Suppress reserved user defined literal warning within namespace std.

[EricWF]

Summary

Currently Clang warns if a UDL is defined with a name reserved for future standardization. However this warning can cause problems with the standard library headers. Most of the time the STL headers are treated as system headers and this warning is ignored.

Unfortunately when the STL headers are not treated as system headers this diagnostic issues a dozen or so false positives in *every* TU that includes an STL header. Arguably these diagnostics are false positives because they warn that UDL's in namespace std are supposed to be using the reserved names.

Although rare there are users who effectively use the libc++ headers with -Wsystem-headers (including other LLVM projects). Instead of forcing them to add "-Wno-user-defined-literals" this patch fixes the diagnostic in clang by preventing the emission of false positives.

lib/Sema/SemaDeclCXX.cpp

            diag::err_literal_operator_default_argument)
         << Param->getDefaultArgRange();
       break;
     }
   }
 
   StringRef LiteralName
     = FnDecl->getDeclName().getCXXLiteralIdentifier()->getName();
-  if (LiteralName[0] != '_') {
+  if (LiteralName[0] != '_' && !FnDecl->isInStdNamespace()) {
     // C++11 [usrlit.suffix]p1:
     //   Literal suffix identifiers that do not start with an underscore
     //   are reserved for future standardization.
     Diag(FnDecl->getLocation(), diag::warn_user_literal_reserved)
       << NumericLiteralParser::isValidUDSuffix(getLangOpts(), LiteralName);
   }
 
   return false;

test/CXX/lex/lex.literal/lex.ext/p10.cpp

 // RUN: %clang_cc1 -std=c++11 -verify %s
 
 using size_t = decltype(sizeof(int));
 void operator "" wibble(const char *); // expected-warning {{user-defined literal suffixes not starting with '_' are reserved; no literal will invoke this operator}}
 void operator "" wibble(const char *, size_t); // expected-warning {{user-defined literal suffixes not starting with '_' are reserved; no literal will invoke this operator}}
 
 template<typename T>
 void f() {
   // A program containing a reserved ud-suffix is ill-formed.
   123wibble; // expected-error {{invalid suffix 'wibble'}}
   123.0wibble; // expected-error {{invalid suffix 'wibble'}}
   const char *p = ""wibble; // expected-error {{invalid suffix on literal; C++11 requires a space between literal and identifier}} expected-error {{expected ';'}}
   const char *q = R"x("hello")x"wibble; // expected-error {{invalid suffix on literal; C++11 requires a space between literal and identifier}} expected-error {{expected ';'}}
 }
+
+namespace std {
+  void operator "" stdwibble(const char*);
+}