[gcc r13-6133] c++: static_assert (false) in template [DR2518]

[gcc r13-6133] c++: static_assert (false) in template [DR2518]

[Jason Merrill]

https://gcc.gnu.org/g:9944ca17c0766623bce260684edc614def7ea761

commit r13-6133-g9944ca17c0766623bce260684edc614def7ea761
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Feb 10 16:16:45 2023 -0800

    c++: static_assert (false) in template [DR2518]
    
    For a long time, people have expected to be able to write
    static_assert (false) in a template and only have it diagnosed if the
    template is instantiated, but we (and other implementations) gave an error
    about the uninstantiated template because the standard says that if no valid
    instantiation of the template is possible, the program is ill-formed, no
    diagnostic required, and we try to diagnose IFNDR things when feasible.
    
    At the meeting last week we were looking at CWG2518, which wanted to specify
    that an implementation must not accept a program containing a failing #error
    or static_assert.  We also looked at P2593, which proposed allowing
    static_assert in an uninstantiated template.  We ended up combining these
    two in order to avoid requiring implementations to reject programs with
    static_assert (false) in uninstantiated templates.
    
    The committee accepted this as a DR, so I'm making the change to all
    standard modes.  This behavior was also conformant previously, since no
    diagnostic was required in this case.
    
    We continue to diagnose non-constant or otherwise ill-formed conditions, so
    no changes to existing tests were needed.
    
            DR 2518
            PR c++/52809
            PR c++/53638
            PR c++/87389
            PR c++/89741
            PR c++/92099
            PR c++/104041
            PR c++/104691
    
    gcc/cp/ChangeLog:
    
            * semantics.cc (finish_static_assert): Don't diagnose in
            template context.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/DRs/dr2518.C: New test.

Diff:
---
 gcc/cp/semantics.cc               | 17 ++++++++++-------
 gcc/testsuite/g++.dg/DRs/dr2518.C |  7 +++++++
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index c2df0b69b30..79b7cc72f21 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -11232,14 +11232,16 @@ finish_static_assert (tree condition, tree message, location_t location,
   if (check_for_bare_parameter_packs (condition))
     condition = error_mark_node;
 
+  /* Save the condition in case it was a concept check.  */
+  tree orig_condition = condition;
+
   if (instantiation_dependent_expression_p (condition))
     {
       /* We're in a template; build a STATIC_ASSERT and put it in
          the right place. */
-      tree assertion;
-
-      assertion = make_node (STATIC_ASSERT);
-      STATIC_ASSERT_CONDITION (assertion) = condition;
+    defer:
+      tree assertion = make_node (STATIC_ASSERT);
+      STATIC_ASSERT_CONDITION (assertion) = orig_condition;
       STATIC_ASSERT_MESSAGE (assertion) = message;
       STATIC_ASSERT_SOURCE_LOCATION (assertion) = location;
 
@@ -11253,9 +11255,6 @@ finish_static_assert (tree condition, tree message, location_t location,
       return;
     }
 
-  /* Save the condition in case it was a concept check.  */
-  tree orig_condition = condition;
-
   /* Fold the expression and convert it to a boolean value. */
   condition = contextual_conv_bool (condition, complain);
   condition = fold_non_dependent_expr (condition, complain,
@@ -11270,6 +11269,10 @@ finish_static_assert (tree condition, tree message, location_t location,
 
       if (integer_zerop (condition))
 	{
+	  /* CWG2518: static_assert failure in a template is not IFNDR.  */
+	  if (processing_template_decl)
+	    goto defer;
+
 	  int sz = TREE_INT_CST_LOW (TYPE_SIZE_UNIT
 				     (TREE_TYPE (TREE_TYPE (message))));
 	  int len = TREE_STRING_LENGTH (message) / sz - 1;
diff --git a/gcc/testsuite/g++.dg/DRs/dr2518.C b/gcc/testsuite/g++.dg/DRs/dr2518.C
new file mode 100644
index 00000000000..240186211e6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2518.C
@@ -0,0 +1,7 @@
+// CWG 2518
+// { dg-do compile { target c++11 } }
+
+template <class T> void f()
+{
+  static_assert (false, "");
+}