[gcc r15-3913] c++/modules: Allow imported references in constant expressions

[Nathaniel Shead via Gcc-cvs]

https://gcc.gnu.org/g:1a0b33ebc57ebcc9595b19050f5c36c1f9d39e3e

commit r15-3913-g1a0b33ebc57ebcc9595b19050f5c36c1f9d39e3e
Author: Nathaniel Shead <nathanielosh...@gmail.com>
Date:   Thu Sep 12 20:06:39 2024 +1000

    c++/modules: Allow imported references in constant expressions
    
    Currently the streaming code uses TREE_CONSTANT to determine whether an
    entity will have a definition that is interesting to stream out.  This
    is not sufficient, however; we also need to write the definition of
    references, since although not TREE_CONSTANT they can still be usable in
    constant expressions.
    
    As such this patch uses the existing decl_maybe_constant_var function
    which correctly handles this case.
    
    gcc/cp/ChangeLog:
    
            * module.cc (has_definition): Use decl_maybe_constant_var
            instead of TREE_CONSTANT.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/cexpr-5_a.C: New test.
            * g++.dg/modules/cexpr-5_b.C: New test.
    
    Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
    Reviewed-by: Jason Merrill <ja...@redhat.com>

Diff:
---
 gcc/cp/module.cc                         |  2 +-
 gcc/testsuite/g++.dg/modules/cexpr-5_a.C | 13 +++++++++++++
 gcc/testsuite/g++.dg/modules/cexpr-5_b.C |  9 +++++++++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index f5df9e875d3a..65b37b4b5544 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -11829,7 +11829,7 @@ has_definition (tree decl)
               since there's no TU to emit them in otherwise.  */
            return true;
 
-         if (!TREE_CONSTANT (decl))
+         if (!decl_maybe_constant_var_p (decl))
            return false;
 
          return true;
diff --git a/gcc/testsuite/g++.dg/modules/cexpr-5_a.C 
b/gcc/testsuite/g++.dg/modules/cexpr-5_a.C
new file mode 100644
index 000000000000..3a9f00523f65
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/cexpr-5_a.C
@@ -0,0 +1,13 @@
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi M }
+
+export module M;
+
+int x = 123;
+void f() {}
+
+int& xr = x;
+auto& fr = f;
+
+constexpr int& cxr = xr;
+constexpr auto& cfr = fr;
diff --git a/gcc/testsuite/g++.dg/modules/cexpr-5_b.C 
b/gcc/testsuite/g++.dg/modules/cexpr-5_b.C
new file mode 100644
index 000000000000..4b1b901104bc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/cexpr-5_b.C
@@ -0,0 +1,9 @@
+// { dg-additional-options "-fmodules-ts" }
+
+module M;
+
+constexpr auto& use_xr = xr;
+constexpr auto& use_fr = fr;
+
+static_assert(&cxr == &use_xr);
+static_assert(&cfr == &use_fr);