https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104230
Bug ID: 104230
Summary: Non-type template arguments of reference and pointer
type fail when initialized by pointer to member
operator
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: anton at socialhacker dot com
Target Milestone: ---
I believe that GCC incorrectly rejects non-type template arguments of pointer
and reference type that were generated by using the pointer to member operator.
My reading of "P1907R1: Inconsistencies with non-type template parameters" [1]
makes me think that these uses should be acceptable. In particular the change
to "13.4.2 [temp.arg.nontype] paragraph 2" moves the restriction of passing a
reference to a subobject so that it only applies to subobjects of already
disallowed values (temporaries, string literals ...). And it looks like that's
working when using direct member access operators, but not when using pointer
to member operators. And nothing there seems to indicate how the reference or
pointer is generated, so presumably any valid constant expression that
generates a valid pointer or reference should be acceptable.
Below is a simple test case that fails on both the trunk [2] and 11.2 [3]
versions on compiler explorer.
struct T { int a; int b; } t;
template <int *> struct U {};
U<&t.a> u1;
U<&t.b> u2;
U<(&(t.*(&T::a)))> u3; // Interestingly doesn't fail, good?
U<(&(t.*(&T::b)))> u4; // Fails, shouldn't fail in C++20 I believe
template <int &> struct V {};
V<t.a> v1;
V<t.b> v2;
V<(t.*(&T::a))> v3; // Fails, shouldn't fail in C++20 I believe
V<(t.*(&T::b))> v4; // Fails, shouldn't fail in C++20 I believe
Both compiler version produce the same output when run with `--std=c++20
-Wall`:
<source>:8:18: error: '(((int*)(& t)) + 4)' is not a valid template
argument for 'int*' because it is not the address of a variable
8 | U<(&(t.*(&T::b)))> u4;
| ^
<source>:14:15: error: '&(int&)(& t)' is not a valid template argument of
type 'int&' because '(int&)(& t)' is not a variable
14 | V<(t.*(&T::a))> v3;
| ^
<source>:15:15: error: '&(((int&)(& t)) + 4)' is not a valid template
argument of type 'int&' because '(((int&)(& t)) + 4)' is not a variable
15 | V<(t.*(&T::b))> v4;
| ^
[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1907r1.html
[2] gcc version 12.0.1 20220125 (experimental)
(Compiler-Explorer-Build-gcc-bb99171b9b0f01a46bfca2d3cbd52fc6faf6cbaa-binutils-2.36.1)
[3] gcc version 11.2.0 (Compiler-Explorer-Build-gcc--binutils-2.36.1)
