[PATCH] PR9804 - __is_signed conflicts with libstdc++

[• chatur01]

Summary

Extends the hackish recovery in r130399 to deal with the case of bool being hidden behind a typedef.

Fixes PR9804

[rsmith]

What is the motivation for this? Every version of libstdc++ I can find uses the keyword bool and not a typedef.

[• chatur01]

In D9157#159383, @rsmith wrote:

What is the motivation for this? Every version of libstdc++ I can find uses the keyword bool and not a typedef.

Sorry for the late reply Richard... I've also lost the previous thread I had with. I originally suggested the motivation was for cases like,

typedef bool BOOL;

class Foo {
    static const BOOL __is_signed;
  };

#include <map>

int main() {}

But as you pointed out, this is ill-formed. I found a smaller example, which I'm confident is well-formed yet rejected by Clang,

typedef bool Bool;
#define bool Bool

#include <algorithm>

int main(void) {}

I see the following errors

In file included from blentest.c:4:
In file included from /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/algorithm:61:
In file included from /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/stl_algobase.h:63:
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/ext/numeric_traits.h:63:25: error: expected member name or ';' after declaration specifiers
      static const bool __is_signed = __glibcxx_signed(_Value);
      ~~~~~~~~~~~~~~~~~ ^
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/ext/numeric_traits.h:74:50: error: expected unqualified-id
    const bool __numeric_traits_integer<_Value>::__is_signed;
                                                 ^
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/ext/numeric_traits.h:106:25: error: expected member name or ';' after declaration specifiers
      static const bool __is_signed = true;
      ~~~~~~~~~~~~~~~~~ ^
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/ext/numeric_traits.h:115:51: error: expected unqualified-id
    const bool __numeric_traits_floating<_Value>::__is_signed;
                                                  ^
In file included from blentest.c:4:
In file included from /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/algorithm:61:
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/stl_algobase.h:941:49: error: expected unqualified-id
         && !__gnu_cxx::__numeric_traits<_ValueType1>::__is_signed
                                                       ^
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/stl_algobase.h:946:45: error: non-type template argument of type 'Bool' (aka 'bool') is not an integral constant expression
      return std::__lexicographical_compare<__simple>::__lc(__first1, __last1,
                                            ^~~~~~~~
6 errors generated.

The only restrictions I can see stated in the C++ standard (N4296, 2014-11-19) are in section 17.6.4.3.1, and they only cover that you're not allowed to #define or #undef names declared in a standard library header, or override, final and some other bits about attributes. bool is a fundamental type, so I think my example above should be accepted.

I think reopening PR9804 for this was incorrect, it looks quite similar, but perhaps I should open a new PR for this issue. Do you agree?

Thank you for your time. :)

lib/Parse/ParseDecl.cpp

     case tok::kw___is_signed:
       // GNU libstdc++ 4.4 uses __is_signed as an identifier, but Clang
       // typically treats it as a trait. If we see __is_signed as it appears
       // in libstdc++, e.g.,
       //
       //   static const bool __is_signed;
       //
       // then treat __is_signed as an identifier rather than as a keyword.
-      if (DS.getTypeSpecType() == TST_bool &&
-          DS.getTypeQualifiers() == DeclSpec::TQ_const &&
-          DS.getStorageClassSpec() == DeclSpec::SCS_static)
-        TryKeywordIdentFallback(true);
-
-      // We're done with the declaration-specifiers.
-      goto DoneWithDeclSpec;
-
+      switch (DS.getTypeSpecType()) {
+      case TST_typename:
+        {
+          auto SeenType = Sema::GetTypeFromParser(DS.getRepAsType());
+          if (!SeenType->isBooleanType())
+            goto DoneWithDeclSpec;
+          // fall through
+        }
+      case TST_bool:
+        if (DS.getTypeQualifiers() == DeclSpec::TQ_const &&
+            DS.getStorageClassSpec() == DeclSpec::SCS_static)
+          TryKeywordIdentFallback(true);
+        // fall through
+      default:
+        goto DoneWithDeclSpec;
+      }
       // typedef-name
     case tok::kw___super:
     case tok::kw_decltype:
     case tok::identifier: {
       // This identifier can only be a typedef name if we haven't already seen
       // a type-specifier.  Without this check we misparse:
       //  typedef int X; struct Y { short X; };  as 'short int'.
       if (DS.hasTypeSpecifier())

test/SemaCXX/libstdcxx_is_pod_hack.cpp

 
 // Another, similar egregious hack for __is_signed, which is a type
 // trait in Embarcadero's compiler but is used as an identifier in
 // libstdc++.
 struct test_is_signed {
   static const bool __is_signed = true; // expected-warning {{keyword '__is_signed' will be made available as an identifier}}
 };
 
+// The case where `bool` above is hidden behind a typedef is tested
+// in libstdcxx_is_signed_hack.cpp
+
 bool check_signed = test_is_signed::__is_signed;
 
 template<bool B> struct must_be_true {};
 template<> struct must_be_true<false>;
 
 void foo() {
   bool b = __is_pod(int);
   must_be_true<__is_pod(int)> mbt;

test/SemaCXX/libstdcxx_is_signed_hack.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// libstdc++ 4.4 uses __is_signed as an identifier, while Clang treats it
+// as a keyword for the __is_signed type trait. Cope with this conflict
+// via some hackish recovery: if we see a declaration of the form
+//
+// static const T __is_signed
+//
+// where T is bool or a typedef that reduces to bool, then we stop treating
+// __is_signed as a keyword and instead treat it as an identifier.
+
+typedef bool BOOL;
+struct test_is_signed_typedef {
+  static const BOOL __is_signed = true; // expected-warning {{keyword '__is_signed' will be made available as an identifier}}
+};