Implement C++20 operator<=>.

There are three major pieces to this support: scalar operator<=>,
synthesis of comparison operators, and rewritten/reversed overload
resolution (e.g. a < b becomes 0 > b <=> a).

Unlike other defaulted functions, where we use synthesized_method_walk to
semi-simulate what the definition of the function will be like, this patch
determines the characteristics of a comparison operator by trying to define
it.

My handling of non-dependent rewritten operators in templates can still use
some work: build_min_non_dep_op_overload can't understand the rewrites and
crashes, so I'm avoiding it for now by clearing *overload.  This means we'll
do name lookup again at instantiation time, which can incorrectly mean a
different result.  I'll poke at this more in stage 3.

I'm leaving out a fourth section ("strong structural equality") even though
I've implemented it, because it seems likely to change radically tomorrow.

Thanks to Tim van Deurzen and Jakub for implementing lexing of the <=>
operator, and Jonathan for the initial <compare> header.

gcc/cp/
	* cp-tree.h (struct lang_decl_fn): Add maybe_deleted bitfield.
	(DECL_MAYBE_DELETED): New.
	(enum special_function_kind): Add sfk_comparison.
	(LOOKUP_REWRITTEN, LOOKUP_REVERSED): New.
	* call.c (struct z_candidate): Add rewritten and reversed methods.
	(add_builtin_candidate): Handle SPACESHIP_EXPR.
	(add_builtin_candidates): Likewise.
	(add_candidates): Don't add a reversed candidate if the parms are
	the same.
	(add_operator_candidates): Split out from build_new_op_1.  Handle
	rewritten and reversed candidates.
	(add_candidate): Swap conversions of reversed candidate.
	(build_new_op_1): Swap them back.  Build a second operation for
	rewritten candidates.
	(extract_call_expr): Handle rewritten calls.
	(same_fn_or_template): New.
	(joust): Handle rewritten and reversed candidates.
	* class.c (add_implicitly_declared_members): Add implicit op==.
	(classtype_has_op, classtype_has_defaulted_op): New.
	* constexpr.c (cxx_eval_binary_expression): Handle SPACESHIP_EXPR.
	(cxx_eval_constant_expression, potential_constant_expression_1):
	Likewise.
	* cp-gimplify.c (genericize_spaceship): New.
	(cp_genericize_r): Use it.
	* cp-objcp-common.c (cp_common_init_ts): Handle SPACESHIP_EXPR.
	* decl.c (finish_function): Handle deleted function.
	* decl2.c (grokfield): SET_DECL_FRIEND_CONTEXT on defaulted friend.
	(mark_used): Check DECL_MAYBE_DELETED.  Remove assumption that
	defaulted functions are non-static members.
	* error.c (dump_expr): Handle SPACESHIP_EXPR.
	* method.c (type_has_trivial_fn): False for sfk_comparison.
	(enum comp_cat_tag, struct comp_cat_info_t): New types.
	(comp_cat_cache): New array variable.
	(lookup_comparison_result, lookup_comparison_category)
	(is_cat, cat_tag_for, spaceship_comp_cat)
	(spaceship_type, genericize_spaceship)
	(common_comparison_type, early_check_defaulted_comparison)
	(comp_info, build_comparison_op): New.
	(synthesize_method): Handle sfk_comparison.  Handle deleted.
	(get_defaulted_eh_spec, maybe_explain_implicit_delete)
	(explain_implicit_non_constexpr, implicitly_declare_fn)
	(defaulted_late_check, defaultable_fn_check): Handle sfk_comparison.
	* name-lookup.c (get_std_name_hint): Add comparison categories.
	* tree.c (special_function_p): Add sfk_comparison.
	* typeck.c (cp_build_binary_op): Handle SPACESHIP_EXPR.

2019-11-05  Tim van Deurzen  <tim@kompiler.org>

	Add new tree code for the spaceship operator.
gcc/cp/
	* cp-tree.def: Add new tree code.
	* operators.def: New binary operator.
	* parser.c: Add new token and tree code.
libcpp/
	* cpplib.h: Add spaceship operator for C++.
	* lex.c: Implement conditional lexing of spaceship operator for C++20.

2019-11-05  Jonathan Wakely  <jwakely@redhat.com>

libstdc++-v3/
	* libsupc++/compare: New header.
	* libsupc++/Makefile.am (std_HEADERS): Add compare.
	* include/std/version: Define __cpp_lib_three_way_comparison.
	* include/std/functional: #include <compare>.

From-SVN: r277865

3 files changed, 14 insertions, 1 deletions

diff --git a/libcpp/ChangeLog b/libcpp/ChangeLog
index a13f7508599..8be84386d6c 100644
--- a/libcpp/ChangeLog
+++ b/libcpp/ChangeLog
@@ -1,3 +1,8 @@
+2019-11-05  Tim van Deurzen  <tim@kompiler.org>
+
+	* cpplib.h: Add spaceship operator for C++.
+	* lex.c: Implement conditional lexing of spaceship operator for C++20.
+
 2019-10-31  Jakub Jelinek  <jakub@redhat.com>
 
 	PR preprocessor/92296
diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index c655d3ffc90..ed108f17bfa 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -78,6 +78,7 @@ struct _cpp_file;
   OP(NOT_EQ,		"!=")						\
   OP(GREATER_EQ,	">=")						\
   OP(LESS_EQ,		"<=")						\
+  OP(SPACESHIP,		"<=>")						\
 									\
   /* These two are unary + / - in preprocessor expressions.  */		\
   OP(PLUS_EQ,		"+=")	/* math */				\
diff --git a/libcpp/lex.c b/libcpp/lex.c
index 3e7d1c37ff5..e95eda3f44e 100644
--- a/libcpp/lex.c
+++ b/libcpp/lex.c
@@ -2980,7 +2980,13 @@ _cpp_lex_direct (cpp_reader *pfile)
 
       result->type = CPP_LESS;
       if (*buffer->cur == '=')
-	buffer->cur++, result->type = CPP_LESS_EQ;
+	{
+	  buffer->cur++, result->type = CPP_LESS_EQ;
+	  if (*buffer->cur == '>'
+	      && CPP_OPTION (pfile, cplusplus)
+	      && CPP_OPTION (pfile, lang) >= CLK_GNUCXX2A)
+	    buffer->cur++, result->type = CPP_SPACESHIP;
+	}
       else if (*buffer->cur == '<')
 	{
 	  buffer->cur++;
@@ -3491,6 +3497,7 @@ cpp_avoid_paste (cpp_reader *pfile, const cpp_token *token1,
 				|| (CPP_OPTION (pfile, objc)
 				    && token1->val.str.text[0] == '@'
 				    && (b == CPP_NAME || b == CPP_STRING)));
+    case CPP_LESS_EQ:	return c == '>';
     case CPP_STRING:
     case CPP_WSTRING:
     case CPP_UTF8STRING: