libstdc++: Fix ambiguous expression in std::array<T, 0>::front() [PR110139]

For 32-bit targets using -pedantic (or using Clang) makes the expression
_M_elems[0] ambiguous.  The overloaded operator[] that we want to call
has a size_t parameter, but 0 is type ptrdiff_t for many ILP32 targets,
so using the implicit conversion from _M_elems to T* and then
subscripting that is also viable.

Change the 0 to (size_type)0 and also make the conversion to T*
explicit, so that's it's not viable here. The latter change requires a
static_cast in data() where we really do want to convert _M_elems to a
pointer.

libstdc++-v3/ChangeLog:

	PR libstdc++/110139
	* include/std/array (__array_traits<T, 0>::operator T*()): Make
	conversion operator explicit.
	(array::front): Use size_type as subscript operand.
	(array::data): Use static_cast to make conversion explicit.
	* testsuite/23_containers/array/element_access/110139.cc: New
	test.

diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
index 27354d9e3fb040e7ea020f0dd01870a91b617045..70280c1beeb45d2fa8bef7f9aa26b99e05b67918 100644 (file)
--- a/libstdc++-v3/include/std/array
+++ b/libstdc++-v3/include/std/array
@@ -69,7 +69,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
        // Conversion to a pointer produces a null pointer.
        __attribute__((__always_inline__))
-       constexpr operator _Tp*() const noexcept { return nullptr; }
+       constexpr explicit operator _Tp*() const noexcept { return nullptr; }
      };
 
      using _Is_swappable = true_type;
@@ -240,7 +240,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       front() noexcept
       {
        __glibcxx_requires_nonempty();
-       return _M_elems[0];
+       return _M_elems[(size_type)0];
       }
 
       [[__nodiscard__]]
@@ -250,7 +250,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 201402L
        __glibcxx_requires_nonempty();
 #endif
-       return _M_elems[0];
+       return _M_elems[(size_type)0];
       }
 
       [[__nodiscard__]]
@@ -274,12 +274,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       [[__nodiscard__, __gnu__::__const__, __gnu__::__always_inline__]]
       _GLIBCXX17_CONSTEXPR pointer
       data() noexcept
-      { return _M_elems; }
+      { return static_cast<pointer>(_M_elems); }
 
       [[__nodiscard__]]
       _GLIBCXX17_CONSTEXPR const_pointer
       data() const noexcept
-      { return _M_elems; }
+      { return static_cast<const_pointer>(_M_elems); }
     };
 
 #if __cpp_deduction_guides >= 201606

diff --git a/libstdc++-v3/testsuite/23_containers/array/element_access/110139.cc b/libstdc++-v3/testsuite/23_containers/array/element_access/110139.cc
new file mode 100644 (file)
index 0000000..6f09aac
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/array/element_access/110139.cc
@@ -0,0 +1,5 @@
+// { dg-options "-pedantic" }
+// { dg-do compile { target c++11 } }
+#include <array>
+template class std::array<int, 0>;
+// { dg-bogus "ambiguous overload" "" { target *-*-* } 0 }