probably there is a relation to bug 48882 I do not attach the precompiled file (.ii), because it is too big. #include<algorithm> // std::max std::min #include<iostream> template<typename T> const T& myMin(const T& a, const T& b) { if(a < b) return a; return b; } template<typename T> const T& myMax(const T& a, const T& b) { if(a > b) return a; return b; } int main() { int a = 1000; int b = 2000; bool even = true; const int& (*fp)(const int&, const int&); fp = even ? myMax<int> : myMin<int>; /* yields link error */ std::cout << fp(a,b) << std::endl; } does not link. Output : Es werden eingebaute Spezifikationen verwendet. COLLECT_GCC=/home/me/programme/gcc47/bin/g++ COLLECT_LTO_WRAPPER=/home/me/programme/gcc47/libexec/gcc/i686-pc-linux-gnu/4.7.0/lto-wrapper Ziel: i686-pc-linux-gnu Konfiguriert mit: ./configure --prefix=/home/me/programme/gcc47 Thread-Modell: posix gcc-Version 4.7.0 20110430 (experimental) (GCC) COLLECT_GCC_OPTIONS='-v' '-std=c++0x' '-Wall' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=pentiumpro' /home/me/programme/gcc47/libexec/gcc/i686-pc-linux-gnu/4.7.0/cc1plus -E -quiet -v -D_GNU_SOURCE bug2.cpp -mtune=generic -march=pentiumpro -std=c++0x -Wall -fpch-preprocess -o bug2.ii nicht vorhandenes Verzeichnis »/home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/../../../../i686-pc-linux-gnu/include« wird ignoriert #include "..." - Suche beginnt hier: #include <...> - Suche beginnt hier: /home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/../../../../include/c++/4.7.0 /home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/../../../../include/c++/4.7.0/i686-pc-linux-gnu /home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/../../../../include/c++/4.7.0/backward /home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/include /usr/local/include /home/me/programme/gcc47/include /home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/include-fixed /usr/include Ende der Suchliste. COLLECT_GCC_OPTIONS='-v' '-std=c++0x' '-Wall' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=pentiumpro' /home/me/programme/gcc47/libexec/gcc/i686-pc-linux-gnu/4.7.0/cc1plus -fpreprocessed bug2.ii -quiet -dumpbase bug2.cpp -mtune=generic -march=pentiumpro -auxbase bug2 -Wall -std=c++0x -version -o bug2.s GNU C++ (GCC) Version 4.7.0 20110430 (experimental) (i686-pc-linux-gnu) kompiliert von GNU-C-Version 4.7.0 20110430 (experimental), GMP-Version 5.0.1, MPFR-Version 3.0.0-p3, MPC-Version 0.8.2. GGC-Heuristik: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 GNU C++ (GCC) Version 4.7.0 20110430 (experimental) (i686-pc-linux-gnu) kompiliert von GNU-C-Version 4.7.0 20110430 (experimental), GMP-Version 5.0.1, MPFR-Version 3.0.0-p3, MPC-Version 0.8.2. GGC-Heuristik: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 Compiler executable checksum: c95befb475332395eb1695ad61c47f68 COLLECT_GCC_OPTIONS='-v' '-std=c++0x' '-Wall' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=pentiumpro' as --32 -o bug2.o bug2.s COMPILER_PATH=/home/me/programme/gcc47/libexec/gcc/i686-pc-linux-gnu/4.7.0/:/home/me/programme/gcc47/libexec/gcc/i686-pc-linux-gnu/4.7.0/:/home/me/programme/gcc47/libexec/gcc/i686-pc-linux-gnu/:/home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/:/home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/ LIBRARY_PATH=/home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/:/home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-std=c++0x' '-Wall' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=pentiumpro' /home/me/programme/gcc47/libexec/gcc/i686-pc-linux-gnu/4.7.0/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o /home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/crtbegin.o -L/home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0 -L/home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/../../.. bug2.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /home/me/programme/gcc47/lib/gcc/i686-pc-linux-gnu/4.7.0/crtend.o /usr/lib/crtn.o bug2.o: In function `main': bug2.cpp:(.text+0x26): undefined reference to `int const& myMax<int>(int const&, int const&)' bug2.cpp:(.text+0x2d): undefined reference to `int const& myMin<int>(int const&, int const&)' collect2: ld gab 1 als Ende-Status zurück
However if(even) fp = myMax<int>; else fp = myMin<int>; does work as expected.
reduced: template<typename T> T myMax(T a, T b) { if(a < b) return a; return b; } int main() { bool even = true; int (*fp)(int, int); fp = even ? myMax<int> : myMax<int>; /* yields link error */ } The function template specialization should be implicitly instantiated
Is this related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49609#c2 ? The ternary operator is not one of the contexts listed in 13.4 [over.over] p1 either. The explicit argument list uniquely identifies the specialization, so myMax<int> is an lvalue for the function template, but I don't see any requirement that it gets instantiated
*** Bug 49609 has been marked as a duplicate of this bug. ***
Author: jason Date: Fri Jul 1 20:24:08 2011 New Revision: 175764 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=175764 Log: PR c++/48883 PR c++/49609 * pt.c (resolve_nondeduced_context): Call mark_used. Added: trunk/gcc/testsuite/g++.dg/template/explicit-args4.C Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/pt.c trunk/gcc/testsuite/ChangeLog
Fixed for 4.7.