[clang-tidy] Fix `readability-redundant-declaration` false positive for template friend declaration

[fwolff]

Summary

Fixes PR#48086. The problem is that the current matcher uses hasParent() to detect friend declarations, but for a template friend declaration, the immediate parent of the FunctionDecl is a FunctionTemplateDecl, not the FriendDecl. Therefore, I have replaced the matcher with hasAncestor().

[aaron.ballman]

Hmm, the test case you added passes without the patch applied: https://godbolt.org/z/T9TerMYGz

I think the issue is that the class needs to be an instantiated template type: https://godbolt.org/z/bhznPdsvf, and a second (interesting but contrived) test case would be:

template <typename>
struct Friendly {
  template <typename T>
  friend void generic_friend() {}
};

template <typename T>
void generic_friend();

int main() {
  Friendly<int> f;
  generic_friend<int>();
}

(We do NOT want to diagnose the second declaration of generic_friend as being redundant -- it is necessary to make the definition visible.)

[fwolff]

In D114299#3164127, @aaron.ballman wrote:

Hmm, the test case you added passes without the patch applied: https://godbolt.org/z/T9TerMYGz

You are right; I have fixed the test now, and I've also added your other example as a second test.

[aaron.ballman]

I'm a bit worried about using hasAncestor() (that has a tendency to match in surprising nested locations), but I can't come up with a test case that would be an issue, so this LGTM.

clang-tools-extra/clang-tidy/readability/RedundantDeclarationCheck.cpp

 }
 
 void RedundantDeclarationCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
       namedDecl(anyOf(varDecl(unless(isDefinition())),
                       functionDecl(unless(anyOf(
                           isDefinition(), isDefaulted(),
                           doesDeclarationForceExternallyVisibleDefinition(),
-                          hasParent(friendDecl()))))))
+                          hasAncestor(friendDecl()))))))
           .bind("Decl"),
       this);
 }
 
 void RedundantDeclarationCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *D = Result.Nodes.getNodeAs<NamedDecl>("Decl");
   const auto *Prev = D->getPreviousDecl();
   if (!Prev)

clang-tools-extra/test/clang-tidy/checkers/readability-redundant-declaration.cpp

 
 struct Friendly {
   friend void best_friend();
   friend void enemy();
 };
 
 void enemy();
 
+template <typename>
+struct TemplateFriendly {
+  template <typename T>
+  friend void generic_friend();
+};
+
+template <typename T>
+void generic_friend() {}
+
+TemplateFriendly<int> template_friendly;
+
+template <typename>
+struct TemplateFriendly2 {
+  template <typename T>
+  friend void generic_friend2() {}
+};
+
+template <typename T>
+void generic_friend2();
+
+void generic_friend_caller() {
+  TemplateFriendly2<int> f;
+  generic_friend2<int>();
+}
+
+
 namespace macros {
 #define DECLARE(x) extern int x
 #define DEFINE(x) extern int x; int x = 42
 DECLARE(test);
 DEFINE(test);
 // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: redundant 'test' declaration
 // CHECK-FIXES: {{^}}#define DECLARE(x) extern int x{{$}}
 // CHECK-FIXES: {{^}}#define DEFINE(x) extern int x; int x = 42{{$}}