libstdc++: Allow unique_ptr<Incomplete[]>::operator[] [PR 101236]

PR libstdc++/101236 shows that LLVM depends on being able to use
unique_ptr<T[]>::operator[] when T is incomplete. This is undefined, but
previously worked with libstdc++. When I added the conditional noexcept
to that operator we started to diagnose the incomplete type.

This change restores support for that case, by making the noexcept
condition check that the type is complete before checking whether
indexing on the pointer can throw.  A workaround for PR c++/101239 is
needed to avoid a bogus error where G++ fails to do SFINAE on the
ill-formed p[n] expression and gets an ICE. Instead of checking that the
p[n] expression is valid in the trailing-return-type, we only check that
the element_type is complete.

libstdc++-v3/ChangeLog:

	PR libstdc++/101236
	* include/bits/unique_ptr.h (unique_ptr<T[], D>::operator[]):
	Fail gracefully if element_type is incomplete.
	* testsuite/20_util/unique_ptr/cons/incomplete.cc: Clarify that
	the standard doesn't require this test to work for array types.
	* testsuite/20_util/unique_ptr/lwg2762.cc: Check that incomplete
	types can be used with array specialization.
	* testsuite/20_util/unique_ptr/101236.cc: New test.

libstdc++-v3/include/bits/unique_ptr.h

@@ -491,6 +491,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  = __and_< is_base_of<_Tp, _Up>,
 		    __not_<is_same<__remove_cv<_Tp>, __remove_cv<_Up>>> >;
 
+      // This checks whether p[n] is noexcept, but fails gracefully when
+      // element_type is incomplete. The standard requires a complete type
+      // for unique_ptr<T[], D>, but we try to support it anyway (PR 101236).
+      template<typename _Ptr, typename _Elt>
+	static constexpr auto
+	_S_nothrow_deref(size_t __n)
+	-> decltype(sizeof(_Elt) != 0) // PR c++/101239
+	{ return noexcept(std::declval<_Ptr>()[__n]); }
+
+      template<typename _Ptr, typename _Elt>
+	static constexpr bool
+	_S_nothrow_deref(...)
+	{ return false; }
+
     public:
       using pointer	  = typename __uniq_ptr_impl<_Tp, _Dp>::pointer;
       using element_type  = _Tp;
@@ -655,7 +669,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       /// Access an element of owned array.
       typename std::add_lvalue_reference<element_type>::type
       operator[](size_t __i) const
-      noexcept(noexcept(std::declval<pointer>()[std::declval<size_t&>()]))
+      noexcept(_S_nothrow_deref<pointer, element_type>(0))
       {
 	__glibcxx_assert(get() != pointer());
 	return get()[__i];

libstdc++-v3/testsuite/20_util/unique_ptr/101236.cc

@@ -0,0 +1,13 @@
+// { dg-do compile { target c++11 } }
+#include <memory>
+
+struct Incomplete;
+struct pr101236
+{
+  // The standard says "T shall be a complete type" for unique_ptr<T[], D>
+  // so this is a GCC extension.
+  std::unique_ptr<Incomplete[]> p;
+
+  Incomplete& f() { return p[0]; }
+};
+struct Incomplete { };

libstdc++-v3/testsuite/20_util/unique_ptr/cons/incomplete.cc

@@ -24,9 +24,17 @@ struct Incomplete;
 void f(void** p)
 {
   ::new (p[0]) std::unique_ptr<Incomplete>();
-  ::new (p[1]) std::unique_ptr<Incomplete[]>();
 
   // PR libstdc++/87704
-  ::new (p[2]) std::unique_ptr<Incomplete>(nullptr);
-  ::new (p[3]) std::unique_ptr<Incomplete[]>(nullptr);
+  ::new (p[1]) std::unique_ptr<Incomplete>(nullptr);
+}
+
+// The standard says "T shall be a complete type" for unique_ptr<T[], D>
+// so this is a GCC extension.
+void f_array(void** p)
+{
+  ::new (p[0]) std::unique_ptr<Incomplete[]>();
+
+  // PR libstdc++/87704
+  ::new (p[1]) std::unique_ptr<Incomplete[]>(nullptr);
 }

libstdc++-v3/testsuite/20_util/unique_ptr/lwg2762.cc

@@ -32,12 +32,24 @@ template<typename T, bool Nothrow>
   using UPtr = std::unique_ptr<T, deleter<Nothrow>>;
 
 // noexcept-specifier depends on the pointer type
-static_assert( noexcept(*std::declval<UPtr<int, true>&>()), "" );
-static_assert( ! noexcept(*std::declval<UPtr<int, false>&>()), "" );
+static_assert( noexcept(*std::declval<UPtr<int, true>&>()), "LWG 2762" );
+static_assert( ! noexcept(*std::declval<UPtr<int, false>&>()), "LWG 2762" );
 
 // This has always been required, even in C++11.
-static_assert( noexcept(std::declval<UPtr<int, false>&>().operator->()), "" );
+static_assert( noexcept(std::declval<std::unique_ptr<long>>().operator->()),
+	       "operator-> is always noexcept" );
+static_assert( noexcept(std::declval<UPtr<int, false>&>().operator->()),
+	       "operator-> is always noexcept" );
 
-// This is not required by the standard
-static_assert( noexcept(std::declval<UPtr<int[], true>&>()[0]), "" );
-static_assert( ! noexcept(std::declval<UPtr<int[], false>&>()[0]), "" );
+// This is not required by the standard, but we make it depend on the pointer.
+static_assert( noexcept(std::declval<std::unique_ptr<long[]>>()[0]), "QoI" );
+static_assert( noexcept(std::declval<UPtr<int[], true>&>()[0]), "QoI" );
+static_assert( ! noexcept(std::declval<UPtr<int[], false>&>()[0]), "QoI" );
+
+// This is forbidden by the standard ("T shall be a complete type")
+// but we try to support it anyway, see PR libstdc++/101236.
+struct Incomplete;
+static_assert( ! noexcept(std::declval<UPtr<Incomplete[], true>>()[0]),
+	       "this would be noexcept if the type was complete");
+static_assert( ! noexcept(std::declval<UPtr<Incomplete[], false>>()[0]),
+	       "this would still be noexcept(false) if the type was complete");