[clang-tidy] Fix a false positive about C++17 deduced class template types in unused-using-decl check.

[merge_guards_bot]

Unit tests: pass. 62415 tests passed, 0 failed and 839 were skipped.

clang-tidy: pass.

clang-format: fail. Please format your changes with clang-format by running git-clang-format HEAD^ or applying this patch.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

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[gribozavr2]

clang-tools-extra/test/clang-tidy/checkers/misc-unused-using-decls-cxx17.cpp

10

s/hint/guide/

[hokein]

address comments.

[merge_guards_bot]

Unit tests: pass. 62415 tests passed, 0 failed and 839 were skipped.

clang-tidy: pass.

clang-format: fail. Please format your changes with clang-format by running git-clang-format HEAD^ or applying this patch.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

Pre-merge checks is in beta. Report issue. Please join beta or enable it for your project.

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

for (const auto &Arg : TemplateArgs) {
     if (InnerMatcher.matches(Arg, Finder, &ArgBuilder)) {
       Matched = true;
       Result.addMatch(ArgBuilder);
     }
   }
   *Builder = std::move(Result);
   return Matched;
 }
+
+AST_MATCHER_P(DeducedTemplateSpecializationType, refsToTemplatedDecl,
+              clang::ast_matchers::internal::Matcher<NamedDecl>, DeclMatcher) {
+  if (const auto *TD = Node.getTemplateName().getAsTemplateDecl())
+    return DeclMatcher.matches(*TD, Finder, Builder);
+  return false;
+}
 } // namespace
 
 // A function that helps to tell whether a TargetDecl in a UsingDecl will be
 // checked. Only variable, function, function template, class template, class,
 // enum declaration and enum constant declaration are considered.
 static bool ShouldCheckDecl(const Decl *TargetDecl) {
   return isa<RecordDecl>(TargetDecl) || isa<ClassTemplateDecl>(TargetDecl) ||
          isa<FunctionDecl>(TargetDecl) || isa<VarDecl>(TargetDecl) ||
          isa<FunctionTemplateDecl>(TargetDecl) || isa<EnumDecl>(TargetDecl) ||
          isa<EnumConstantDecl>(TargetDecl);
 }
 
 void UnusedUsingDeclsCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(usingDecl(isExpansionInMainFile()).bind("using"), this);
   auto DeclMatcher = hasDeclaration(namedDecl().bind("used"));
   Finder->addMatcher(loc(enumType(DeclMatcher)), this);
   Finder->addMatcher(loc(recordType(DeclMatcher)), this);
   Finder->addMatcher(loc(templateSpecializationType(DeclMatcher)), this);
+  Finder->addMatcher(loc(deducedTemplateSpecializationType(
+                         refsToTemplatedDecl(namedDecl().bind("used")))),
+                     this);
   Finder->addMatcher(declRefExpr().bind("used"), this);
   Finder->addMatcher(callExpr(callee(unresolvedLookupExpr().bind("used"))),
                      this);
   Finder->addMatcher(
       callExpr(hasDeclaration(functionDecl(
           forEachTemplateArgument(templateArgument().bind("used"))))),
       this);
   Finder->addMatcher(loc(templateSpecializationType(forEachTemplateArgument(

clang-tools-extra/test/clang-tidy/checkers/misc-unused-using-decls-cxx17.cpp

+// RUN: %check_clang_tidy -std=c++17-or-later %s misc-unused-using-decls %t --
+// -- -fno-delayed-template-parsing -isystem %S/Inputs/
+
+namespace ns {
+
+template <typename T> class Foo {
+public:
+  Foo(T);
+};
+// Deduction hint (CTAD)
+template <typename T> Foo(T t) -> Foo<T>;
+
+template <typename T> class Bar {
+public:
+  Bar(T);
+};
+
+template <typename T> class Unused {};
+
+} // namespace ns
+
+using ns::Bar;
+using ns::Foo;
+using ns::Unused; // Unused
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: using decl 'Unused' is unused
+// CHECK-FIXES: {{^}}// Unused
+
+void f() {
+  Foo(123);
+  Bar(1);
+}