[Sema] @IBAction implies @objc.

Per the core team discussion, @IBAction is tied to the Objective-C
runtime and should imply @objc. This was an oversight when the
attribute was added pre-1.0. Fixes rdar://problem/24750631.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -2607,7 +2607,7 @@ ERROR(objc_enum_case_multi,none,
       "'@objc' enum case declaration defines multiple enum cases with the same Objective-C name", ())
 
 // If you change this, also change enum ObjCReason
-#define OBJC_ATTR_SELECT "select{marked @_cdecl|marked dynamic|marked @objc|marked @IBOutlet|marked @NSManaged|a member of an @objc protocol|implicitly @objc|an @objc override|an implementation of an @objc requirement}"
+#define OBJC_ATTR_SELECT "select{marked @_cdecl|marked dynamic|marked @objc|marked @IBOutlet|marked @IBAction|marked @NSManaged|a member of an @objc protocol|implicitly @objc|an @objc override|an implementation of an @objc requirement}"
 
 ERROR(objc_invalid_on_var,none,
       "property cannot be %" OBJC_ATTR_SELECT "0 "

lib/Sema/TypeCheckDecl.cpp

@@ -2035,11 +2035,13 @@ static Optional<ObjCReason> shouldMarkAsObjC(TypeChecker &TC,
   // explicitly declared @objc.
   if (VD->getAttrs().hasAttribute<ObjCAttr>())
     return ObjCReason::ExplicitlyObjC;
-  // dynamic, @IBOutlet and @NSManaged imply @objc.
+  // dynamic, @IBOutlet, @IBAction, and @NSManaged imply @objc.
   else if (VD->getAttrs().hasAttribute<DynamicAttr>())
     return ObjCReason::ExplicitlyDynamic;
   else if (VD->getAttrs().hasAttribute<IBOutletAttr>())
     return ObjCReason::ExplicitlyIBOutlet;
+  else if (VD->getAttrs().hasAttribute<IBActionAttr>())
+    return ObjCReason::ExplicitlyIBAction;
   else if (VD->getAttrs().hasAttribute<NSManagedAttr>())
     return ObjCReason::ExplicitlyNSManaged;
   // A member of an @objc protocol is implicitly @objc.

lib/Sema/TypeChecker.h

@@ -404,6 +404,7 @@ enum class ObjCReason {
   ExplicitlyDynamic,
   ExplicitlyObjC,
   ExplicitlyIBOutlet,
+  ExplicitlyIBAction,
   ExplicitlyNSManaged,
   MemberOfObjCProtocol,
   ImplicitlyObjC,

test/attr/attr_ibaction.swift

@@ -65,13 +65,21 @@ protocol CP2 : class { }
 
   // Protocol types
   @IBAction func action7(_: P1) {} // expected-error{{argument to @IBAction method cannot have non-object type 'P1'}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
+  // expected-note@-2{{protocol 'P1' is not '@objc'}}
   @IBAction func action8(_: CP1) {} // expected-error{{argument to @IBAction method cannot have non-object type 'CP1'}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
+  // expected-note@-2{{protocol 'CP1' is not '@objc'}}
   @IBAction func action9(_: OP1) {}
   @IBAction func action10(_: P1?) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action11(_: CP1?) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action12(_: OP1?) {}
   @IBAction func action13(_: P1!) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action14(_: CP1!) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action15(_: OP1!) {}
 
   // Class metatype
@@ -86,18 +94,27 @@ protocol CP2 : class { }
 
   // Protocol types
   @IBAction func action21(_: P1.Type) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action22(_: CP1.Type) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action23(_: OP1.Type) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
   @IBAction func action24(_: P1.Type?) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action25(_: CP1.Type?) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action26(_: OP1.Type?) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
   @IBAction func action27(_: P1.Type!) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  //expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action28(_: CP1.Type!) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  //expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
   @IBAction func action29(_: OP1.Type!) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
 
   // Other bad cases
   @IBAction func action30(_: S) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
+  // expected-note@-2{{Swift structs cannot be represented in Objective-C}}
   @IBAction func action31(_: E) {} // expected-error{{argument to @IBAction method cannot have non-object type}}
-
+  // expected-error@-1{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
+  // expected-note@-2{{non-'@objc' enums cannot be represented in Objective-C}}
   init() { }
 }

test/attr/attr_objc.swift

@@ -1606,6 +1606,20 @@ class HasIBOutlet {
   // CHECK-LABEL: {{^}}  @IBOutlet var badOutlet: PlainStruct
 }
 
+//===---
+//===--- @IBAction implies @objc
+//===---
+
+// CHECK-LABEL: {{^}}class HasIBAction {
+class HasIBAction {
+  @IBAction func goodAction(_ sender: AnyObject?) { }
+  // CHECK: {{^}}  @IBAction @objc func goodAction(_ sender: AnyObject?) {
+
+  @IBAction func badAction(_ sender: PlainStruct?) { }
+  // expected-error@-1{{argument to @IBAction method cannot have non-object type 'PlainStruct?'}}
+  // expected-error@-2{{method cannot be marked @IBAction because the type of the parameter cannot be represented in Objective-C}}
+}
+
 //===---
 //===--- @NSManaged implies @objc
 //===---