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From: "nabijaczleweli at nabijaczleweli dot xyz" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/114165] New: &scalar+1 and array+1 rejected as template
 parameters
Date: Thu, 29 Feb 2024 11:34:05 +0000
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X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114165

            Bug ID: 114165
           Summary: &scalar+1 and array+1 rejected as template parameters
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nabijaczleweli at nabijaczleweli dot xyz
  Target Milestone: ---

Given:
    void consoom(void(*)());

    template<int *, unsigned>
    void withN() {}

    template<int *, int *>
    void withP() {}


    static int scalar;
    static int array[1];


    int main() {
        consoom(withN<&scalar, 1>);
        consoom(withN<array,   1>);
        consoom(withP<&scalar, &scalar + 1>);
        consoom(withP<array,   array + 1>);
    }
clang trunk and gcc trunk -std=3Dc++{11,14,17} agree that=20
    <source>: In function 'int main()':
    <source>:17:12: error: no matches converting function 'withP' to type '=
void
(*)()'
       17 |     consoom(withP<&scalar, &scalar + 1>);
          |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    <source>:7:6: note: candidate is: 'template<int* <anonymous>, int*
<anonymous> > void withP()'
        7 | void withP() {}
          |      ^~~~~
    <source>:18:12: error: no matches converting function 'withP' to type '=
void
(*)()'
       18 |     consoom(withP<array,   array + 1>);
          |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
    <source>:7:6: note: candidate is: 'template<int* <anonymous>, int*
<anonymous> > void withP()'
        7 | void withP() {}
          |      ^~~~~
    Compiler returned: 1
or
    <source>:17:5: error: no matching function for call to 'consoom'
       17 |     consoom(withP<&scalar, &scalar + 1>);
          |     ^~~~~~~
    <source>:1:6: note: candidate function not viable: no overload of 'with=
P'
matching 'void (*)()' for 1st argument
        1 | void consoom(void(*)());
          |      ^       ~~~~~~~~~
    <source>:18:5: error: no matching function for call to 'consoom'
       18 |     consoom(withP<array,   array + 1>);
          |     ^~~~~~~
    <source>:1:6: note: candidate function not viable: no overload of 'with=
P'
matching 'void (*)()' for 1st argument
        1 | void consoom(void(*)());
          |      ^       ~~~~~~~~~
    2 errors generated.
    Compiler returned: 1

clang trunk -std=3Dc++20 accepts the code.
GCC trunk -std=3Dc++20 rejects the code with the same error.

https://godbolt.org/z/63xzf1MT6

Per my reading of [temp.arg.nontype].6, this is allowed in C++20 because it=
's
not rejected by [temp.arg.nontype].6.{1,2,3,4,5}.=