Use C++14-style return type deduction in clang.

Summary:
Simplifies the C++11-style "-> decltype(...)" return-type deduction.

Note that you have to be careful about whether the function return type
is `auto` or `decltype(auto)`.  The difference is that bare `auto`
strips const and reference, just like lambda return type deduction.  In
some cases that's what we want (or more likely, we know that the return
type is a value type), but whenever we're wrapping a templated function
which might return a reference, we need to be sure that the return type
is decltype(auto).

No functional change.

Reviewers: bkramer, MaskRay, martong, shafik

Subscribers: martong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74423

clang/lib/AST/ASTImporter.cpp

@@ -204,9 +204,7 @@ namespace clang {
 
     // Wrapper for an overload set.
     template <typename ToDeclT> struct CallOverloadedCreateFun {
-      template <typename... Args>
-      auto operator()(Args &&... args)
-          -> decltype(ToDeclT::Create(std::forward<Args>(args)...)) {
+      template <typename... Args> decltype(auto) operator()(Args &&... args) {
         return ToDeclT::Create(std::forward<Args>(args)...);
       }
     };

clang/lib/StaticAnalyzer/Checkers/GCDAntipatternChecker.cpp

@@ -52,18 +52,16 @@ public:
                         BugReporter &BR) const;
 };
 
-auto callsName(const char *FunctionName)
-    -> decltype(callee(functionDecl())) {
+decltype(auto) callsName(const char *FunctionName) {
   return callee(functionDecl(hasName(FunctionName)));
 }
 
-auto equalsBoundArgDecl(int ArgIdx, const char *DeclName)
-    -> decltype(hasArgument(0, expr())) {
+decltype(auto) equalsBoundArgDecl(int ArgIdx, const char *DeclName) {
   return hasArgument(ArgIdx, ignoringParenCasts(declRefExpr(
                                  to(varDecl(equalsBoundNode(DeclName))))));
 }
 
-auto bindAssignmentToDecl(const char *DeclName) -> decltype(hasLHS(expr())) {
+decltype(auto) bindAssignmentToDecl(const char *DeclName) {
   return hasLHS(ignoringParenImpCasts(
                          declRefExpr(to(varDecl().bind(DeclName)))));
 }

clang/lib/StaticAnalyzer/Checkers/OSObjectCStyleCast.cpp

@@ -55,8 +55,7 @@ static void emitDiagnostics(const BoundNodes &Nodes,
     CE->getSourceRange());
 }
 
-static auto hasTypePointingTo(DeclarationMatcher DeclM)
-    -> decltype(hasType(pointerType())) {
+static decltype(auto) hasTypePointingTo(DeclarationMatcher DeclM) {
   return hasType(pointerType(pointee(hasDeclaration(DeclM))));
 }
 

clang/lib/StaticAnalyzer/Checkers/ObjCAutoreleaseWriteChecker.cpp

@@ -100,8 +100,7 @@ static inline std::vector<llvm::StringRef> toRefs(std::vector<std::string> V) {
   return std::vector<llvm::StringRef>(V.begin(), V.end());
 }
 
-static auto callsNames(std::vector<std::string> FunctionNames)
-    -> decltype(callee(functionDecl())) {
+static decltype(auto) callsNames(std::vector<std::string> FunctionNames) {
   return callee(functionDecl(hasAnyName(toRefs(FunctionNames))));
 }
 

clang/lib/StaticAnalyzer/Checkers/PointerSortingChecker.cpp

@@ -54,7 +54,7 @@ static void emitDiagnostics(const BoundNodes &Match, const Decl *D,
                      OS.str(), Location, Range);
 }
 
-auto callsName(const char *FunctionName) -> decltype(callee(functionDecl())) {
+decltype(auto) callsName(const char *FunctionName) {
   return callee(functionDecl(hasName(FunctionName)));
 }