[Code completion] For a Selector argument, provide #selector(<#objc method#>).

Author: DougGregor

lib/IDE/CodeCompletion.cpp

@@ -1985,7 +1985,7 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
     Builder.addRightParen();
   }
 
-  void addPoundSelector() {
+  void addPoundSelector(bool needPound) {
     // #selector is only available when the Objective-C runtime is.
     if (!Ctx.LangOpts.EnableObjCInterop) return;
 
@@ -1994,7 +1994,10 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
                                   CodeCompletionResult::ResultKind::Keyword,
                                   SemanticContextKind::ExpressionSpecific,
                                   ExpectedTypes);
-    Builder.addTextChunk("selector");
+    if (needPound)
+      Builder.addTextChunk("#selector");
+    else
+      Builder.addTextChunk("selector");
     Builder.addLeftParen();
     Builder.addSimpleTypedParameter("@objc method", /*isVarArg=*/false);
     Builder.addRightParen();
@@ -3152,6 +3155,19 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
     }
 
     addValueLiteralCompletions();
+
+    // If the expected type is ObjectiveC.Selector, add #selector.
+    if (Ctx.LangOpts.EnableObjCInterop) {
+      for (auto T : ExpectedTypes) {
+        if (auto structDecl = T->getStructOrBoundGenericStruct()) {
+          if (structDecl->getName() == Ctx.Id_Selector &&
+              structDecl->getParentModule()->getName() == Ctx.Id_ObjectiveC) {
+            addPoundSelector(/*needPound=*/true);
+            break;
+          }
+        }
+      }
+    }
   }
 
   struct LookupByName : public swift::VisibleDeclConsumer {
@@ -4584,7 +4600,7 @@ void CodeCompletionCallbacksImpl::doneParsing() {
 
   case CompletionKind::AfterPound: {
     Lookup.addPoundAvailable(ParentStmtKind);
-    Lookup.addPoundSelector();
+    Lookup.addPoundSelector(/*needPound=*/false);
     break;
   }
   }

test/IDE/complete_pound_selector.swift

@@ -1,10 +1,26 @@
-// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=AFTER_POUND | FileCheck %s
+// RUN: %target-swift-ide-test(mock-sdk: %clang-importer-sdk) -code-completion -source-filename %s -code-completion-token=AFTER_POUND | FileCheck -check-prefix=CHECK-AFTER_POUND %s
+
+// RUN: %target-swift-ide-test(mock-sdk: %clang-importer-sdk) -code-completion -source-filename %s -code-completion-token=SELECTOR_ARG1 | FileCheck -check-prefix=CHECK-SELECTOR_ARG %s
+
+// RUN: %target-swift-ide-test(mock-sdk: %clang-importer-sdk) -code-completion -source-filename %s -code-completion-token=SELECTOR_ARG2 | FileCheck -check-prefix=CHECK-SELECTOR_ARG %s
 
 // REQUIRES: objc_interop
 
+import Foundation
+
 {
   if ##^AFTER_POUND^#
 }
 
-// CHECK: Keyword/ExprSpecific:               available({#Platform...#}, *); name=available(Platform..., *)
-// CHECK: Keyword/ExprSpecific:               selector({#@objc method#}); name=selector(@objc method)
+func selectorArg1(obj: NSObject) {
+  obj.doSelector(#^SELECTOR_ARG1^#
+}
+
+func selectorArg2(obj: NSObject) {
+  obj.messageSomeObject(obj, selector:#^SELECTOR_ARG2^#
+}
+
+// CHECK-AFTER_POUND: Keyword/ExprSpecific:               available({#Platform...#}, *); name=available(Platform..., *)
+// CHECK-AFTER_POUND: Keyword/ExprSpecific:               selector({#@objc method#}); name=selector(@objc method)
+
+// CHECK-SELECTOR_ARG: Keyword/ExprSpecific:               #selector({#@objc method#}); name=#selector(@objc method)

test/Inputs/clang-importer-sdk/usr/include/Foundation.h

@@ -992,3 +992,7 @@ int variadicFunc2(int A, ...);
 
 extern NSString *NSGlobalConstant;
 extern void NSGlobalFunction(void);
+
+@interface NSObject (Selectors)
+-(void)messageSomeObject:(nonnull id)object selector:(SEL)selector;
+@end