[CSSimplify] Allow conversion from caller isolated to parameter isola…

lib/Sema/CSSimplify.cpp

@@ -3267,6 +3267,21 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
   SmallVector<AnyFunctionType::Param, 8> func2Params;
   func2Params.append(func2->getParams().begin(), func2->getParams().end());
 
+  // Support conversion from `@execution(caller)` to a function type
+  // with an isolated parameter.
+  if (subKind == ConstraintKind::Subtype &&
+      func1->getIsolation().isNonIsolatedCaller() &&
+      func2->getIsolation().isParameter()) {
+    // `@execution(caller)` function gets an implicit isolation parameter
+    // introduced during SILGen and thunk is going to forward an isolation
+    // from the caller to it.
+    // Let's remove the isolated parameter from consideration, function
+    // types have to match on everything else.
+    llvm::erase_if(func2Params, [](const AnyFunctionType::Param &param) {
+      return param.isIsolated();
+    });
+  }
+
   // Add a very narrow exception to SE-0110 by allowing functions that
   // take multiple arguments to be passed as an argument in places
   // that expect a function that takes a single tuple (of the same

test/Concurrency/attr_execution/conversions.swift

@@ -44,11 +44,26 @@ func testUpcast(arr: [@execution(caller) () async -> Void]) {
 }
 
 // Isolated parameter
-func testParameterIsolation(fn: @escaping (isolated (any Actor)?) async -> Void) {
+func testParameterIsolation(fn: @escaping (isolated (any Actor)?) async -> Void, caller: @escaping @execution(caller) (String) async -> Void) {
   let _: @execution(caller) () async -> Void = fn
   // expected-error@-1 {{cannot convert value of type '(isolated (any Actor)?) async -> Void' to specified type '@execution(caller) () async -> Void'}}
   let _: @execution(concurrent) () async -> Void = fn
   // expected-error@-1 {{cannot convert value of type '(isolated (any Actor)?) async -> Void' to specified type '() async -> Void'}}
+
+  let _: (isolated (any Actor)?) async -> Void = callerTest // Ok
+  let _: (isolated (any Actor)?) -> Void = callerTest
+  // expected-error@-1 {{invalid conversion from 'async' function of type '@execution(caller) () async -> ()' to synchronous function type '(isolated (any Actor)?) -> Void'}}
+  let _: (isolated (any Actor)?) async -> Void = concurrentTest
+  // expected-error@-1 {{cannot convert value of type '() async -> ()' to specified type '(isolated (any Actor)?) async -> Void'}}
+  let _: (isolated (any Actor)?, Int) async -> Void = callerTest
+  // expected-error@-1 {{cannot convert value of type '@execution(caller) () async -> ()' to specified type '(isolated (any Actor)?, Int) async -> Void'}}
+
+  let _: (String, isolated any Actor) async -> Void = caller // Ok
+  let _: (isolated (any Actor)?, String) async -> Void = caller // Ok
+
+  let _: (Int, isolated any Actor) async -> Void = { @execution(caller) x in } // Ok
+  let _: (Int, isolated any Actor) async -> Void = { @execution(caller) (x: Int) in } // Ok
+  let _: (isolated any Actor, Int, String) async -> Void = { @execution(caller) (x: Int, _: String) in } // Ok
 }
 
 // Non-conversion situations