c++: Fix constexpr evaluation of SPACESHIP_EXPR [PR96497]

The following valid testcase is rejected, because cxx_eval_binary_expression
is called on the SPACESHIP_EXPR with lval = true, as the address of the
spaceship needs to be passed to a method call.
After recursing on the operands and calling genericize_spaceship which turns
it into a TARGET_EXPR with initialization, we call cxx_eval_constant_expression
on it which succeeds, but then we fall through into code that will
VERIFY_CONSTANT (r) which FAILs because it is an address of a variable.  Rather
than avoiding that for lval = true and SPACESHIP_EXPR, the patch just tail
calls cxx_eval_constant_expression - I believe that call should perform all
the needed verifications.

2020-08-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/96497
	* constexpr.c (cxx_eval_binary_expression): For SPACESHIP_EXPR, tail
	call cxx_eval_constant_expression after genericize_spaceship to avoid
	undesirable further VERIFY_CONSTANT.

	* g++.dg/cpp2a/spaceship-constexpr3.C: New test.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index ab747a5..8ee0f2a 100644 (file)
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3085,8 +3085,8 @@ cxx_eval_binary_expression (const constexpr_ctx *ctx, tree t,
   else if (code == SPACESHIP_EXPR)
     {
       r = genericize_spaceship (type, lhs, rhs);
-      r = cxx_eval_constant_expression (ctx, r, lval, non_constant_p,
-                                       overflow_p);
+      return cxx_eval_constant_expression (ctx, r, lval, non_constant_p,
+                                          overflow_p);
     }
 
   if (r == NULL_TREE)

diff --git a/gcc/testsuite/g++.dg/cpp2a/spaceship-constexpr3.C b/gcc/testsuite/g++.dg/cpp2a/spaceship-constexpr3.C
new file mode 100644 (file)
index 0000000..95f03e6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/spaceship-constexpr3.C
@@ -0,0 +1,7 @@
+// PR c++/96497
+// { dg-do compile { target c++20 } }
+
+#include <compare>
+
+static_assert(std::partial_ordering(std::strong_ordering::less) < 0);
+static_assert(std::partial_ordering(1 <=> 2) < 0);