https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97600
--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jason Merrill <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:9324f7a25c7161a813bfae6cc2d180784b165740 commit r11-5954-g9324f7a25c7161a813bfae6cc2d180784b165740 Author: Jason Merrill <ja...@redhat.com> Date: Fri Dec 11 14:37:09 2020 -0500 c++: Avoid considering some conversion ops [PR97600] Patrick's earlier patch to check convertibility before constraints for conversion ops wasn't suitable because checking convertibility can also lead to unwanted instantiations, but it occurs to me that there's a smaller check we can do to avoid doing normal consideration of the conversion ops in this case: since we're in the middle of a user-defined conversion, we can exclude from consideration any conversion ops that return a type that would need an additional user-defined conversion to reach the desired type: namely, a type that differs in class-ness from the desired type. [temp.inst]/9 allows optimizations like this: "If the function selected by overload resolution can be determined without instantiating a class template definition, it is unspecified whether that instantiation actually takes place." gcc/cp/ChangeLog: PR libstdc++/97600 * call.c (build_user_type_conversion_1): Avoid considering conversion functions that return a clearly unsuitable type. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/concepts-conv3.C: New test.
