[Bug c++/105344] New: std::source_location::curent() seemingly treated as a pure function in template specializations

[Bug c++/105344] New: std::source_location::curent() seemingly treated as a pure function in template specializations

[jan at twosigma dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105344

            Bug ID: 105344
           Summary: std::source_location::curent() seemingly treated as a
                    pure function in template specializations
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jan at twosigma dot com
  Target Milestone: ---

When using std::source_location::current().line() in certain contexts on two
different lines, gcc seems to incorrectly think the expression refers to the
same value.

Testcase:

#include <source_location>
#include <utility>

template <int i, class = void> struct foo;

// Two following two specializations are different, yet gcc errors out,
// claiming they are the same.
template <int i>
struct foo<i, std::enable_if_t<i == std::source_location::current().line()>> {
    static constexpr int num = i;
};

template <int i>
struct foo<i, std::enable_if_t<i == std::source_location::current().line()>> {
    static constexpr int num = i;
};

This outputs:

<source>:14:8: error: redefinition of 'struct foo<i, typename std::enable_if<(i
== std::source_location::current().std::source_location::line()), void>::type>'
   14 | struct foo<i, std::enable_if_t<i ==
std::source_location::current().line()>> {
      |       
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:9:8: note: previous definition of 'struct foo<i, typename
std::enable_if<(i ==
std::source_location::current().std::source_location::line()), void>::type>'
    9 | struct foo<i, std::enable_if_t<i ==
std::source_location::current().line()>> {
      |       
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Compiler returned: 1

https://godbolt.org/z/ozf1MbG3n shows this code works fine under MSVC.

[Bug c++/105344] std::source_location::curent() seemingly treated as a pure function in template specializations

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105344

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jan Polasek from comment #0) 
> https://godbolt.org/z/ozf1MbG3n shows this code works fine under MSVC.

You don't try to use foo afterwards.
Once you do, MSVC will have an ICE.
try:
```
#include <source_location>
#include <utility>

template <int i, class = void> struct foo;

// Two following two specializations are different, yet gcc errors out,
// claiming they are the same.
#line 9
template <int i>
struct foo<i, std::enable_if_t<i == std::source_location::current().line()>> {
    static constexpr int num = i;
};

#line 14
template <int i>
struct foo<i, std::enable_if_t<i == std::source_location::current().line()>> {
    static constexpr int num = i;
};


int main(void)
{
    foo<9> t;
    foo<14> t;
}
```

So I don't know if this is valid or not.