https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80748
Bug ID: 80748
Summary: [concepts] noexcept specifier operands are allowed but
ignored in compound requirements
Product: gcc
Version: c++-concepts
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: tom at honermann dot net
CC: andrew.n.sutton at gmail dot com, asutton at gcc dot gnu.org
Target Milestone: ---
It appears that an operand provided to the noexcept specifier in compound
requirements is ignored; the presence of any exception specification appears to
be interpreted as introducing an exception constraint:
$ cat t.cpp
template<typename T>
concept bool C = requires { { T::smf() } noexcept(false); };
struct S1 {
static void smf();
};
struct S2 {
static void smf() noexcept;
};
static_assert(C<S1>);
static_assert(C<S2>);
$ g++ --version
g++ (GCC) 8.0.0 20170513 (experimental)
...
$ g++ -c -fconcepts t.cpp
t.cpp:9:1: error: static assertion failed
static_assert(C<S1>);
^~~~~~~~~~~~~
In the above example, the expectation is that the compound requirement having a
'noexcept(false)' specifier will confer no exception constraint requirement and
that both S1 and S2 should satisfy constraint checks. However, S1 is rejected.
The test case above is ill-formed according to the Concepts TS (N4641) given
that compound requirements are specified to only accept an optional 'noexcept'
specifier (without operands):
ยง 5.1.4.3 Compound requirements [expr.prim.req.compound]
compound-requirement:
{ expression } noexcept[opt] trailing-return-type[opt] ;
gcc allows an operand to be specified, but appears not to evaluate it. If the
intent of the TS is to disallow operands to the noexcept specifier in compound
requirements, then gcc should reject them. Otherwise, if the intent of the TS
is to allow operands, then gcc should evaluate them and conditionally apply
exception constraints (and an issue opened with the Concepts TS).
