c++: Fix ICE with static_cast when converting from int[] [PR93862]

This ICEs since my patch for P0388, which allowed conversions to arrays
of unknown bound, but not the reverse, so these two static_casts are
ill-formed.

[expr.static.cast]/3 says that "cv1 T1" and "cv2 T2" have to be
reference-compatible and the comment in build_static_cast_1 says it too
but then we actually use reference_related_p...  Fixed thus.

2020-02-26  Marek Polacek  <polacek@redhat.com>

	PR c++/93862 - ICE with static_cast when converting from int[].
	* call.c (reference_compatible_p): No longer static.
	* cp-tree.h (reference_compatible_p): Declare.
	* typeck.c (build_static_cast_1): Use reference_compatible_p instead
	of reference_related_p.

	* g++.dg/cpp0x/rv-cast7.C: New test.

gcc/cp/ChangeLog

 2020-02-26  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/93862 - ICE with static_cast when converting from int[].
+	* call.c (reference_compatible_p): No longer static.
+	* cp-tree.h (reference_compatible_p): Declare.
+	* typeck.c (build_static_cast_1): Use reference_compatible_p instead
+	of reference_related_p.
+
+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/93803 - ICE with constexpr init and [[no_unique_address]].
 	* constexpr.c (reduced_constant_expression_p): Don't crash on a null
 	field.

gcc/cp/call.c

@@ -204,7 +204,6 @@ static struct z_candidate *add_candidate
 	 conversion **, tree, tree, int, struct rejection_reason *, int);
 static tree source_type (conversion *);
 static void add_warning (struct z_candidate *, struct z_candidate *);
-static bool reference_compatible_p (tree, tree);
 static conversion *direct_reference_binding (tree, conversion *);
 static bool promoted_arithmetic_type_p (tree);
 static conversion *conditional_conversion (tree, tree, tsubst_flags_t);
@@ -1554,7 +1553,7 @@ reference_related_p (tree t1, tree t2)
 
 /* Returns nonzero if T1 is reference-compatible with T2.  */
 
-static bool
+bool
 reference_compatible_p (tree t1, tree t2)
 {
   /* [dcl.init.ref]

gcc/cp/cp-tree.h

@@ -6357,6 +6357,7 @@ extern tree make_temporary_var_for_ref_to_temp	(tree, tree);
 extern bool type_has_extended_temps		(tree);
 extern tree strip_top_quals			(tree);
 extern bool reference_related_p			(tree, tree);
+extern bool reference_compatible_p		(tree, tree);
 extern int remaining_arguments			(tree);
 extern tree perform_implicit_conversion		(tree, tree, tsubst_flags_t);
 extern tree perform_implicit_conversion_flags	(tree, tree, tsubst_flags_t, int);

gcc/cp/typeck.c

@@ -7383,7 +7383,7 @@ build_static_cast_1 (location_t loc, tree type, tree expr, bool c_cast_p,
   if (TYPE_REF_P (type)
       && TYPE_REF_IS_RVALUE (type)
       && (clk = real_lvalue_p (expr))
-      && reference_related_p (TREE_TYPE (type), intype)
+      && reference_compatible_p (TREE_TYPE (type), intype)
       && (c_cast_p || at_least_as_qualified_p (TREE_TYPE (type), intype)))
     {
       if (processing_template_decl)

gcc/testsuite/ChangeLog

 2020-02-26  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/93862 - ICE with static_cast when converting from int[].
+	* g++.dg/cpp0x/rv-cast7.C: New test.
+
+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
 	DR 1423, Convertibility of nullptr to bool.
 	* g++.dg/DRs/dr1423.C: New test.
 

gcc/testsuite/g++.dg/cpp0x/rv-cast7.C

+// PR c++/93862 - ICE with static_cast when converting from int[].
+// { dg-do compile { target c++11 } }
+
+int(&&intu_rvref)[]{1,2,3,4};
+int(&int4_lvref)[4] = static_cast<int(&)[4]>(intu_rvref); // { dg-error "invalid .static_cast." }
+int(&&int4_rvref)[4] = static_cast<int(&&)[4]>(intu_rvref); // { dg-error "invalid .static_cast." }