[gcc r12-6267] libstdc++: Fix overconstrained std::string constructor [PR103919]

[Jonathan Wakely]

https://gcc.gnu.org/g:6aa0859afaf28f4fb13121352225bc5877e02a44

commit r12-6267-g6aa0859afaf28f4fb13121352225bc5877e02a44
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jan 5 15:16:33 2022 +0000

    libstdc++: Fix overconstrained std::string constructor [PR103919]
    
    The C++17 basic_string(const T&, size_t, size_t) constructor is
    overconstrained, so it can't be used for a NTBS and a temporary string
    gets constructed (potentially allocating memory). There is no
    corresponding constructor taking an NTBS, so no need to disambiguate
    from it. Accepting an NTBS avoids the temporary (and potential
    allocation) and is what the standard requires.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/103919
            * include/bits/basic_string.h (basic_string(const T&, size_t, size_t)):
            Relax constraints on string_view parameter.
            * include/bits/cow_string.h (basic_string(const T&, size_t, size_t)):
            Likewise.
            * testsuite/21_strings/basic_string/cons/char/103919.cc: New test.

Diff:
---
 libstdc++-v3/include/bits/basic_string.h           |  3 +-
 libstdc++-v3/include/bits/cow_string.h             |  3 +-
 .../21_strings/basic_string/cons/char/103919.cc    | 43 ++++++++++++++++++++++
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 463cef25b6e..a91ba5114b1 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -766,7 +766,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __n   The number of characters to copy from __t.
        *  @param  __a   Allocator to use.
        */
-      template<typename _Tp, typename = _If_sv<_Tp, void>>
+      template<typename _Tp,
+	       typename = enable_if_t<is_convertible_v<const _Tp&, __sv_type>>>
 	_GLIBCXX20_CONSTEXPR
 	basic_string(const _Tp& __t, size_type __pos, size_type __n,
 		     const _Alloc& __a = _Alloc())
diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index 8d0b7727be4..84aab2f33c6 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -690,7 +690,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __n   The number of characters to copy from __t.
        *  @param  __a   Allocator to use.
        */
-      template<typename _Tp, typename = _If_sv<_Tp, void>>
+      template<typename _Tp,
+	       typename = enable_if_t<is_convertible_v<const _Tp&, __sv_type>>>
 	basic_string(const _Tp& __t, size_type __pos, size_type __n,
 		     const _Alloc& __a = _Alloc())
 	: basic_string(_S_to_string_view(__t).substr(__pos, __n), __a) { }
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/103919.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/103919.cc
new file mode 100644
index 00000000000..94400e319ff
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/103919.cc
@@ -0,0 +1,43 @@
+// { dg-do run { target c++17 } }
+
+#include <string>
+#include <new>
+#include <cstdlib>
+#include <cstring>
+#include <testsuite_hooks.h>
+
+std::size_t counter = 0;
+
+void* operator new(std::size_t n)
+{
+  counter += n;
+  return std::malloc(n);
+}
+
+void operator delete(void* p)
+{
+  std::free(p);
+}
+
+void operator delete(void* p, std::size_t)
+{
+  std::free(p);
+}
+
+int main()
+{
+  const char* str = "A string that is considerably longer than the SSO buffer";
+
+  // PR libstdc++/103919
+  // basic_string(const T&, size_t, size_t) constructor is overconstrained
+  counter = 0;
+  std::string s(str, 2, 6);
+  VERIFY( s == "string" );
+#if _GLIBCXX_USE_CXX11_ABI
+  // The string fits in the SSO buffer, so nothing is allocated.
+  VERIFY( counter == 0 );
+#else
+  // The COW string allocates a string rep and 7 chars.
+  VERIFY( counter < std::strlen(str) );
+#endif
+}