c++: quadratic constexpr behavior for left-assoc logical exprs [PR102780]

In the testcase below the two left fold expressions each expand into a
constant logical expression with 1024 terms, for which potential_const_expr
takes more than a minute to return true.  This happens because p_c_e_1
performs trial evaluation of the first operand of a &&/|| in order to
determine whether to consider the potentiality of the second operand.
And because the expanded expression is left-associated, this trial
evaluation causes p_c_e_1 to be quadratic in the number of terms of the
expression.

This patch fixes this quadratic behavior by making p_c_e_1 preemptively
compute potentiality of the second operand of a &&/||, and perform trial
evaluation of the first operand only if the second operand isn't
potentially constant.  We must be careful to avoid emitting bogus
diagnostics during the preemptive computation; to that end, we perform
this shortcut only when tf_error is cleared, and when tf_error is set we
now first check potentiality of the whole expression quietly and replay
the check noisily for diagnostics.

Apart from fixing the quadraticness for left-associated logical exprs,
this change also reduces compile time for the libstdc++ testcase
20_util/variant/87619.cc by about 15% even though our <variant> uses
right folds instead of left folds.  Likewise for the testcase in the PR,
for which compile time is reduced by 30%.  The reason for these speedups
is that p_c_e_1 no longer performs expensive trial evaluation of each term
of large constant logical expressions when determining their potentiality.

	PR c++/102780

gcc/cp/ChangeLog:

	* constexpr.c (potential_constant_expression_1) <case TRUTH_*_EXPR>:
	When tf_error isn't set, preemptively check potentiality of the
	second operand before performing trial evaluation of the first
	operand.
	(potential_constant_expression_1): When tf_error is set, first check
	potentiality quietly and return true if successful, otherwise
	proceed noisily to give errors.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/fold13.C: New test.

gcc/cp/constexpr.c

@@ -8892,13 +8892,18 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
       tmp = boolean_false_node;
     truth:
       {
-	tree op = TREE_OPERAND (t, 0);
-	if (!RECUR (op, rval))
+	tree op0 = TREE_OPERAND (t, 0);
+	tree op1 = TREE_OPERAND (t, 1);
+	if (!RECUR (op0, rval))
 	  return false;
+	if (!(flags & tf_error) && RECUR (op1, rval))
+	  /* When quiet, try to avoid expensive trial evaluation by first
+	     checking potentiality of the second operand.  */
+	  return true;
 	if (!processing_template_decl)
-	  op = cxx_eval_outermost_constant_expr (op, true);
-	if (tree_int_cst_equal (op, tmp))
-	  return RECUR (TREE_OPERAND (t, 1), rval);
+	  op0 = cxx_eval_outermost_constant_expr (op0, true);
+	if (tree_int_cst_equal (op0, tmp))
+	  return (flags & tf_error) ? RECUR (op1, rval) : false;
 	else
 	  return true;
       }
@@ -9107,6 +9112,17 @@ bool
 potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 				 tsubst_flags_t flags)
 {
+  if (flags & tf_error)
+    {
+      /* Check potentiality quietly first, as that could be performed more
+	 efficiently in some cases (currently only for TRUTH_*_EXPR).  If
+	 that fails, replay the check noisily to give errors.  */
+      flags &= ~tf_error;
+      if (potential_constant_expression_1 (t, want_rval, strict, now, flags))
+	return true;
+      flags |= tf_error;
+    }
+
   tree target = NULL_TREE;
   return potential_constant_expression_1 (t, want_rval, strict, now,
 					  flags, &target);

gcc/testsuite/g++.dg/cpp1z/fold13.C

@@ -0,0 +1,29 @@
+// { dg-do compile { target c++17 } }
+// Verify constexpr evaluation of a large left fold logical expression
+// isn't quadratic in the size of the expanded expression.
+
+template<int> struct S { static constexpr bool value = true; };
+
+template<class T, T...> struct integer_sequence { };
+
+template<class T, T N>
+using make_integer_sequence
+#if __has_builtin(__make_integer_seq)
+  = __make_integer_seq<integer_sequence, T, N>;
+#else
+  = integer_sequence<T, __integer_pack(N)...>;
+#endif
+
+template<int... Is>
+constexpr bool f_impl(integer_sequence<int, Is...>) {
+  return (... && S<Is>::value);
+}
+
+static_assert(f_impl(make_integer_sequence<int, 1024>()));
+
+template<int... Is>
+constexpr bool g_impl(integer_sequence<int, Is...>) {
+  return (... || !S<Is>::value);
+}
+
+static_assert(!g_impl(make_integer_sequence<int, 1024>()));