From: Nathaniel Shead <nathanieloshead@gmail.com>
Date: Thu, 12 Sep 2024 10:06:39 +0000 (+1000)
Subject: c++/modules: Allow imported references in constant expressions
X-Git-Tag: basepoints/gcc-16~5640
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=1a0b33ebc57ebcc9595b19050f5c36c1f9d39e3e;p=thirdparty%2Fgcc.git

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 <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
---

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index f5df9e875d3..65b37b4b554 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 00000000000..3a9f00523f6
--- /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 00000000000..4b1b901104b
--- /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);