The following code is compiled without errors template<typename> struct B { }; template<typename... Types> struct A : public B<Types>... { using B<Types>::operator=...; using B<Types>::B...; }; int main() {} But when you change the order of using declarations compilation fails (the same error occurs in all versions of gcc on godbolt.org) template<typename> struct B { }; template<typename... Types> struct A : public B<Types>... { using B<Types>::B...; <--- using B<Types>::operator=...; <--- }; int main() {} gcc -v -std=c++17 -Wall -Wextra main.cpp Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper OFFLOAD_TARGET_NAMES=nvptx-none:hsa OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.2.1-17ubuntu1~18.04.1' --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 9.2.1 20191102 (Ubuntu 9.2.1-17ubuntu1~18.04.1) COLLECT_GCC_OPTIONS='-v' '-std=c++17' '-Wall' '-Wextra' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/9/cc1plus -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE main.cpp -quiet -dumpbase main.cpp -mtune=generic -march=x86-64 -auxbase main -Wall -Wextra -std=c++17 -version -fasynchronous-unwind-tables -fstack-protector-strong -Wformat-security -o /tmp/ccVHNdqy.s GNU C++17 (Ubuntu 9.2.1-17ubuntu1~18.04.1) version 9.2.1 20191102 (x86_64-linux-gnu) compiled by GNU C version 9.2.1 20191102, GMP version 6.1.2, MPFR version 4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/9" ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/9/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/include/c++/9 /usr/include/x86_64-linux-gnu/c++/9 /usr/include/c++/9/backward /usr/lib/gcc/x86_64-linux-gnu/9/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/9/include-fixed /usr/include/x86_64-linux-gnu /usr/include End of search list. GNU C++17 (Ubuntu 9.2.1-17ubuntu1~18.04.1) version 9.2.1 20191102 (x86_64-linux-gnu) compiled by GNU C version 9.2.1 20191102, GMP version 6.1.2, MPFR version 4.0.1, MPC version 1.1.0, isl version isl-0.19-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 33caeb1da5df6f896d4e495951c13eae main.cpp:8:11: error: expected nested-name-specifier before ‘B’ 8 | using B<Types>::operator=...; | ^ ----------------------------- The following code is also compiled with an error if there is no comment template<typename T> struct B { void foo() {} }; template<typename... Types> struct A : public B<Types>... { // using B<Types>::B...; <--- void bar() { (B<Types>::foo() , ...); } }; int main() {} gcc -std=c++17 -Wall -Wextra b.cpp b.cpp: In member function ‘void A<Types>::bar()’: b.cpp:11:11: error: expected primary-expression before ‘>’ token 11 | (B<Types>::foo() , ...); | ^ b.cpp:11:14: error: ‘::foo’ has not been declared 11 | (B<Types>::foo() , ...); | ^~~ b.cpp:11:11: error: binary expression in operand of fold-expression 11 | (B<Types>::foo() , ...); | ~~~~~~~^~~~~~~~ b.cpp:11:11: error: operand of fold expression has no unexpanded parameter packs
Confirmed. Doesn't seem like a regression.
The difference stems from the fact that in this test: template<typename> struct B { }; template<typename... Types> struct A : public B<Types>... { using B<Types>::B...; // #1 using B<Types>::operator=...; // #2 }; int main() {} when parsing B<Types> in #2 cp_parser_lookup_name finds #1, which is a dependent USING_DECL. strip_using_decl doesn't do anything with such a USING_DECL, so we think that we haven't found a template name.
Potential fix: --- a/gcc/cp/parser.c +++ b/gcc/cp/parser.c @@ -16982,6 +16982,15 @@ cp_parser_template_name (cp_parser* parser, decl = strip_using_decl (decl); + if (TREE_CODE (decl) == USING_DECL + && TREE_CODE (USING_DECL_SCOPE (decl)) == TYPE_PACK_EXPANSION) + { + tree t = PACK_EXPANSION_PATTERN (USING_DECL_SCOPE (decl)); + t = get_template_info (t); + if (t) + decl = TI_TEMPLATE (t); + } + /* If DECL is a template, then the name was a template-name. */ if (TREE_CODE (decl) == TEMPLATE_DECL) {
This looks fixed in GCC 11/12, ever since r11-6942
Dup of bug 80417. *** This bug has been marked as a duplicate of bug 80417 ***