Magic number checker shouldn't warn on user defined string literals

Fixes a false positive brought up by PR40633.

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

@@ -122,10 +122,21 @@ bool MagicNumbersCheck::isConstant(const MatchFinder::MatchResult &Result,
   return llvm::any_of(
       Result.Context->getParents(ExprResult),
       [&Result](const DynTypedNode &Parent) {
-        return isUsedToInitializeAConstant(Result, Parent) ||
-               // Ignore this instance, because this match reports the location
-               // where the template is defined, not where it is instantiated.
-               Parent.get<SubstNonTypeTemplateParmExpr>();
+        if (isUsedToInitializeAConstant(Result, Parent))
+          return true;
+
+        // Ignore this instance, because this match reports the location
+        // where the template is defined, not where it is instantiated.
+        if (Parent.get<SubstNonTypeTemplateParmExpr>())
+          return true;
+
+        // Don't warn on string user defined literals:
+        // std::string s = "Hello World"s;
+        if (const auto *UDL = Parent.get<UserDefinedLiteral>())
+          if (UDL->getLiteralOperatorKind() == UserDefinedLiteral::LOK_String)
+            return true;
+
+        return false;
       });
 }
 

clang-tools-extra/test/clang-tidy/readability-magic-numbers-userliteral.cpp

@@ -0,0 +1,16 @@
+// RUN: %check_clang_tidy -std=c++14-or-later %s readability-magic-numbers %t --
+
+namespace std {
+  class string {};
+  using size_t = decltype(sizeof(int));
+  string operator ""s(const char *, std::size_t);
+  int operator "" s(unsigned long long);
+}
+
+void UserDefinedLiteral() {
+  using std::operator ""s;
+  "Hello World"s;
+  const int i = 3600s;
+  int j = 3600s;
+  // CHECK-MESSAGES: :[[@LINE-1]]:11: warning: 3600s is a magic number; consider replacing it with a named constant [readability-magic-numbers]
+}