAnyToNonIsolated_sync(fn: @Sendable @isolated(any) () -> ()) {
+  // expected-error @+1 {{cannot convert value of type '@isolated(any) @Sendable () -> ()' to expected argument type '@Sendable () -> ()}}
+  requireSendableNonIsolated_sync(fn)
+}
+
 func requireSendableGlobalActor(_ fn: @Sendable @MainActor () -> ()) {}
 func testConvertIsolatedAnyToMainActor(fn: @Sendable @isolated(any) () -> ()) {
   // expected-error @+1 {{cannot convert value of type '@isolated(any) @Sendable () -> ()' to expected argument type '@MainActor @Sendable () -> ()'}}