[Sema]: fix @discardableResult interaction with implicit @sendable conversions

Updates existing diagnostic handling to look through function
conversions when suppressing diagnostics for unused functions that are
return values from callees marked with @discardableResult.

Author: jamieQ

lib/Sema/TypeCheckStmt.cpp

@@ -1886,7 +1886,26 @@ void TypeChecker::checkIgnoredExpr(Expr *E) {
                     : valueE;
 
     if (auto *Fn = dyn_cast<ApplyExpr>(expr)) {
-      if (auto *calledValue = Fn->getCalledValue()) {
+      /// Some concurrency-related things may have intermediate conversions that
+      /// we want to look through, e.g.
+      ///
+      /// ```swift
+      /// @discardableResult
+      /// @MainActor
+      /// func foo() -> () -> Void {
+      ///   return {}
+      /// }
+      ///
+      /// @MainActor
+      /// func test() {
+      ///   foo()
+      ///   // ^ return value implicitly wapped in a `@Sendable` conversion
+      /// }
+      /// ```
+      /// Attempt to look through function conversions when resolving the
+      /// called value.
+      if (auto *calledValue =
+              Fn->getCalledValue(/*skipFunctionConversions=*/true)) {
         if (auto *FD = dyn_cast<AbstractFunctionDecl>(calledValue)) {
           if (FD->getAttrs().hasAttribute<DiscardableResultAttr>()) {
             isDiscardable = true;

test/Concurrency/attr_discardableResult_async_await.swift

@@ -14,3 +14,23 @@ func mainActorAsyncDiscardable() async -> Int { 0 }
 func consumesMainActorAsyncDiscardable() async {
   await mainActorAsyncDiscardable() // ok
 }
+
+// https://github.com/swiftlang/swift/issues/83463
+
+@MainActor
+@discardableResult
+func returnsDiscardableFunc() -> () -> Void { return {} }
+
+@MainActor
+func testDiscardsSyncFuncWithImplicitSendableConversion() {
+    returnsDiscardableFunc()
+}
+
+@MainActor
+@discardableResult
+func mainActorAsyncReturnsDiscardableFunc() async -> () -> Void { return {} }
+
+@MainActor
+func discardsAsyncFuncWithImplicitSendableConversion() async {
+  await mainActorAsyncReturnsDiscardableFunc()
+}