https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101715
--- Comment #19 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:3abcbf243239f9576a60f4ce7f8ee4b3fa14784b commit r12-6802-g3abcbf243239f9576a60f4ce7f8ee4b3fa14784b Author: Marek Polacek <pola...@redhat.com> Date: Fri Jan 14 17:41:49 2022 -0500 c++: ICE with noexcept and canonical types [PR101715] This is a "canonical types differ for identical types" ICE, which started with r11-4682. It's a bit tricky to explain. Consider: template <typename T> struct S { S<T> bar() noexcept(T::value); // #1 S<T> foo() noexcept(T::value); // #2 }; template <typename T> S<T> S<T>::foo() noexcept(T::value) {} // #3 We ICE because #3 and #2 have the same type, but their canonical types differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1. The member functions #1 and #2 have the same type. However, since their noexcept-specifier is deferred, when parsing them, we create a variant for both of them, because DEFERRED_PARSE cannot be compared. In other words, build_cp_fntype_variant's tree v = TYPE_MAIN_VARIANT (type); for (; v; v = TYPE_NEXT_VARIANT (v)) if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late)) return v; will *not* find an existing variant when creating a method_type for #2, so we have to create a new one. But then we perform delayed parsing and call fixup_deferred_exception_variants for #1 and #2. f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly parsed noexcept-specifier. It also sets TYPE_CANONICAL (#2) to #1. Both noexcepts turned out to be the same, so now we have two equivalent variants in the list! I.e., +-----------------+ +-----------------+ +-----------------+ | main | | #2 | | #1 | | S S::<T379>(S*) |----->| S S::<T37c>(S*) |----->| S S::<T37a>(S*) |----->NULL | - | | noex(T::value) | | noex(T::value) | +-----------------+ +-----------------+ +-----------------+ Then we get to #3. As for #1 and #2, grokdeclarator calls build_memfn_type, which ends up calling build_cp_fntype_variant, which will use the loop above to look for an existing variant. The first one that matches cp_check_qualified_type will be used, so we use #2 rather than #1, and the TYPE_CANONICAL mismatch follows. Hopefully that makes sense. As for the fix, I didn't think I could rewrite the method_type #2 with #1 because the type may have escaped via decltype. So my approach is to elide #2 from the list, so when looking for a matching variant, we always find #1 (#2 remains live though, which admittedly sounds sort of dodgy). PR c++/101715 gcc/cp/ChangeLog: * tree.cc (fixup_deferred_exception_variants): Remove duplicate variants after parsing the exception specifications. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/noexcept72.C: New test. * g++.dg/cpp0x/noexcept73.C: New test.
