[clang-tidy] Fix fix-it overlaps in readability-static-definition-in-anonymous-namespace (#86599)

Because check emitted multiple warnings for every template instance
fix-it couldn't be applied due to overlaps.

Using TK_IgnoreUnlessSpelledInSource and restricting check to C++ only.

Author: PiotrZSL

clang-tools-extra/clang-tidy/readability/StaticDefinitionInAnonymousNamespaceCheck.h

@@ -24,6 +24,12 @@ class StaticDefinitionInAnonymousNamespaceCheck : public ClangTidyCheck {
       : ClangTidyCheck(Name, Context) {}
   void registerMatchers(ast_matchers::MatchFinder *Finder) override;
   void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+    return LangOpts.CPlusPlus;
+  }
+  std::optional<TraversalKind> getCheckTraversalKind() const override {
+    return TK_IgnoreUnlessSpelledInSource;
+  }
 };
 
 } // namespace clang::tidy::readability

clang-tools-extra/docs/ReleaseNotes.rst

@@ -253,6 +253,11 @@ Changes in existing checks
   mode by resolving symbolic links to header files. Fixed handling of Hungarian
   Prefix when configured to `LowerCase`.
 
+- Improved :doc:`readability-static-definition-in-anonymous-namespace
+  <clang-tidy/checks/readability/static-definition-in-anonymous-namespace>`
+  check by resolving fix-it overlaps in template code by disregarding implicit
+  instances.
+
 Removed checks
 ^^^^^^^^^^^^^^
 

clang-tools-extra/test/clang-tidy/checkers/readability/static-definition-in-anonymous-namespace.cpp

@@ -51,6 +51,17 @@ static int c = 1;
 } // namespace deep_inner
 } // namespace inner
 
+template<typename T>
+static void printTemplate(T&&) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:13: warning: 'printTemplate' is a static definition in anonymous namespace; static is redundant here [readability-static-definition-in-anonymous-namespace]
+// CHECK-FIXES: {{^}}void printTemplate(T&&) {}
+
+void testTemplate() {
+  printTemplate(5);
+  printTemplate(5U);
+  printTemplate("some string");
+}
+
 } // namespace
 
 namespace N {