[gcc r15-3937] c++/modules: Propagate purview/import for templates in duplicate_decls [PR116803]

[Nathaniel Shead]

https://gcc.gnu.org/g:cf9efe5ec14fea3ad5746fbefb22544bb9424d9d

commit r15-3937-gcf9efe5ec14fea3ad5746fbefb22544bb9424d9d
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Fri Sep 27 18:58:27 2024 +1000

    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>

Diff:
---
 gcc/cp/decl.cc                            | 10 ++++++++++
 gcc/testsuite/g++.dg/modules/merge-18_a.H |  8 ++++++++
 gcc/testsuite/g++.dg/modules/merge-18_b.H | 13 +++++++++++++
 gcc/testsuite/g++.dg/modules/merge-18_c.C | 10 ++++++++++
 4 files changed, 41 insertions(+)

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 6a7ba416cf8a..07fb9855cd20 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 000000000000..8d86ad980ba5
--- /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 000000000000..2a762e2ac498
--- /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 000000000000..b90d85f75024
--- /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);
+}