New test failure with gcc 16 (possibly c++20 related)

[ellert]

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Description

Fedora rawhide recently updated gcc to version 16, which uses c++20 as default c++ version.
After this the gtest-roofit-roofitcore-testRooFuncWrapper started failing with the following error in root 6.38.00.

[ RUN      ] RooFuncWrapper/FactoryTest.NLLFit/RecursiveFraction
RooFitResult::isIdentical: final parameter a1 differs in value:	0.223611 vs.	0.22502	(-0.00626406)
RooFitResult::isIdentical: final parameter a1 differs in error:	0.122657 vs.	0.128778	(-0.0475309)
RooFitResult::isIdentical: final parameter a2 differs in value:	0.134579 vs.	0.132792	(0.0134604)
RooFitResult::isIdentical: final parameter a2 differs in error:	0.0497441 vs.	0.0571375	(-0.129397)
RooFitResult::isIdentical: final parameter sigma1 differs in value:	0.753925 vs.	0.747673	(0.00836104)
RooFitResult::isIdentical: final parameter sigma2 differs in error:	0.224634 vs.	0.220331	(0.019528)
RooFitResult::isIdentical: final parameter sigma3 differs in value:	2.09943 vs.	2.03224	(0.0330612)
RooFitResult::isIdentical: final parameter sigma3 differs in error:	1.14436 vs.	1.79891	(-0.363859)
RooFitResult::isIdentical: final parameter sigma4 differs in value:	2.15177 vs.	2.18057	(-0.0132066)
RooFitResult::isIdentical: final parameter sigma4 differs in error:	0.515142 vs.	0.725729	(-0.290173)
/builddir/build/BUILD/root-6.38.00-build/root-6.38.00/roofit/roofitcore/test/testRooFuncWrapper.cxx:215: Failure
Value of: resultAd->isIdenticalNoCov(*resultRef, tol, tol)
  Actual: false
Expected: true
[  FAILED  ] RooFuncWrapper/FactoryTest.NLLFit/RecursiveFraction, where GetParam() = 112-byte object <A0-F7 75-42 F4-55 00-00 11-00 00-00 00-00 00-00 11-00 00-00 00-00 00-00 6C-61 56-61 72-00 00-2E D0-F3 75-42 F4-55 00-00 00-00 00-00 00-00 00-00 C0-E4 5C-01 F4-55 00-00 90-C4 5C-01 F4-55 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 D0-8F 5C-01 F4-55 00-00 E0-B1 5C-01 F4-55 00-00 7B-14 AE-47 E1-7A 74-3F 01-6E 63-57 72-61 70-70> (81 ms)

I don't know if this is related to the new compiler version of the new c++ standard version.
Failure does not seem to be architecture dependent: seen on x86_64, aarch64 and ppc64le (these architectures are the ones that are built by koschei.fedoraproject.org).

Reproducer

Build root, run tests.

ROOT version

6.38.00

Installation method

built from source (when building rpm)

Operating system

Fedora Rowhide (Fedora 44)

Additional context

Compiler installed from Fedora rawhide rpm gcc-c++-16.0.0-0.4.fc44.

[dpiparo]

Hi @ellert , thanks for the report. We recently added fedora rawhide to our battery of platforms, but the generation of the image for the CI has been broken for the last 3 days because of R: this is perhaps the reason why we did not see the issue ourselves.
Said that, we take your report seriously and will look into it.

[ellert]

Fixed by #20841