[Bug c++/58969] New: bogus error: the value of 'kName' is not usable in a constant expression

[ppluzhnikov at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58969

            Bug ID: 58969
           Summary: bogus error: the value of 'kName' is not usable in a
                    constant expression
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppluzhnikov at google dot com

Google ref: b/11479502

/// --- cut ---
template <const char name[]>
class BaseObject {
 virtual const char* GetName() const {
   return name;
 }
};

const char kName[] = "name";
class Object : public BaseObject<kName> {
};

int main() {
 return 0;
}
/// --- cut ---

Using g++ (GCC) 4.9.0 20131028 (experimental)

g++ -c t.cc
t.cc:9:34: error: 'kName' cannot appear in a constant-expression
 class Object : public BaseObject<kName> {
                                  ^
t.cc:9:39: error: template argument 1 is invalid
 class Object : public BaseObject<kName> {
                                       ^

g++ -c t.cc -std=c++11
t.cc:9:34: error: the value of 'kName' is not usable in a constant expression
 class Object : public BaseObject<kName> {
                                  ^
t.cc:8:12: note: 'kName' was not declared 'constexpr'
 const char kName[] = "name";
            ^

Compiles with Clang.

Analysis by Richard Smith:

This is a GCC bug. It appears to be some sort of confusion in the way GCC
handles internal linkage non-type template parameters of pointer type. You can
also get GCC to accept the code by marking kName as 'extern'.


Obligatory standards references (relative to N3797):

[temp.param](14.1)/8: "A non-type template-parameter of type "array of T" [...]
is adjusted to be of type "pointer to T".

[temp.arg.nontype](14.3.2)/1: "A template-argument for a non-type, non-template
template-parameter shall be [...] a constant expression that designates the
address of a complete object with static storage duration and external or
internal linkage [...] expressed [...] as & id-expression, where the
id-expression is the name of an object [...], except that the & may be omitted
if the name refers to a[n] [...] array"

Note that "kName" is a constant expression, because it is a glvalue core
constant expression whose value refers to an object with static storage
duration (see [expr.const](5.19)/4).

[temp.arg.nontype](14.3.2)/5: "The following conversions are performed on each
expression used as a non-type template-argument. [...] For a non-type
template-parameter of type pointer to object, [...] the array-to-pointer
conversion [is] applied."


So BaseObject has a template parameter of type 'const char*', and that
parameter can bind to the template argument kName (after array-to-pointer
decay). There's no requirement that kName be declared 'constexpr'; its value is
not used here, only its address is used.