Thank you for testing.

I agree that the build errors with Clang should be handled in a different report.

IIUC, the risk of applying the suggested patch is low. Maybe, it won't work with all versions of autoconf (I haven't tested, and future changes in autoconf might cause that the un-documented workaround might no longer work). So, I pushed the change to the stable branch:
https://hg.savannah.gnu.org/hgweb/octave/rev/fe201b17eb86

In a worst case scenario, we could still revert it for a patch release.

Marking as ready for test.

The attached patch skips the configure test that is broken for C++20.

If no one opposes, I'd like to push that to the stable branch to avoid potential issues during the next year (until Octave 10 will be released).

@Dmitri: Does that change work for you?


(file #55761)

Googling around -- it looks like there are some c++20 incompatibilities with c++17 (and before).

const char* blah = u8"Foo 😀 Bar" // fine in c++17, error in c++20

cited as one of them.

Dmitri.
--

> First, if multiple "-std=..." options are present, what does GCC > do?  What would clang++ or any other compiler do?

I think the last one (on the right) overwrites the previous ones.
So for some reason "g++ -std=gnu++20" fails the C++11 test:

configure:11200: checking for g++ -std=gnu++20 option to enable C++11 features
configure:11436: g++ -std=gnu++20  -c -Wall -ggdb3 -O2 -march=native -mtune=native -mavx -mavx2 -flto=auto  conftest.cpp >&5
conftest.cpp: In function 'int main()':
conftest.cpp:173:8: warning: unused variable 'a1' [-Wunused-variable]
  173 |   auto a1 = 6538;
      |        ^~
conftest.cpp:175:8: warning: unused variable 'a3' [-Wunused-variable]
  175 |   auto a3 = "String literal";
      |        ^~
conftest.cpp:177:16: warning: unused variable 'a4' [-Wunused-variable]
  177 |   decltype(a2) a4 = 34895.034;
      |                ^~
conftest.cpp:181:9: warning: unused variable 'sa' [-Wunused-variable]
  181 |   short sa[cxx11test::get_val()] = { 0 };
      |         ^~
conftest.cpp:185:23: warning: unused variable 'il' [-Wunused-variable]
  185 |   cxx11test::testinit il = { 4323, 435234.23544 };
      |                       ^~
conftest.cpp:211:8: warning: unused variable 'match' [-Wunused-variable]
  211 |   bool match = std::regex_search(testmatch, filename_regex);
      |        ^~~~~
conftest.cpp:215:35: warning: unused variable 'size' [-Wunused-variable]
  215 |   cxx11test::int_array::size_type size = array.size();
      |                                   ^~~~
conftest.cpp:220:25: warning: empty parentheses were disambiguated as a function declaration [-Wvexing-parse]
  220 |   cxx11test::delegate d2();
      |                         ^~
conftest.cpp:220:25: note: remove parentheses to default-initialize a variable
  220 |   cxx11test::delegate d2();
      |                         ^~
      |                         --
conftest.cpp:220:25: note: or replace parentheses with braces to value-initialize a variable
conftest.cpp:229:9: warning: unused variable 'c' [-Wunused-variable]
  229 |   char *c = nullptr;
      |         ^
conftest.cpp:237:22: error: invalid conversion from 'const char8_t*' to 'const char*' [-fpermissive]
  237 |   char const *utf8 = u8"UTF-8 string \u2500";
      |                      ^~~~~~~~~~~~~~~~~~~~~~~
      |                      |
      |                      const char8_t*
conftest.cpp:237:15: warning: unused variable 'utf8' [-Wunused-variable]
  237 |   char const *utf8 = u8"UTF-8 string \u2500";
      |               ^~~~
conftest.cpp:238:19: warning: unused variable 'utf16' [-Wunused-variable]
  238 |   char16_t const *utf16 = u"UTF-8 string \u2500";
      |                   ^~~~~
conftest.cpp:239:19: warning: unused variable 'utf32' [-Wunused-variable]
  239 |   char32_t const *utf32 = U"UTF-32 string \u2500";
      |                   ^~~~~
configure:11436: $? = 1
configure: failed program was:

So it adds "-std=c++11" which later would require "-std=c++17"
because of the pmr.

Dmitri.
--

Attached.

Dmitri.
--


(file #55718)

I compiled octave 10 (hgid 359f922209d4) with 'CXX=g++ -std=gnu++20', yet
+verbose+
octave:4> _octave_config_info_("CXX")
ans = g++ -std=gnu++20 -std=gnu++11 -std=gnu++17
-verbose-

That looks wrong.

Dmitri.
--