[libc++][Android] Rename user NS to User to avoid conflict with struct

[rprichard]

Summary

Bionic's sys/user.h declares a "struct user". The header tends to be
included, and when it is, it conflicts with "namespace user". Rename
user to User.

[rprichard]

The capitalized namespace User spelling was already used in libcxx/test/std/algorithms/alg.nonmodifying/alg.search/search.pass.cpp (D72640).

In NDK r25b, including <system_error> includes sys/user.h via this chain:

/usr/include/c++/v1/system_error
/usr/include/c++/v1/stdexcept
/usr/include/c++/v1/iosfwd
/usr/include/c++/v1/wchar.h
/usr/local/include/wchar.h
/usr/include/wchar.h
/usr/include/time.h
/usr/include/sys/time.h
/usr/include/sys/select.h
/usr/include/signal.h
/usr/include/sys/ucontext.h
/usr/include/sys/user.h

[ldionne]

That's pretty terrible (note that Darwin also has its share of similar sins). Would you be able to report this as a bug to Android? Perhaps the struct can be removed when a macro is provided and they can start some sort of deprecation path?

I won't block this, but it would be really nice to at least get the ball rolling on fixing this issue in the system headers.

[danalbert]

Would you be able to report this as a bug to Android? Perhaps the struct can be removed when a macro is provided and they can start some sort of deprecation path?

We can add a macro, but I doubt we'll ever default to hiding this struct. Every time we've tried to make our headers more hygenic our users have told us they'd prefer it if we didn't because it breaks their builds (in trivially fixable ways, but they don't care; they don't want it to break regardless).

Our plan since then has been to ignore these types of problems until modules can solve them properly.

libcxx/test/std/diagnostics/syserr/syserr.errcode/syserr.errcode.constructors/lwg3629.pass.cpp

 // Regression test for https://github.com/llvm/llvm-project/issues/57614
 
 int make_error_code; // It's important that this comes before <system_error>
 
 #include <system_error>
 #include <cassert>
 #include <type_traits>
 
-namespace user {
+namespace User {
   enum Err {};
 
   std::error_code make_error_code(Err) { return std::error_code(42, std::generic_category()); }
 }
 
 namespace std {
   template <>
-  struct is_error_code_enum<user::Err> : true_type {};
+  struct is_error_code_enum<User::Err> : true_type {};
 }
 
 int main(int, char**) {
-  std::error_code e((user::Err()));
+  std::error_code e((User::Err()));
   assert(e.value() == 42);
   assert(e.category() == std::generic_category());
 
   return 0;
 }

libcxx/test/std/diagnostics/syserr/syserr.errcode/syserr.errcode.modifiers/lwg3629.pass.cpp

 // Regression test for https://github.com/llvm/llvm-project/issues/57614
 
 int make_error_code; // It's important that this comes before <system_error>
 
 #include <system_error>
 #include <cassert>
 #include <type_traits>
 
-namespace user {
+namespace User {
   enum Err {};
 
   std::error_code make_error_code(Err) { return std::error_code(42, std::generic_category()); }
 }
 
 namespace std {
   template <>
-  struct is_error_code_enum<user::Err> : true_type {};
+  struct is_error_code_enum<User::Err> : true_type {};
 }
 
 int main(int, char**) {
   std::error_code e;
-  e = user::Err();
+  e = User::Err();
   assert(e.value() == 42);
   assert(e.category() == std::generic_category());
 
   return 0;
 }

libcxx/test/std/diagnostics/syserr/syserr.errcondition/syserr.errcondition.constructors/lwg3629.pass.cpp

 // Regression test for https://github.com/llvm/llvm-project/issues/57614
 
 int make_error_condition; // It's important that this comes before <system_error>
 
 #include <system_error>
 #include <cassert>
 #include <type_traits>
 
-namespace user {
+namespace User {
   enum Err {};
 
   std::error_condition make_error_condition(Err) { return std::error_condition(42, std::generic_category()); }
 }
 
 namespace std {
   template <>
-  struct is_error_condition_enum<user::Err> : true_type {};
+  struct is_error_condition_enum<User::Err> : true_type {};
 }
 
 int main(int, char**) {
-  std::error_condition e((user::Err()));
+  std::error_condition e((User::Err()));
   assert(e.value() == 42);
   assert(e.category() == std::generic_category());
 
   return 0;
 }

libcxx/test/std/diagnostics/syserr/syserr.errcondition/syserr.errcondition.modifiers/lwg3629.pass.cpp

 int make_error_condition; // It's important that this comes before <system_error>
 
 #include <system_error>
 #include <cassert>
 #include <type_traits>
 
 #include "test_macros.h"
 
-namespace user {
+namespace User {
   enum Err {};
 
   std::error_condition make_error_condition(Err) { return std::error_condition(42, std::generic_category()); }
 }
 
 namespace std {
   template <>
-  struct is_error_condition_enum<user::Err> : true_type {};
+  struct is_error_condition_enum<User::Err> : true_type {};
 }
 
 int main(int, char**) {
   std::error_condition e;
-  e = user::Err();
+  e = User::Err();
   assert(e.value() == 42);
   assert(e.category() == std::generic_category());
   return 0;
 }