[Concurrency] Converting from nonisolated to @execution(caller) does not cross an isolation boundary

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -2734,7 +2734,16 @@ namespace {
           // Converting to `@execution(caller)` function type
           case FunctionTypeIsolation::Kind::NonIsolatedCaller: {
             switch (fromIsolation.getKind()) {
-            case FunctionTypeIsolation::Kind::NonIsolated:
+            case FunctionTypeIsolation::Kind::NonIsolated: {
+              // nonisolated -> @execution(caller) doesn't cross
+              // an isolation boundary.
+              if (!fromFnType->isAsync())
+                break;
+
+              // @execution(concurrent) -> @execution(caller)
+              // crosses an isolation boundary.
+              LLVM_FALLTHROUGH;
+            }
             case FunctionTypeIsolation::Kind::GlobalActor:
             case FunctionTypeIsolation::Kind::Erased:
               diagnoseNonSendableParametersAndResult(toFnType);

test/Concurrency/attr_execution_conversions.swift

@@ -103,13 +103,11 @@ func testNonSendableDiagnostics(
   let _: @execution(caller) () async -> NonSendable = erased2 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
   // expected-error@-1 {{cannot convert '@isolated(any) @Sendable () async -> NonSendable' to '@execution(caller) () async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
 
-  let _: @execution(caller) (NonSendable) async -> Void = nonIsolated1 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
-  // expected-error@-1 {{annot convert '@Sendable (NonSendable) -> Void' to '@execution(caller) (NonSendable) async -> Void' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
+  let _: @execution(caller) (NonSendable) async -> Void = nonIsolated1 // Ok
   let _: @execution(caller) (NonSendable) async -> Void = nonIsolated2 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
   // expected-error@-1 {{cannot convert '@Sendable (NonSendable) async -> Void' to '@execution(caller) (NonSendable) async -> Void' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
 
-  let _: @execution(caller) () async -> NonSendable = nonIsolated3 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
-  // expected-error@-1 {{cannot convert '@Sendable () -> NonSendable' to '@execution(caller) () async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
+  let _: @execution(caller) () async -> NonSendable = nonIsolated3 // Ok
   let _: @execution(caller) () async -> NonSendable = nonIsolated4 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
   // expected-error@-1 {{cannot convert '@Sendable () async -> NonSendable' to '@execution(caller) () async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}