PR c++/56973, DR 696 - capture constant variables only as needed.

	* expr.c (mark_use): Split out from mark_rvalue_use and
	mark_lvalue_use.  Handle lambda capture of constant variables.
	(mark_lvalue_use_nonread): New.
	* semantics.c (process_outer_var_ref): Don't capture a constant
	variable until forced.
	* pt.c (processing_nonlambda_template): New.
	* call.c (build_this): Check it.
	* decl2.c (grok_array_decl): Call mark_rvalue_use and
	mark_lvalue_use_nonread.
	* init.c (constant_value_1): Don't call mark_rvalue_use.
	* typeck.c (build_static_cast): Handle lambda capture.

From-SVN: r253266

1 files changed, 8 insertions, 16 deletions

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 4e87e47..d96423f 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -3282,7 +3282,7 @@ outer_automatic_var_p (tree decl)
    rewrite it for lambda capture.  */
 
 tree
-process_outer_var_ref (tree decl, tsubst_flags_t complain)
+process_outer_var_ref (tree decl, tsubst_flags_t complain, bool force_use)
 {
   if (cp_unevaluated_operand)
     /* It's not a use (3.2) if we're in an unevaluated context.  */
@@ -3303,6 +3303,12 @@ process_outer_var_ref (tree decl, tsubst_flags_t complain)
   if (parsing_nsdmi ())
     containing_function = NULL_TREE;
 
+  /* Core issue 696: Only an odr-use of an outer automatic variable causes a
+     capture (or error), and a constant variable can decay to a prvalue
+     constant without odr-use.  So don't capture yet.  */
+  if (decl_constant_var_p (decl) && !force_use)
+    return decl;
+
   if (containing_function && LAMBDA_FUNCTION_P (containing_function))
     {
       /* Check whether we've already built a proxy.  */
@@ -3314,7 +3320,7 @@ process_outer_var_ref (tree decl, tsubst_flags_t complain)
 	    return d;
 	  else
 	    /* We need to capture an outer proxy.  */
-	    return process_outer_var_ref (d, complain);
+	    return process_outer_var_ref (d, complain, force_use);
 	}
     }
 
@@ -3353,20 +3359,6 @@ process_outer_var_ref (tree decl, tsubst_flags_t complain)
       && uses_template_parms (DECL_TI_ARGS (containing_function)))
     return decl;
 
-  /* Core issue 696: "[At the July 2009 meeting] the CWG expressed
-     support for an approach in which a reference to a local
-     [constant] automatic variable in a nested class or lambda body
-     would enter the expression as an rvalue, which would reduce
-     the complexity of the problem"
-
-     FIXME update for final resolution of core issue 696.  */
-  if (decl_constant_var_p (decl))
-    {
-      tree t = maybe_constant_value (convert_from_reference (decl));
-      if (TREE_CONSTANT (t))
-	return t;
-    }
-
   if (lambda_expr && VAR_P (decl)
       && DECL_ANON_UNION_VAR_P (decl))
     {