c++/modules: Stream unmergeable temporaries by value again [PR114856]

In r14-9266-g2823b4d96d9ec4 I gave all temporary vars a DECL_CONTEXT,
including those at namespace or global scope, so that they could be
properly merged across importers.  However, not all of these temporary
vars are actually supposed to be mergeable.

For instance, in the attached testcase we have an unnamed temporary var
used in the NSDMI of a class member, which cannot properly merged -- but
it also doesn't need to be, as it'll be thrown away when the class type
itself is merged anyway.

This patch reverts the change made above and instead makes a weaker
adjustment that only causes temporary vars with linkage have a
DECL_CONTEXT to merge from.  This way these unnamed, "unmergeable"
temporaries are properly streamed by value again.

	PR c++/114856

gcc/cp/ChangeLog:

	* call.cc (make_temporary_var_for_ref_to_temp): Set context for
	temporaries with linkage.
	* init.cc (create_temporary_var): Revert to only set context
	when in a function decl.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr114856.h: New test.
	* g++.dg/modules/pr114856_a.H: New test.
	* g++.dg/modules/pr114856_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
(cherry picked from commit e60032b382364897a58e67994baac896bcd03327)

gcc/cp/call.cc

@@ -13800,6 +13800,9 @@ make_temporary_var_for_ref_to_temp (tree decl, tree type)
       tree name = mangle_ref_init_variable (decl);
       DECL_NAME (var) = name;
       SET_DECL_ASSEMBLER_NAME (var, name);
+
+      /* Set the context to make the variable mergeable in modules.  */
+      DECL_CONTEXT (var) = current_scope ();
     }
   else
     /* Create a new cleanup level if necessary.  */

gcc/cp/init.cc

@@ -4287,7 +4287,7 @@ create_temporary_var (tree type)
   TREE_USED (decl) = 1;
   DECL_ARTIFICIAL (decl) = 1;
   DECL_IGNORED_P (decl) = 1;
-  DECL_CONTEXT (decl) = current_scope ();
+  DECL_CONTEXT (decl) = current_function_decl;
 
   return decl;
 }

gcc/testsuite/g++.dg/modules/pr114856.h

@@ -0,0 +1,12 @@
+// PR c++/114856
+
+#include <initializer_list>
+struct A {
+  ~A();
+};
+struct V {
+  V(std::initializer_list<A>);
+};
+struct data {
+  V v{{}};
+};

gcc/testsuite/g++.dg/modules/pr114856_a.H

@@ -0,0 +1,5 @@
+// PR c++/114856
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+#include "pr114856.h"

gcc/testsuite/g++.dg/modules/pr114856_b.C

@@ -0,0 +1,5 @@
+// PR c++/114856
+// { dg-additional-options "-fmodules-ts" }
+
+#include "pr114856.h"
+import "pr114856_a.H";