P1064R0 - Allowing Virtual Function Calls in Constant Expressions

	P1064R0 - Allowing Virtual Function Calls in Constant Expressions
	* call.c (build_over_call): No longer check if we're outside a template
	function.
	* class.c (build_vtbl_initializer): Build vtable's constructor with
	indexes.
	* constexpr.c (cxx_eval_constant_expression): Don't ignore _vptr's
	initializer.  Handle OBJ_TYPE_REF.
	(potential_constant_expression_1): Handle OBJ_TYPE_REF.
	* decl.c (maybe_commonize_var): Bail out for any DECL_ARTIFICIAL.
	(initialize_artificial_var): Mark the variable as constexpr.
	(grokdeclarator): Change error to pedwarn.  Only warn when
	pedantic and not C++2a.

	* gimple-fold.c (gimple_get_virt_method_for_vtable): Adjust assert.

	* g++.dg/cpp0x/constexpr-virtual5.C: Adjust dg-error.
	* g++.dg/cpp2a/constexpr-virtual1.C: New test.
	* g++.dg/cpp2a/constexpr-virtual2.C: New test.
	* g++.dg/cpp2a/constexpr-virtual3.C: New test.
	* g++.dg/cpp2a/constexpr-virtual4.C: New test.
	* g++.dg/cpp2a/constexpr-virtual5.C: New test.
	* g++.dg/cpp2a/constexpr-virtual6.C: New test.
	* g++.dg/cpp2a/constexpr-virtual7.C: New test.
	* g++.dg/cpp2a/constexpr-virtual8.C: New test.
	* g++.dg/cpp2a/constexpr-virtual9.C: New test.
	* g++.dg/diagnostic/virtual-constexpr.C: Skip for C++2a.  Use
	-pedantic-errors.  Adjust dg-error.

From-SVN: r264408

5 files changed, 72 insertions, 14 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index fdc1247..7ea4b6d 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,18 @@
+2018-09-18  Marek Polacek  <polacek@redhat.com>
+
+	P1064R0 - Allowing Virtual Function Calls in Constant Expressions
+	* call.c (build_over_call): No longer check if we're outside a template
+	function.
+	* class.c (build_vtbl_initializer): Build vtable's constructor with
+	indexes.
+	* constexpr.c (cxx_eval_constant_expression): Don't ignore _vptr's
+	initializer.  Handle OBJ_TYPE_REF.
+	(potential_constant_expression_1): Handle OBJ_TYPE_REF.
+	* decl.c (maybe_commonize_var): Bail out for any DECL_ARTIFICIAL.
+	(initialize_artificial_var): Mark the variable as constexpr.
+	(grokdeclarator): Change error to pedwarn.  Only warn when
+	pedantic and not C++2a.
+
 2018-09-18  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/85065
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 69503ca..ddf0ed0 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -8399,10 +8399,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
       && DECL_BUILT_IN_CLASS (fn) == BUILT_IN_NORMAL)
     maybe_warn_class_memaccess (input_location, fn, args);
 
-  if (DECL_VINDEX (fn) && (flags & LOOKUP_NONVIRTUAL) == 0
-      /* Don't mess with virtual lookup in instantiate_non_dependent_expr;
-	 virtual functions can't be constexpr.  */
-      && !in_template_function ())
+  if (DECL_VINDEX (fn) && (flags & LOOKUP_NONVIRTUAL) == 0)
     {
       tree t;
       tree binfo = lookup_base (TREE_TYPE (TREE_TYPE (argarray[0])),
diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index e950a74..9ca4644 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -9266,6 +9266,7 @@ build_vtbl_initializer (tree binfo,
       tree vcall_index;
       tree fn, fn_original;
       tree init = NULL_TREE;
+      tree idx = size_int (jx++);
 
       fn = BV_FN (v);
       fn_original = fn;
@@ -9369,7 +9370,7 @@ build_vtbl_initializer (tree binfo,
 	  int i;
 	  if (init == size_zero_node)
 	    for (i = 0; i < TARGET_VTABLE_USES_DESCRIPTORS; ++i)
-	      CONSTRUCTOR_APPEND_ELT (*inits, NULL_TREE, init);
+	      CONSTRUCTOR_APPEND_ELT (*inits, idx, init);
 	  else
 	    for (i = 0; i < TARGET_VTABLE_USES_DESCRIPTORS; ++i)
 	      {
@@ -9377,11 +9378,11 @@ build_vtbl_initializer (tree binfo,
 				     fn, build_int_cst (NULL_TREE, i));
 		TREE_CONSTANT (fdesc) = 1;
 
-		CONSTRUCTOR_APPEND_ELT (*inits, NULL_TREE, fdesc);
+		CONSTRUCTOR_APPEND_ELT (*inits, idx, fdesc);
 	      }
 	}
       else
-	CONSTRUCTOR_APPEND_ELT (*inits, NULL_TREE, init);
+	CONSTRUCTOR_APPEND_ELT (*inits, idx, init);
     }
 }
 
