https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70380
Bug ID: 70380
Summary: SFINAE error with constexpr expressions referencing a
non-const variable
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
The following program attempts to use SFINAE to choose between two overloads of
a function template. Whether one or the other overload is viable depends on
whether or not an expression used in its declaration is a core constant
expression. I believe the program is valid as is (with the invalid lines
commented out) and should be accepted. However, GCC 6 and all versions before
it rejects it. (Clang rejects it as well, due to what I think is also a bug.)
$ cat t.cpp && /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -xc++ t.cpp
int x;
struct A {
constexpr A (bool b) : m (b ? 42 : x) { }
constexpr int foo () const { return m; }
int m;
};
struct B {
constexpr B (bool b) : m (b ? x : 42) { }
constexpr int bar () const { return m; }
int m;
};
// constexpr int a0 = A (false).foo (); // not constant, rejected
constexpr int a1 = A (true).foo (); // valid, accepted
constexpr int b0 = B (false).bar (); // valid, accepted
// constexpr int b1 = B (true).bar (); // not constant, rejected
template <bool X>
int f (int (*)[A (X).foo ()] = 0) { return !X; }
template <bool X>
int f (int (*)[B (X).bar ()] = 0) { return X; }
constexpr int f0 = f<0>(); // valid, rejected
constexpr int f1 = f<1>(); // valid, rejected
t.cpp:26:24: error: call to non-constexpr function ‘int f(int
(*)[(B)(X).B::bar()]) [with bool X = false]’
constexpr int f0 = f<0>(); // valid, rejected
~~~~^~
t.cpp:27:24: error: call to non-constexpr function ‘int f(int
(*)[(A)(X).A::foo()]) [with bool X = true]’
constexpr int f1 = f<1>(); // valid, rejected
~~~~^~
