[clang-tidy] Fix readability-redundant-string-init for c++17/c++2a

Summary:
`readability-redundant-string-init` was one of several clang-tidy checks documented as failing for C++17. (The failure mode in C++17 is that it changes `std::string Name = ""`; to `std::string Name = Name;`, which actually compiles but crashes at run-time.)

Analyzing the AST with `clang -Xclang -ast-dump` showed that the outer `CXXConstructExprs` that previously held the correct SourceRange were being elided in C++17/2a, but the containing `VarDecl` expressions still had all the relevant information. So this patch changes the fix to get its source ranges from `VarDecl`.

It adds one test `std::string g = "u", h = "", i = "uuu", j = "", k;` to confirm proper warnings and fixit replacements in a single `DeclStmt` where some strings require replacement and others don't. The readability-redundant-string-init.cpp and readability-redundant-string-init-msvc.cpp tests now pass for C++11/14/17/2a.

Reviewers: gribozavr, etienneb, alexfh, hokein, aaron.ballman, gribozavr2

Patch by: poelmanc

Subscribers: NoQ, MyDeveloperDay, Eugene.Zelenko, dylanmckay, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D69238

clang-tools-extra/clang-tidy/modernize/UseOverrideCheck.cpp

@@ -20,11 +20,13 @@ namespace modernize {
 UseOverrideCheck::UseOverrideCheck(StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),
       IgnoreDestructors(Options.get("IgnoreDestructors", false)),
+      AllowOverrideAndFinal(Options.get("AllowOverrideAndFinal", false)),
       OverrideSpelling(Options.get("OverrideSpelling", "override")),
       FinalSpelling(Options.get("FinalSpelling", "final")) {}
 
 void UseOverrideCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
   Options.store(Opts, "IgnoreDestructors", IgnoreDestructors);
+  Options.store(Opts, "AllowOverrideAndFinal", AllowOverrideAndFinal);
   Options.store(Opts, "OverrideSpelling", OverrideSpelling);
   Options.store(Opts, "FinalSpelling", FinalSpelling);
 }
@@ -103,7 +105,8 @@ void UseOverrideCheck::check(const MatchFinder::MatchResult &Result) {
   bool OnlyVirtualSpecified = HasVirtual && !HasOverride && !HasFinal;
   unsigned KeywordCount = HasVirtual + HasOverride + HasFinal;
 
-  if (!OnlyVirtualSpecified && KeywordCount == 1)
+  if ((!OnlyVirtualSpecified && KeywordCount == 1) ||
+      (!HasVirtual && HasOverride && HasFinal && AllowOverrideAndFinal))
     return; // Nothing to do.
 
   std::string Message;
@@ -113,8 +116,9 @@ void UseOverrideCheck::check(const MatchFinder::MatchResult &Result) {
     Message = "annotate this function with '%0' or (rarely) '%1'";
   } else {
     StringRef Redundant =
-        HasVirtual ? (HasOverride && HasFinal ? "'virtual' and '%0' are"
-                                              : "'virtual' is")
+        HasVirtual ? (HasOverride && HasFinal && !AllowOverrideAndFinal
+                          ? "'virtual' and '%0' are"
+                          : "'virtual' is")
                    : "'%0' is";
     StringRef Correct = HasFinal ? "'%1'" : "'%0'";
 
@@ -211,7 +215,7 @@ void UseOverrideCheck::check(const MatchFinder::MatchResult &Result) {
     Diag << FixItHint::CreateInsertion(InsertLoc, ReplacementText);
   }
 
-  if (HasFinal && HasOverride) {
+  if (HasFinal && HasOverride && !AllowOverrideAndFinal) {
     SourceLocation OverrideLoc = Method->getAttr<OverrideAttr>()->getLocation();
     Diag << FixItHint::CreateRemoval(
         CharSourceRange::getTokenRange(OverrideLoc, OverrideLoc));

clang-tools-extra/clang-tidy/modernize/UseOverrideCheck.h

@@ -26,6 +26,7 @@ public:
 
 private:
   const bool IgnoreDestructors;
+  const bool AllowOverrideAndFinal;
   const std::string OverrideSpelling;
   const std::string FinalSpelling;
 };

clang-tools-extra/docs/ReleaseNotes.rst

@@ -164,6 +164,12 @@ Improvements to clang-tidy
   Finds non-static member functions that can be made ``const``
   because the functions don't use ``this`` in a non-const way.
 
+- Improved :doc:`modernize-use-override
+  <clang-tidy/checks/modernize-use-override>` check.
+
+  The check now supports the ``AllowOverrideAndFinal`` option to eliminate
+  conflicts with ``gcc -Wsuggest-override`` or ``gcc -Werror=suggest-override``.
+
 Improvements to include-fixer
 -----------------------------
 

clang-tools-extra/docs/clang-tidy/checks/modernize-use-override.rst

@@ -27,6 +27,14 @@ Options
 
    If set to non-zero, this check will not diagnose destructors. Default is `0`.
 
+.. option:: AllowOverrideAndFinal
+
+   If set to non-zero, this check will not diagnose ``override`` as redundant
+   with ``final``. This is useful when code will be compiled by a compiler with
+   warning/error checking flags requiring ``override`` explicitly on overriden
+   members, such as ``gcc -Wsuggest-override``/``gcc -Werror=suggest-override``.
+   Default is `0`.
+
 .. option:: OverrideSpelling
 
    Specifies a macro to use instead of ``override``. This is useful when