[gcc r16-8112] libstdc++: Improve diagnostics for std::allocator<incomplete type>

[Jonathan Wakely]

https://gcc.gnu.org/g:f60dbb57967982f456f48d6f904246f73c6bc905

commit r16-8112-gf60dbb57967982f456f48d6f904246f73c6bc905
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Mar 9 12:15:19 2026 +0000

    libstdc++: Improve diagnostics for std::allocator<incomplete type>
    
    Using requires { sizeof(T); } in __new_allocator::allocate gives a
    better diagnostic when the static_assert fails.
    
    We can also reduce the total number of diagnostics due to invalid
    sizeof(T) and alignof(T) by using the same requires-expression to make
    the body of the function a discarded statement when the static_assert
    already failed. This fixes a regression in diagnostic quality due to
    making __new_allocator::allocate constexpr in r16-7271-g7197d0cce70525,
    which caused a cascade of seven errors instead of just one for
    std::allocator<incomplete-type>().allocate(1).
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/allocator.h (allocator::allocate): Use specific
            feature test macro for constexpr allocate and deallocate. Make
            consteval path a discarded statement if sizeof(T) is ill-formed.
            * include/bits/new_allocator.h (__new_allocator::allocate): Use
            requires-expression for static_cast. Make function body a
            discarded stament if sizeof(T) is ill-formed. Use if-constexpr
            for alignment checks.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/allocator.h     |  5 ++++-
 libstdc++-v3/include/bits/new_allocator.h | 22 +++++++++++++++++-----
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/bits/allocator.h b/libstdc++-v3/include/bits/allocator.h
index a5e115265911..9c22c805ebe7 100644
--- a/libstdc++-v3/include/bits/allocator.h
+++ b/libstdc++-v3/include/bits/allocator.h
@@ -188,11 +188,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif
       ~allocator() _GLIBCXX_NOTHROW { }
 
-#if __cplusplus > 201703L
+#if __cpp_constexpr_dynamic_alloc // >= C++20
       [[nodiscard,__gnu__::__always_inline__]]
       constexpr _Tp*
       allocate(size_t __n)
       {
+#if __cpp_concepts
+	if constexpr (requires { sizeof(_Tp); })
+#endif
 	if (std::__is_constant_evaluated())
 	  {
 	    if (__builtin_mul_overflow(__n, sizeof(_Tp), &__n))
diff --git a/libstdc++-v3/include/bits/new_allocator.h b/libstdc++-v3/include/bits/new_allocator.h
index 4fe67f99e5c0..fbe03e392aae 100644
--- a/libstdc++-v3/include/bits/new_allocator.h
+++ b/libstdc++-v3/include/bits/new_allocator.h
@@ -120,6 +120,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 # define _GLIBCXX_OPERATOR_DELETE ::operator delete
 #endif
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
+
       // NB: __n is permitted to be 0.  The C++ standard says nothing
       // about what the return value is when __n == 0.
       _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR _Tp*
@@ -128,9 +131,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 201103L
 	// _GLIBCXX_RESOLVE_LIB_DEFECTS
 	// 3308. std::allocator<void>().allocate(n)
+#if ! __cpp_concepts
 	static_assert(sizeof(_Tp) != 0, "cannot allocate incomplete types");
-#endif
+#else
+	static_assert(requires { sizeof(_Tp); },
+	  "cannot allocate incomplete types");
 
+	if constexpr (!requires { sizeof(_Tp); })
+	  return nullptr; // static_assert already failed
+	else
+#endif
+#endif
 	if (__builtin_expect(__n > this->_M_max_size(), false))
 	  {
 	    // _GLIBCXX_RESOLVE_LIB_DEFECTS
@@ -139,16 +150,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      std::__throw_bad_array_new_length();
 	    std::__throw_bad_alloc();
 	  }
-
 #if __cpp_aligned_new && __cplusplus >= 201103L
-	if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
+	else if constexpr (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
 	  {
 	    std::align_val_t __al = std::align_val_t(alignof(_Tp));
 	    return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n * sizeof(_Tp),
 							   __al));
 	  }
 #endif
-	return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n * sizeof(_Tp)));
+	else
+	  return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n * sizeof(_Tp)));
       }
 
       // __p is not permitted to be a null pointer.
@@ -162,7 +173,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif
 
 #if __cpp_aligned_new && __cplusplus >= 201103L
-	if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
+	if constexpr (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
 	  {
 	    _GLIBCXX_OPERATOR_DELETE(_GLIBCXX_SIZED_DEALLOC(__p, __n),
 				     std::align_val_t(alignof(_Tp)));
@@ -172,6 +183,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	_GLIBCXX_OPERATOR_DELETE(_GLIBCXX_SIZED_DEALLOC(__p, __n));
       }
 
+#pragma GCC diagnostic pop
 #undef _GLIBCXX_SIZED_DEALLOC
 #undef _GLIBCXX_OPERATOR_DELETE
 #undef _GLIBCXX_OPERATOR_NEW