A better fix for C++23 P2266R1

After 6d6a143913603b040c10a5db83c2103557899011 "Address some of the sprintf in
vcl/source/fontsubset/cff.cxx", --with-latest-c++ builds that pick up a C++23
compiler that implements
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2266r1.html> "P2266R1:
Simpler implicit move" started to fail with something like

> vcl/source/fontsubset/cff.cxx:2061:16: error: no viable conversion from returned value of type 'char[64]' to function return type 'OString'
>         return aDefaultGlyphName;
>                ^~~~~~~~~~~~~~~~~
[...]
> include/rtl/string.hxx:313:5: note: candidate constructor [with T = char[64]] not viable: expects an lvalue for 1st argument
>     OString( T& value, typename libreoffice_internal::NonConstCharArrayDetector< T, libreoffice_internal::Dummy >::Type = libreoffice_internal::Dummy() )
>     ^

etc.  So I figured there should be something better than
433ab39b2175bdadb4916373cd2dc8e1aabc08a5 "Adapt implicit OString return value
construction to C++23 P2266R1" (which this commit reverts, modulo its conflicts
in comphelper/source/xml/xmltools.cxx and
sc/source/filter/xcl97/XclExpChangeTrack.cxx) to address the underlying issue in
a way that keeps code that works up to C++20 also working in C++23.

(The fix is only relevant for non-explicit constructors that involve
NonConstCharArrayDetector and non-const lvalue references, not for other
functions involving those.  OUString has a similar constructor but which is
explicit, and OUStringBuffer doesn't have any similar constructors at all, so
this only affects OString and OStringBuffer constructors.)

Change-Id: I31cf16b9507899f5999243f8467dfa24bc94c5ac
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/142455
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/sal/qa/rtl/strings/nonconstarray.cxx b/sal/qa/rtl/strings/nonconstarray.cxx
index 4b66e4e311c3..d759049582d0 100644
--- a/sal/qa/rtl/strings/nonconstarray.cxx
+++ b/sal/qa/rtl/strings/nonconstarray.cxx
@@ -12,11 +12,24 @@
 #include <cppunit/TestAssert.h>
 #include <cppunit/TestFixture.h>
 #include <cppunit/extensions/HelperMacros.h>
+#include <rtl/strbuf.hxx>
 #include <rtl/string.hxx>
 #include <rtl/ustring.hxx>
 
 namespace
 {
+OString returnOString()
+{
+    char buf[] = "foo\0bar";
+    return buf;
+}
+
+OStringBuffer returnOStringBuffer()
+{
+    char buf[] = "foo\0bar";
+    return buf;
+}
+
 class Test : public CppUnit::TestFixture
 {
 private:
@@ -81,10 +94,17 @@ private:
         }
     }
 
+    void testP2266R3()
+    {
+        CPPUNIT_ASSERT_EQUAL(OString("foo"), returnOString());
+        CPPUNIT_ASSERT_EQUAL(OString("foo"), returnOStringBuffer().makeStringAndClear());
+    }
+
     CPPUNIT_TEST_SUITE(Test);
     CPPUNIT_TEST(testOString);
     CPPUNIT_TEST(testOUStringChar);
     CPPUNIT_TEST(testOUStringChar16t);
+    CPPUNIT_TEST(testP2266R3);
     CPPUNIT_TEST_SUITE_END();
 };