c++: Fix static initialization from <=>.

Constant evaluation of genericize_spaceship produced a CONSTRUCTOR, which we
then wanted to bind to a reference, which we can't do.  So wrap the result
in a TARGET_EXPR so we get something with an address.

We also need to handle treating the result of cxx_eval_binary_expression as
a glvalue for SPACESHIP_EXPR.

My earlier change to add uid_sensitive to maybe_constant_value was wrong; we
don't even look at the cache when manifestly_const_eval, and I failed to
adjust the later call to cxx_eval_outermost_constant_expr.

gcc/cp/ChangeLog
2020-02-11  Jason Merrill  <jason@redhat.com>

	PR c++/93650
	PR c++/90691
	* constexpr.c (maybe_constant_value): Correct earlier change.
	(cxx_eval_binary_expression) [SPACESHIP_EXPR]: Pass lval through.
	* method.c (genericize_spaceship): Wrap result in TARGET_EXPR.

gcc/cp/ChangeLog

+2020-02-11  Jason Merrill  <jason@redhat.com>
+
+	PR c++/93650
+	PR c++/90691
+	* constexpr.c (maybe_constant_value): Correct earlier change.
+	(cxx_eval_binary_expression) [SPACESHIP_EXPR]: Pass lval through.
+	* method.c (genericize_spaceship): Wrap result in TARGET_EXPR.
+
 2020-02-12  Patrick Palka  <ppalka@redhat.com>
 
 	PR c++/69448

gcc/cp/constexpr.c

@@ -2834,7 +2834,7 @@ cxx_fold_pointer_plus_expression (const constexpr_ctx *ctx, tree t,
 
 static tree
 cxx_eval_binary_expression (const constexpr_ctx *ctx, tree t,
-			    bool /*lval*/,
+			    bool lval,
 			    bool *non_constant_p, bool *overflow_p)
 {
   tree r = NULL_TREE;
@@ -2902,7 +2902,7 @@ 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, false, non_constant_p,
+      r = cxx_eval_constant_expression (ctx, r, lval, non_constant_p,
 					overflow_p);
     }
 
@@ -6686,13 +6686,11 @@ maybe_constant_value (tree t, tree decl, bool manifestly_const_eval,
 	  r = unshare_expr_without_location (r);
 	  protected_set_expr_location (r, EXPR_LOCATION (t));
 	}
-      if (r != t || TREE_CONSTANT (t) || !manifestly_const_eval)
-	return r;
-      /* If we cached this as non-constant and we need a constant value, try
-	 again; we might have failed before due to UID_SENSITIVE.  */
+      return r;
     }
 
-  r = cxx_eval_outermost_constant_expr (t, true, true, false, false, decl);
+  r = cxx_eval_outermost_constant_expr (t, true, true, false, false,
+					decl, uid_sensitive);
   gcc_checking_assert (r == t
 		       || CONVERT_EXPR_P (t)
 		       || TREE_CODE (t) == VIEW_CONVERT_EXPR

gcc/cp/method.c

@@ -1075,6 +1075,9 @@ genericize_spaceship (tree type, tree op0, tree op1)
   comp = fold_build2 (EQ_EXPR, boolean_type_node, op0, op1);
   r = fold_build3 (COND_EXPR, type, comp, eq, r);
 
+  /* Wrap the whole thing in a TARGET_EXPR like build_conditional_expr_1.  */
+  r = get_target_expr (r);
+
   return r;
 }
 

gcc/testsuite/g++.dg/cpp2a/spaceship-constexpr2.C

+// PR c++/93650
+// { dg-do compile { target c++2a } }
+
+namespace std {
+  using type = enum _Ord { less };
+  class strong_ordering {
+    type _M_value;
+    constexpr strong_ordering(_Ord) : _M_value() {}
+    static const strong_ordering less;
+    static strong_ordering equal;
+    static strong_ordering greater;
+  } constexpr strong_ordering::less(_Ord::less);
+  auto v = 1 <=> 2;
+}