[committed] libstdc++: Disable std::string{, _view} construction from nullptr (P2166R1)

[Jonathan Wakely via Gcc-patches]

Implement this C++23 feature. Because construction from a null pointer
is undefined, we can implement it for C++11 and up, turning undefined
behaviour into a compilation error.

libstdc++-v3/ChangeLog:

        * include/bits/basic_string.h (basic_string(nullptr_t)): Define
        as deleted.
        (operator=(nullptr_t)): Likewise.
        * include/bits/cow_string.h (basic_string(nullptr_t)): Likewise.
        (operator=(nullptr_t)): Likewise.
        * include/std/string_view (basic_string_view(nullptr_t)):
        Likewise.
        * testsuite/21_strings/basic_string/cons/char/nullptr.cc: New test.
        * testsuite/21_strings/basic_string_view/cons/char/nonnull.cc:
        Change dg-warning to dg-error.
        * testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc:
        Likewise.

Tested powerpc64le-linux. Committed to trunk.


commit cf876562c592193732f869e9f96034a42d0fad89
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Oct 1 21:27:24 2021

    libstdc++: Disable std::string{,_view} construction from nullptr (P2166R1)
    
    Implement this C++23 feature. Because construction from a null pointer
    is undefined, we can implement it for C++11 and up, turning undefined
    behaviour into a compilation error.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h (basic_string(nullptr_t)): Define
            as deleted.
            (operator=(nullptr_t)): Likewise.
            * include/bits/cow_string.h (basic_string(nullptr_t)): Likewise.
            (operator=(nullptr_t)): Likewise.
            * include/std/string_view (basic_string_view(nullptr_t)):
            Likewise.
            * testsuite/21_strings/basic_string/cons/char/nullptr.cc: New test.
            * testsuite/21_strings/basic_string_view/cons/char/nonnull.cc:
            Change dg-warning to dg-error.
            * testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc:
            Likewise.

diff --git a/libstdc++-v3/include/bits/basic_string.h 
b/libstdc++-v3/include/bits/basic_string.h
index 24c454d863a..68c388408f0 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -623,6 +623,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
          _M_construct(__str.begin(), __str.end());
       }
 
+      basic_string(nullptr_t) = delete;
+      basic_string& operator=(nullptr_t) = delete;
 #endif // C++11
 
       /**
diff --git a/libstdc++-v3/include/bits/cow_string.h 
b/libstdc++-v3/include/bits/cow_string.h
index ba4a8cc2e98..f0540128364 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -667,6 +667,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        else
          _M_dataplus._M_p = _S_construct(__str.begin(), __str.end(), __a);
       }
+
+      basic_string(nullptr_t) = delete;
+      basic_string& operator=(nullptr_t) = delete;
 #endif // C++11
 
       /**
diff --git a/libstdc++-v3/include/std/string_view 
b/libstdc++-v3/include/std/string_view
index d8cbee9bee0..996b03f9346 100644
--- a/libstdc++-v3/include/std/string_view
+++ b/libstdc++-v3/include/std/string_view
@@ -168,6 +168,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // C++23
 #endif // C++20
 
+      basic_string_view(nullptr_t) = delete;
+
       constexpr basic_string_view&
       operator=(const basic_string_view&) noexcept = default;
 
diff --git 
a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/nullptr.cc 
b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/nullptr.cc
new file mode 100644
index 00000000000..fdb24aeeb89
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/nullptr.cc
@@ -0,0 +1,4 @@
+// { dg-do compile { target c++11 } }
+#include <string>
+
+std::string s = nullptr; // { dg-error "deleted" "P2166R1" }
diff --git 
a/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/nonnull.cc 
b/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/nonnull.cc
index b33af088a3d..f42f95e2fdd 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/nonnull.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/nonnull.cc
@@ -25,5 +25,5 @@ test01()
 {
   std::string_view s((const char*)nullptr); // { dg-warning "\\\[-Wnonnull" }
   std::string_view t((char*)nullptr);      // { dg-warning "\\\[-Wnonnull" }
-  std::string_view u(nullptr);             // { dg-warning "\\\[-Wnonnull" }
+  std::string_view u(nullptr);             // { dg-error "deleted" }
 }
diff --git 
a/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc 
b/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc
index 4c1013177eb..e480f7c4923 100644
--- 
a/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc
+++ 
b/libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc
@@ -25,5 +25,5 @@ test01()
 {
   std::wstring_view s((const wchar_t*)nullptr);        // { dg-warning 
"\\\[-Wnonnull" }
   std::wstring_view t((wchar_t*)nullptr);      // { dg-warning "\\\[-Wnonnull" 
}
-  std::wstring_view u(nullptr);                        // { dg-warning 
"\\\[-Wnonnull" }
+  std::wstring_view u(nullptr);                        // { dg-error "deleted" 
}
 }