[clang-tidy] Fix FP with readability-redundant-string-init for default arguments

[etienneb]

Summary

Clang-tidy is reporting a warning of redundant string initialisation
on a string parameter initialized with empty string.

See bug: 27087

The reported example is:

#include <string>
void fn(std::string a = "");

[alexfh]

Awesome! Thanks for the fix!

clang-tidy/readability/RedundantStringInitCheck.cpp

void RedundantStringInitCheck::registerMatchers(MatchFinder *Finder) {
   // Examples:
   //     string foo = "";
   //     string bar("");
   Finder->addMatcher(
       namedDecl(varDecl(hasType(cxxRecordDecl(hasName("basic_string"))),
                         hasInitializer(
                             expr(anyOf(EmptyStringCtorExpr,
                                        EmptyStringCtorExprWithTemporaries))
-                            .bind("expr"))))
+                            .bind("expr"))),
+                unless(parmVarDecl()))
           .bind("decl"),
       this);
 }
 
 void RedundantStringInitCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *CtorExpr = Result.Nodes.getNodeAs<Expr>("expr");
   const auto *Decl = Result.Nodes.getNodeAs<NamedDecl>("decl");
   diag(CtorExpr->getExprLoc(), "redundant string initialization")
       << FixItHint::CreateReplacement(CtorExpr->getSourceRange(),
                                       Decl->getName());
 }
 
 } // namespace readability
 } // namespace tidy
 } // namespace clang

test/clang-tidy/readability-redundant-string-init.cpp

void k() {
   std::string a = "", b = "", c = "";
   // CHECK-MESSAGES: [[@LINE-1]]:15: warning: redundant string initialization
   // CHECK-MESSAGES: [[@LINE-2]]:23: warning: redundant string initialization
   // CHECK-MESSAGES: [[@LINE-3]]:31: warning: redundant string initialization
   // CHECK-FIXES: std::string a, b, c;
 
   std::string d = "u", e = "u", f = "u";
 }
+
+// These cases should not generate warnings.
+extern void Param1(std::string param = "");
+extern void Param2(const std::string& param = "");
+void Param3(std::string param = "") {}
+void Param4(STRING param = "") {}
+