[clang-tidy] Don't run misc-unused-using-decls check in C++17.

[hokein]

Summary

It introduces some false positives which are non-trivial to fix.
Ignore running the check in C++17.

[xazax.hun]

clang-tidy/misc/UnusedUsingDeclsCheck.cpp

35

Isn't the check for getLangOpts().CPlusPlus2a redundant? Shouldn't it imply the C++17 flag?

[hokein]

Remove redundant CPlusPlus2a.

[hokein]

clang-tidy/misc/UnusedUsingDeclsCheck.cpp

35

Didn't notice it. Yeah, you are right. If CPlusCplus2a also indicates C++17.

[JonasToth]

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

1

why gnu++17 and not standard?

clang-tidy/misc/UnusedUsingDeclsCheck.cpp

 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) {
+  // FIXME: make the check support C++17 or later. The check relies on the fact
+  // that the used declarations are visited after the "using" declaration, but
+  // it is not ture in C++17's template argument deduction.
+  if (!getLangOpts().CPlusPlus || getLangOpts().CPlusPlus17)
+    return;
   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(declRefExpr().bind("used"), this);
   Finder->addMatcher(callExpr(callee(unresolvedLookupExpr().bind("used"))),
                      this);

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

+// RUN: %check_clang_tidy %s misc-unused-using-decls %t -- -- -fno-delayed-template-parsing -std=gnu++17
+
+namespace ns {
+template <typename K, typename V>
+class KV {
+public:
+  KV(K, V);
+};
+}
+
+using ns::KV;
+
+void f() {
+  KV(1, 2);
+}