c++/modules: Propagate purview/import for templates in duplicate_decls [PR116803]

We need to ensure that for a declaration in the module purview, that the
resulting declaration has PURVIEW_P set and IMPORT_P cleared so that we
understand it might be something requiring exporting.  This is normally
handled for a declaration by set_instantiating_module, but when this
declaration is a redeclaration duplicate_decls needs to propagate this
to olddecl.

This patch only changes the logic for template declarations, because in
the non-template case the whole contents of olddecl's DECL_LANG_SPECIFIC
is replaced with newdecl's (which includes these flags), so there's
nothing to do.

	PR c++/116803

gcc/cp/ChangeLog:

	* decl.cc (duplicate_decls): Propagate DECL_MODULE_PURVIEW_P and
	DECL_MODULE_IMPORT_P for template redeclarations.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/merge-18_a.H: New test.
	* g++.dg/modules/merge-18_b.H: New test.
	* g++.dg/modules/merge-18_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

4 files changed, 41 insertions, 0 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 6a7ba41..07fb985 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -2528,6 +2528,16 @@ duplicate_decls (tree newdecl, tree olddecl, bool hiding, bool was_hidden)
 	    }
 	}
 
+      /* Propagate purviewness and importingness as with
+	 set_instantiating_module.  */
+      if (modules_p ())
+	{
+	  if (DECL_MODULE_PURVIEW_P (new_result))
+	    DECL_MODULE_PURVIEW_P (old_result) = true;
+	  if (!DECL_MODULE_IMPORT_P (new_result))
+	    DECL_MODULE_IMPORT_P (old_result) = false;
+	}
+
       /* If the new declaration is a definition, update the file and
 	 line information on the declaration, and also make
 	 the old declaration the same definition.  */
diff --git a/gcc/testsuite/g++.dg/modules/merge-18_a.H b/gcc/testsuite/g++.dg/modules/merge-18_a.H
new file mode 100644
index 0000000..8d86ad98
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/merge-18_a.H
@@ -0,0 +1,8 @@
+// PR c++/116803
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+namespace ns {
+  template <typename T> void foo();
+  template <typename T> extern const int bar;
+}
diff --git a/gcc/testsuite/g++.dg/modules/merge-18_b.H b/gcc/testsuite/g++.dg/modules/merge-18_b.H
new file mode 100644
index 0000000..2a762e2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/merge-18_b.H
@@ -0,0 +1,13 @@
+// PR c++/116803
+// { dg-additional-options "-fmodule-header -fdump-lang-module" }
+// { dg-module-cmi {} }
+
+import "merge-18_a.H";
+
+namespace ns {
+  template <typename T> void foo() {}
+  template <typename T> const int bar = 123;
+}
+
+// { dg-final { scan-lang-dump {Writing definition '::ns::template foo'} module } }
+// { dg-final { scan-lang-dump {Writing definition '::ns::template bar'} module } }
diff --git a/gcc/testsuite/g++.dg/modules/merge-18_c.C b/gcc/testsuite/g++.dg/modules/merge-18_c.C
new file mode 100644
index 0000000..b90d85f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/merge-18_c.C
@@ -0,0 +1,10 @@
+// PR c++/116803
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts" }
+
+import "merge-18_b.H";
+
+int main() {
+  ns::foo<int>();
+  static_assert(ns::bar<int> == 123);
+}