[CSApply] Key path dynamic member lookup argument is Sendable only if its captures are

lib/Sema/CSApply.cpp

@@ -2556,18 +2556,39 @@ namespace {
     /// Build an implicit argument for keypath based dynamic lookup,
     /// which consists of KeyPath expression and a single component.
     ///
-    /// \param argType The type of the keypath subscript argument.
+    /// \param paramType The type of the keypath subscript parameter
+    ///                  this argument is passed to.
     /// \param dotLoc The location of the '.' preceding member name.
     /// \param memberLoc The locator to be associated with new argument.
-    Expr *buildKeyPathDynamicMemberArgExpr(Type argType, SourceLoc dotLoc,
+    Expr *buildKeyPathDynamicMemberArgExpr(Type paramType, SourceLoc dotLoc,
                                            ConstraintLocator *memberLoc) {
       using Component = KeyPathExpr::Component;
       auto *anchor = getAsExpr(memberLoc->getAnchor());
 
       auto makeKeyPath = [&](ArrayRef<Component> components) -> Expr * {
+        Type keyPathTy = paramType;
+
+        // If parameter of a dynamic member lookup is `& Sendable` type
+        // we need to check key path captures to determine whether the
+        // argument could be `& Sendable` as well or not.
+        if (paramType->isExistentialType() && paramType->isSendableType()) {
+          auto allCapturesAreSendable = [&](const Component &component) {
+            auto *argList = component.getArgs();
+            if (!argList)
+              return true;
+
+            return llvm::all_of(*argList, [&](const auto &arg) {
+              return solution.getResolvedType(arg.getExpr())->isSendableType();
+            });
+          };
+
+          if (!llvm::all_of(components, allCapturesAreSendable))
+            keyPathTy = paramType->getSuperclass();
+        }
+
         auto *kp = KeyPathExpr::createImplicit(ctx, /*backslashLoc*/ dotLoc,
                                                components, anchor->getEndLoc());
-        kp->setType(argType);
+        kp->setType(keyPathTy);
         cs.cacheExprTypes(kp);
 
         // See whether there's an equivalent ObjC key path string we can produce
@@ -2576,7 +2597,7 @@ namespace {
         return kp;
       };
 
-      Type keyPathTy = argType;
+      Type keyPathTy = paramType;
       if (auto *existential = keyPathTy->getAs<ExistentialType>()) {
         keyPathTy = existential->getSuperclass();
         assert(isKnownKeyPathType(keyPathTy));
@@ -3687,6 +3708,8 @@ namespace {
       if (!argExpr)
         return nullptr;
 
+      solution.recordSingleArgMatchingChoice(cs.getConstraintLocator(expr));
+
       // Build an argument list.
       auto *argList =
           ArgumentList::forImplicitSingle(ctx, ctx.Id_dynamicMember, argExpr);

test/Concurrency/sendable_keypaths.swift

@@ -258,3 +258,22 @@ do {
   // TODO(rdar://125948508): This shouldn't be ambiguous (@Sendable version should be preferred)
   let _: () -> Void = forward(Test.fn) // expected-error {{conflicting arguments to generic parameter 'T' ('@Sendable () -> ()' vs. '() -> Void')}}
 }
+
+// https://github.com/swiftlang/swift/issues/77105
+do {
+  @dynamicMemberLookup
+  struct S<T> {
+    subscript<U>(dynamicMember keyPath: KeyPath<T, U> & Sendable) -> U {
+      fatalError()
+    }
+  }
+
+  struct Foo {
+    subscript<T>(bar bar: T) -> Int { 42 }
+  }
+
+  func test(s: S<Foo>) {
+    _ = s[bar: NonSendable()]
+    // expected-warning@-1 {{type 'KeyPath<Foo, Int>' does not conform to the 'Sendable' protocol; this is an error in the Swift 6 language mode}}
+  }
+}