c++: Fix bogus -Wparentheses warning with fold-expression [PR94505]

We issue bogus -Wparentheses warnings (3 of them!) for this fold expression:

  ((B && true) || ...)

Firstly, issuing a warning for a compiler-generated expression is wrong
and secondly, B && true must be wrapped in ( ) otherwise you'll get
error: binary expression in operand of fold-expression.

	PR c++/94505 - bogus -Wparentheses warning with fold-expression.
	* pt.c (fold_expression): Add warning_sentinel for -Wparentheses
	before calling build_x_binary_op.

	* g++.dg/cpp1z/fold11.C: New test.

gcc/cp/ChangeLog

 2020-04-20  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/94505 - bogus -Wparentheses warning with fold-expression.
+	* pt.c (fold_expression): Add warning_sentinel for -Wparentheses
+	before calling build_x_binary_op.
+
+2020-04-20  Marek Polacek  <polacek@redhat.com>
+
 	* coroutines.cc (captures_temporary): Don't assign the result of
 	STRIP_NOPS to the same variable.
 

gcc/cp/pt.c

@@ -12379,6 +12379,7 @@ fold_expression (tree t, tree left, tree right, tsubst_flags_t complain)
   if (FOLD_EXPR_MODIFY_P (t))
     return build_x_modify_expr (input_location, left, code, right, complain);
 
+  warning_sentinel s(warn_parentheses);
   switch (code)
     {
     case COMPOUND_EXPR:

gcc/testsuite/ChangeLog

+2020-04-20  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94505 - bogus -Wparentheses warning with fold-expression.
+	* g++.dg/cpp1z/fold11.C: New test.
+
 2020-04-20  Andreas Krebbel  <krebbel@linux.ibm.com>
 
 	* g++.dg/pr94666.C: New test.

gcc/testsuite/g++.dg/cpp1z/fold11.C

+// PR c++/94505 - bogus -Wparentheses warning with fold-expression.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wparentheses" }
+
+template <bool... B>
+bool foo () {
+    return ((B && true) || ...); // { dg-bogus "suggest parentheses" }
+}
+
+int main () {
+    foo<true, false, false, true> ();
+}