[clang-tidy] Fix misc-unused-using-decls for user-defined literals

Current version complains unused using-declaration even if the target
user-defined literal is used.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D138204

Author: v1nh1shungry

clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.cpp

@@ -54,6 +54,7 @@ void UnusedUsingDeclsCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(loc(templateSpecializationType(forEachTemplateArgument(
                          templateArgument().bind("used")))),
                      this);
+  Finder->addMatcher(userDefinedLiteral().bind("used"), this);
   // Cases where we can identify the UsingShadowDecl directly, rather than
   // just its target.
   // FIXME: cover more cases in this way, as the AST supports it.
@@ -150,7 +151,11 @@ void UnusedUsingDeclsCheck::check(const MatchFinder::MatchResult &Result) {
       if (const auto *USD = dyn_cast<UsingShadowDecl>(ND))
         removeFromFoundDecls(USD->getTargetDecl()->getCanonicalDecl());
     }
+    return;
   }
+  // Check user-defined literals
+  if (const auto *UDL = Result.Nodes.getNodeAs<UserDefinedLiteral>("used"))
+    removeFromFoundDecls(UDL->getCalleeDecl());
 }
 
 void UnusedUsingDeclsCheck::removeFromFoundDecls(const Decl *D) {

clang-tools-extra/test/clang-tidy/checkers/misc/unused-using-decls.cpp

@@ -48,6 +48,7 @@ template <typename T> int UsedInTemplateFunc() { return 1; }
 void OverloadFunc(int);
 void OverloadFunc(double);
 int FuncUsedByUsingDeclInMacro() { return 1; }
+long double operator""_w(long double);
 
 class ostream {
 public:
@@ -106,6 +107,7 @@ using n::UnusedInstance; // UnusedInstance
 using n::UnusedFunc; // UnusedFunc
 // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: using decl 'UnusedFunc' is unused
 // CHECK-FIXES: {{^}}// UnusedFunc
+using n::operator""_w;
 using n::cout;
 using n::endl;
 
@@ -183,6 +185,7 @@ void g() {
   UsedInstance.i;
   UsedFunc();
   UsedTemplateFunc<int>();
+  1.5_w;
   cout << endl;
   Color2 color2;
   int t1 = Color3::Yellow;