https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87748
Bug ID: 87748
Summary: G++-8 treats SFINAE as error
Product: gcc
Version: 8.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: timj at gnu dot org
Target Milestone: ---
Created attachment 44897
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44897&action=edit
g++-8 SFINAE Error Test Case
The following (reduced) code example uses SFINAE to discern argument types in a
visitor pattern. The original bug report is here:
https://github.com/tim-janik/beast/issues/72#issuecomment-432874963
It works well under clang-6 and g++-7.3. With g++-8.1.1 (and reportedly
g++-8.2.1) a template substitution failure during a method presence check on
std::string is promoted to an error, even though a std::false_type substitution
variant exists that should be picked up instead.
Not only do other compiler (versions) compile this correctly, but duplicating a
seemingly unrelated static_assert in the code also "remedies" the g++-8
treatment of the failure as error, tested via -DWITHASSERT below.
EXPECTED:
$ g++ --version
g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0
$ g++ -std=gnu++14 -Wall -O2 aidavisit.cc && ./a.out
visit_string: some_string="A std::string"
BUGGY BEHAVIOUR:
$ g++ --version
g++ (GCC) 8.1.1 20180712 (Red Hat 8.1.1-5)
$ g++ -std=gnu++14 -Wall -O2 aidavisit.cc && ./a.out
aidavisit.cc:19:77: error: 'class std::__cxx11::basic_string<char>' has no
member named '__aida_visit__'
$ g++ -std=gnu++14 -Wall -O2 aidavisit.cc -DWITHASSERT && ./a.out
visit_string: some_string="A std::string"
