[PATCH] c++: Implement CWG2387 - Linkage of const-qualified variable template [PR109126]

[Jakub Jelinek]

Hi!

The following patch attempts to implement DR2387 by making variable
templates including their specialization TREE_PUBLIC when at file
scope and they don't have static storage class.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-08-07  Jakub Jelinek  <ja...@redhat.com>

        PR c++/109126
        * decl.cc (grokvardecl): Implement CWG 2387 - Linkage of
        const-qualified variable template.  Set TREE_PUBLIC on variable
        templates with const qualified types unless static is present.

        * g++.dg/DRs/dr2387.C: New test.
        * g++.dg/DRs/dr2387-aux.cc: New file.

--- gcc/cp/decl.cc.jj   2024-08-06 11:05:29.147469440 +0200
+++ gcc/cp/decl.cc      2024-08-07 11:58:11.275368835 +0200
@@ -11225,6 +11225,8 @@ grokvardecl (tree type,
                                || ! constp
                                || volatilep
                                || inlinep
+                               || in_template_context
+                               || processing_specialization
                                || module_attach_p ()));
       TREE_STATIC (decl) = ! DECL_EXTERNAL (decl);
     }
--- gcc/testsuite/g++.dg/DRs/dr2387.C.jj        2024-08-07 12:11:14.372115971 
+0200
+++ gcc/testsuite/g++.dg/DRs/dr2387.C   2024-08-07 12:13:35.089273604 +0200
@@ -0,0 +1,22 @@
+// DR 2387
+// { dg-do run { target c++14 } }
+// { dg-additional-sources "dr2387-aux.cc" }
+
+template <int N>
+const int a = N;
+template <int N>
+static const int b = N;
+template <int N>
+extern const int c = N;
+template <int N>
+const volatile int d = N;
+template <int N>
+const int e = N;
+template <>
+const int e <43> = 44;
+
+const int *pa = &a <42>;
+const int *pb = &b <42>;
+const int *pc = &c <42>;
+const volatile int *pd = &d <42>;
+const int *pe = &e <43>;
--- gcc/testsuite/g++.dg/DRs/dr2387-aux.cc.jj   2024-08-07 12:11:09.388181223 
+0200
+++ gcc/testsuite/g++.dg/DRs/dr2387-aux.cc      2024-08-07 12:13:25.321401491 
+0200
@@ -0,0 +1,25 @@
+// DR 2387
+
+template <int N>
+extern const int a;
+template <int N>
+static const int b = N;
+template <int N>
+extern const int c;
+template <int N>
+extern const volatile int d;
+template <int N>
+extern const int e;
+extern const int *pa, *pb, *pc, *pe;
+extern const volatile int *pd;
+
+int
+main ()
+{
+  if (pa != &a <42>
+      || pb == &b <42>
+      || pc != &c <42>
+      || pd != &d <42>
+      || pe != &e <43>)
+    __builtin_abort ();
+}

        Jakub