[gcc r11-9435] libstdc++: Fix overconstrained std::string constructor [PR103919]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:4c64143f32642d22590959704e2ec6c686d745ff

commit r11-9435-g4c64143f32642d22590959704e2ec6c686d745ff
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.
            [!_GLIBCXX_USE_CXX11_ABI] (basic_string(const T&, size_t, size_t)):
            Likewise.
            * testsuite/21_strings/basic_string/cons/char/103919.cc: New test.
    
    (cherry picked from commit 6aa0859afaf28f4fb13121352225bc5877e02a44)

Diff:
---
 libstdc++-v3/include/bits/basic_string.h           |  6 ++-
 .../21_strings/basic_string/cons/char/103919.cc    | 43 ++++++++++++++++++++++
 2 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 5d09f774e03..e6fa6eb2354 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -648,7 +648,8 @@ _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>>>
 	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) { }
@@ -3743,7 +3744,8 @@ _GLIBCXX_END_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>>>
 	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
+}