In the following snippet, an error should be emitted - instead, the compiler prints a warning and keeps compiling, and simply skips the second call to f (!) in the resulting executable: #include <stdint.h> #include <stdio.h> struct A { A(int32_t a) {} A(uint32_t a) {} // Comment to make it work explicit A(int64_t a) {} // Comment to make it work }; void f(A a) { printf("hello\n"); } int main(int, char**) { f(2); f(3l); } /opt/gcc62/bin/g++ -std=gnu++11 explicit.cpp explicit.cpp: In function ‘int main(int, char**)’: explicit.cpp:11:6: note: initializing argument 1 of ‘void f(A)’ void f(A a) { printf("hello\n"); } ^ With either marked constructor commented, compiles correctly and gives the expected output ("hello" x 2 or ambiguity error).
gcc 5.3.0 gives a different error - looks odd though, no initializer list in sight: /opt/gcc53/bin/g++ -std=gnu++11 explicit.cpp explicit.cpp: In function ‘int main(int, char**)’: explicit.cpp:15:7: error: converting to ‘A’ from initializer list would use explicit constructor ‘A::A(int64_t)’ f(3l); ^ explicit.cpp:11:6: note: initializing argument 1 of ‘void f(A)’ void f(A a) { printf("hello\n"); } ^ clang++ 3.8 complains about ambiguity: clang++ -std=gnu++11 explicit.cpp explicit.cpp:15:5: error: conversion from 'long' to 'A' is ambiguous f(3l); ^~ explicit.cpp:5:3: note: candidate constructor A(int32_t a) {} ^ explicit.cpp:6:3: note: candidate constructor A(uint32_t a) {} ^ explicit.cpp:11:10: note: passing argument to parameter 'a' here void f(A a) { printf("hello\n"); }
4.5.4 got this right: ex.cc: In function ‘int main()’: ex.cc:15:7: error: conversion from ‘long int’ to ‘A’ is ambiguous ex.cc:6:3: note: candidates are: A::A(uint32_t) ex.cc:5:3: note: A::A(int32_t) Since GCC 4.6 the error has been misleading, and since GCC 6 it's accepts-invalid.
Reduced: struct A { A(int) {} A(unsigned) {} // Comment to make it work explicit A(long) {} // Comment to make it work }; void f(A) { } int main() { f(2); f(3l); }
The 4.5 -> 4.6 difference appeared in r159332: ./cc1plus.159325 -quiet -std=c++0x pr77563.C pr77563.C: In function ‘int main()’: pr77563.C:12:7: error: conversion from ‘long int’ to ‘A’ is ambiguous pr77563.C:3:3: note: candidates are: A::A(unsigned int) pr77563.C:2:3: note: A::A(int) ./cc1plus.159332 -quiet -std=c++0x pr77563.C pr77563.C: In function ‘int main()’: pr77563.C:12:7: error: converting to ‘A’ from initializer list would use explicit constructor ‘A::A(long int)’ then r229283 started to ICE on it instead, and finally r236395 fixed the ICE, but emitted a note without corresponding error/warning: ./cc1plus.236395 -quiet -std=c++0x pr77563.C pr77563.C: In function ‘int main()’: pr77563.C:8:6: note: initializing argument 1 of ‘void f(A)’ void f(A) { } ^
*** Bug 78032 has been marked as a duplicate of this bug. ***
Author: jason Date: Thu Mar 23 12:50:55 2017 New Revision: 246417 URL: https://gcc.gnu.org/viewcvs?rev=246417&root=gcc&view=rev Log: PR c++/77563 - missing ambiguous conversion error. * call.c (convert_like_real): Use LOOKUP_IMPLICIT. Added: trunk/gcc/testsuite/g++.dg/overload/ambig3.C Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/call.c
Fixed on trunk.
Author: jason Date: Wed Apr 12 19:23:30 2017 New Revision: 246893 URL: https://gcc.gnu.org/viewcvs?rev=246893&root=gcc&view=rev Log: PR c++/77563 - missing ambiguous conversion error. * call.c (convert_like_real): Use LOOKUP_IMPLICIT. Added: branches/gcc-6-branch/gcc/testsuite/g++.dg/overload/ambig3.C Modified: branches/gcc-6-branch/gcc/cp/ChangeLog branches/gcc-6-branch/gcc/cp/call.c
Fixed for 6.4. Since the GCC 5 bug is just a strange diagnostic, I'm not going to fix it there.