Fix subscript check and add test for sending result in subscript

Author: stzn

lib/Sema/TypeCheckConcurrency.cpp

@@ -1197,21 +1197,30 @@ bool swift::diagnoseNonSendableTypesInReference(
     }
 
     // Check the result type of a function or subscript.
-    if (auto func = dyn_cast<FuncDecl>(decl)) {
-      if (funcCheckOptions.contains(FunctionCheckKind::Results)) {
-        // only check results if funcCheckKind specifies so
-        Type resultType = func->getResultInterfaceType().subst(subs);
-        auto diag = getSendableResultDiag(refKind);
-        if (diag.ID == diag::non_sendable_result_in_witness.ID &&
-            func->hasSendingResult() && !resultType->hasError()) {
-          return false;
-        }
-        if (diagnoseNonSendableTypes(
-                resultType, fromDC, derivedConformanceType, refLoc,
-                diagnoseLoc.isInvalid() ? refLoc : diagnoseLoc, diag, func,
-                getActorIsolation()))
-          return true;
+    if (funcCheckOptions.contains(FunctionCheckKind::Results)) {
+      Type resultType;
+      bool hasSendingResult;
+      if (auto func = dyn_cast<FuncDecl>(decl)) {
+        resultType = func->getResultInterfaceType().subst(subs);
+        hasSendingResult = func->hasSendingResult();
+        decl = func;
+      } else if (auto subscript = dyn_cast<SubscriptDecl>(decl)) {
+        resultType = subscript->getElementInterfaceType().subst(subs);
+        hasSendingResult = isa<SendingTypeRepr>(subscript->getResultTypeRepr());
       }
+      if (!resultType) {
+        return false;
+      }
+      auto diag = getSendableResultDiag(refKind);
+      if (diag.ID == diag::non_sendable_result_in_witness.ID &&
+          hasSendingResult && !resultType->hasError()) {
+        return false;
+      }
+      if (diagnoseNonSendableTypes(
+              resultType, fromDC, derivedConformanceType, refLoc,
+              diagnoseLoc.isInvalid() ? refLoc : diagnoseLoc, diag, decl,
+              getActorIsolation()))
+        return true;
     }
 
     return false;

test/Concurrency/sendable_conformance_checking_skip_sending.swift

@@ -74,3 +74,17 @@ protocol P6 {
 class P6Class: P6 {
   func bar(_ a: sending NonSendableKlass6, _ b: sending NonSendableKlass6) async -> sending NonSendableKlass6 { a }
 }
+
+class NonSendableKlass7 {}
+
+protocol P7 {
+  subscript(_: sending NonSendableKlass7) -> sending NonSendableKlass7 { get async }
+}
+
+@MainActor
+struct S: P7 {
+  subscript(_: sending NonSendableKlass7) -> sending NonSendableKlass7 {
+    get async { NonSendableKlass7() }
+  }
+}
+