cmake: do not pin the sigc++-2 code path to C++11 - #825
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FindSIGC.cmake and FindSIGC2.cmake put "--std=c++11" into the compile flags of
every target that links libsigc++-2.0. That flag lands last on the command line,
so it overrides CMAKE_CXX_STANDARD and the level cannot be raised from outside
the project.
Qt 6 headers no longer compile as C++11: <QtCore/qnumeric.h> pulls in
qcheckedint_impl.h, which needs constexpr lambdas, three-way comparison and the
qSubOverflow overloads introduced in C++17. Building 26.05.1 against Qt 6.11
therefore fails in async/qt and qtel with, among others:
qcheckedint_impl.h:301:5: error: the type '...::<lambda()>' of 'constexpr'
variable 'f' is not literal
qcheckedint_impl.h:292:35: error: no matching function for call to
'compareThreeWay(int&, int&)'
libsigc++-2.0 builds fine at any level from C++11 upwards, and the sigc++-3.0
branch directly above already asks for C++17, so aligning the 2.x branch on
C++17 costs nothing and unblocks Qt 6.
Found while updating the openSUSE package from 19.09.2 to 26.05.1.
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FindSIGC.cmake and FindSIGC2.cmake put
--std=c++11into the compile flags of every target that links libsigc++-2.0. That flag lands last on the command line, so it overridesCMAKE_CXX_STANDARDand the level cannot be raised from outside the project.Qt 6 headers no longer compile as C++11:
<QtCore/qnumeric.h>pulls inqcheckedint_impl.h, which needs constexpr lambdas, three-way comparison and theqSubOverflowoverloads introduced in C++17. Building 26.05.1 against Qt 6.11 therefore fails inasync/qtandqtelwith, among others:libsigc++-2.0 builds fine at any level from C++11 upwards, and the sigc++-3.0 branch directly above already asks for C++17, so aligning the 2.x branch on C++17 costs nothing and unblocks Qt 6.
Found while updating the openSUSE package from 19.09.2 to 26.05.1; with this change the full build (including qtel against Qt 6.11.1) succeeds.