Make ObjCSelector's lookupDirect return a TinyPtrVector

Returning a MutableArrayRef caused us to
accidentally mutate the lookup table when
re-arranging decls for diagnostics.

Author: hamishknight

include/swift/AST/Decl.h

@@ -4117,8 +4117,8 @@ class ClassDecl final : public NominalTypeDecl {
   ///
   /// \param isInstance Whether we are looking for an instance method
   /// (vs. a class method).
-  MutableArrayRef<AbstractFunctionDecl *> lookupDirect(ObjCSelector selector,
-                                                       bool isInstance);
+  TinyPtrVector<AbstractFunctionDecl *> lookupDirect(ObjCSelector selector,
+                                                     bool isInstance);
 
   /// Record the presence of an @objc method with the given selector.
   void recordObjCMethod(AbstractFunctionDecl *method, ObjCSelector selector);

lib/AST/NameLookup.cpp

@@ -1309,7 +1309,7 @@ void ClassDecl::createObjCMethodLookup() {
   });
 }
 
-MutableArrayRef<AbstractFunctionDecl *>
+TinyPtrVector<AbstractFunctionDecl *>
 ClassDecl::lookupDirect(ObjCSelector selector, bool isInstance) {
   if (!ObjCMethodLookup) {
     createObjCMethodLookup();
@@ -1325,7 +1325,7 @@ ClassDecl::lookupDirect(ObjCSelector selector, bool isInstance) {
     stored.Generation = ctx.getCurrentGeneration();
   }
 
-  return { stored.Methods.begin(), stored.Methods.end() };
+  return stored.Methods;
 }
 
 void ClassDecl::recordObjCMethod(AbstractFunctionDecl *method,

lib/Sema/TypeCheckDeclObjC.cpp

@@ -2073,7 +2073,7 @@ bool swift::diagnoseUnintendedObjCMethodOverrides(SourceFile &sf) {
 }
 
 /// Retrieve the source file for the given Objective-C member conflict.
-static MutableArrayRef<AbstractFunctionDecl *>
+static TinyPtrVector<AbstractFunctionDecl *>
 getObjCMethodConflictDecls(const SourceFile::ObjCMethodConflict &conflict) {
   ClassDecl *classDecl = std::get<0>(conflict);
   ObjCSelector selector = std::get<1>(conflict);
@@ -2150,22 +2150,23 @@ bool swift::diagnoseObjCMethodConflicts(SourceFile &sf) {
 
     // If the first method is in an extension and the second is not, swap them
     // so the primary diagnostic is on the extension method.
+    MutableArrayRef<AbstractFunctionDecl *> methodsRef(methods);
     if (isa<ExtensionDecl>(methods[0]->getDeclContext()) &&
         !isa<ExtensionDecl>(methods[1]->getDeclContext())) {
-      std::swap(methods[0], methods[1]);
+      std::swap(methodsRef[0], methodsRef[1]);
 
     // Within a source file, use our canonical ordering.
     } else if (methods[0]->getParentSourceFile() ==
                methods[1]->getParentSourceFile() &&
               !ordering(methods[0], methods[1])) {
-      std::swap(methods[0], methods[1]);
+      std::swap(methodsRef[0], methodsRef[1]);
     }
 
     // Otherwise, fall back to the order in which the declarations were type
     // checked.
 
     auto originalMethod = methods.front();
-    auto conflictingMethods = methods.slice(1);
+    auto conflictingMethods = methodsRef.slice(1);
 
     auto origDiagInfo = getObjCMethodDiagInfo(originalMethod);
     for (auto conflictingDecl : conflictingMethods) {