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 88c7378..aa33319 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -4209,7 +4209,11 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	 CONST_DECL for aggregate constants.  */
       if (lval)
 	return t;
+      /* is_really_empty_class doesn't take into account _vptr, so initializing
+	 otherwise empty class with { } would overwrite the initializer that
+	 initialize_vtable created for us.  */
       if (COMPLETE_TYPE_P (TREE_TYPE (t))
+	  && !TYPE_POLYMORPHIC_P (TREE_TYPE (t))
 	  && is_really_empty_class (TREE_TYPE (t)))
 	{
 	  /* If the class is empty, we aren't actually loading anything.  */
@@ -4778,7 +4782,6 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
     case MODOP_EXPR:
       /* GCC internal stuff.  */
     case VA_ARG_EXPR:
-    case OBJ_TYPE_REF:
     case NON_DEPENDENT_EXPR:
     case BASELINK:
     case OFFSET_REF:
@@ -4788,6 +4791,34 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       *non_constant_p = true;
       break;
 
+    case OBJ_TYPE_REF:
+      {
+	/* Virtual function call.  Let the constexpr machinery figure out
+	   the dynamic type.  */
+	int token = tree_to_shwi (OBJ_TYPE_REF_TOKEN (t));
+	tree obj = OBJ_TYPE_REF_OBJECT (t);
+	obj = cxx_eval_constant_expression (ctx, obj, lval, non_constant_p,
+					    overflow_p);
+	/* We expect something in the form of &x.D.2103.D.2094; get x. */
+	if (TREE_CODE (obj) != ADDR_EXPR)
+	  {
+	    if (!ctx->quiet)
+	      error_at (cp_expr_loc_or_loc (t, input_location),
+			"expression %qE is not a constant expression", t);
+	    *non_constant_p = true;
+	    return t;
+	  }
+	obj = TREE_OPERAND (obj, 0);
+	while (handled_component_p (obj))
+	  obj = TREE_OPERAND (obj, 0);
+	tree objtype = TREE_TYPE (obj);
+	/* Find the function decl in the virtual functions list.  TOKEN is
+	   the DECL_VINDEX that says which function we're looking for.  */
+	tree virtuals = BINFO_VIRTUALS (TYPE_BINFO (objtype));
+	r = TREE_VALUE (chain_index (token, virtuals));
+	break;
+      }
+
     case PLACEHOLDER_EXPR:
       /* Use of the value or address of the current object.  */
       if (tree ctor = lookup_placeholder (ctx, lval, TREE_TYPE (t)))
@@ -5871,7 +5902,6 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
     case OACC_UPDATE:
       /* GCC internal stuff.  */
     case VA_ARG_EXPR:
-    case OBJ_TYPE_REF:
     case TRANSACTION_EXPR:
     case ASM_EXPR:
     case AT_ENCODE_EXPR:
@@ -5880,6 +5910,14 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	error_at (loc, "expression %qE is not a constant expression", t);
       return false;
 
+    case OBJ_TYPE_REF:
+      if (cxx_dialect >= cxx2a)
+	/* In C++2a virtual calls can be constexpr, don't give up yet.  */
+	return true;
+      else if (flags & tf_error)
+	error_at (loc, "virtual functions cannot be constexpr before C++2a");
+      return false;
+
     case TYPEID_EXPR:
       /* -- a typeid expression whose operand is of polymorphic
             class type;  */
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 50b60e8..827c172 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -5583,11 +5583,13 @@ layout_var_decl (tree decl)
 void
 maybe_commonize_var (tree decl)
 {
+  /* Don't mess with __FUNCTION__ and similar.  */
+  if (DECL_ARTIFICIAL (decl))
+    return;
+
   /* Static data in a function with comdat linkage also has comdat
      linkage.  */
   if ((TREE_STATIC (decl)
-       /* Don't mess with __FUNCTION__.  */
-       && ! DECL_ARTIFICIAL (decl)
        && DECL_FUNCTION_SCOPE_P (decl)
        && vague_linkage_p (DECL_CONTEXT (decl)))
       || (TREE_PUBLIC (decl) && DECL_INLINE_VAR_P (decl)))
@@ -6774,6 +6776,10 @@ initialize_artificial_var (tree decl, vec<constructor_elt, va_gc> *v)
   gcc_assert (TREE_CODE (init) == CONSTRUCTOR);
   DECL_INITIAL (decl) = init;
   DECL_INITIALIZED_P (decl) = 1;
+  /* Mark the decl as constexpr so that we can access its content
+     at compile time.  */
+  DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl) = true;
+  DECL_DECLARED_CONSTEXPR_P (decl) = true;
   determine_visibility (decl);
   layout_var_decl (decl);
   maybe_commonize_var (decl);
@@ -10854,12 +10860,13 @@ grokdeclarator (const cp_declarator *declarator,
 	  storage_class = sc_none;
 	  staticp = 0;
 	}
-      if (constexpr_p)
+      if (constexpr_p && cxx_dialect < cxx2a)
 	{
 	  gcc_rich_location richloc (declspecs->locations[ds_virtual]);
 	  richloc.add_range (declspecs->locations[ds_constexpr]);
-	  error_at (&richloc, "member %qD cannot be declared both %<virtual%> "
-		    "and %<constexpr%>", dname);
+	  pedwarn (&richloc, OPT_Wpedantic, "member %qD can be declared both "
+		   "%<virtual%> and %<constexpr%> only in -std=c++2a or "
+		   "-std=gnu++2a", dname);
 	}
     }
   friendp = decl_spec_seq_has_spec_p (declspecs, ds_friend);