c++: Fix bogus -Wvolatile warning in C++20 [PR98947]

Since most of volatile is deprecated in C++20, we are required to warn
for compound assignments to volatile variables and so on.  But here we
have

  volatile int x, y, z;
  (b ? x : y) = 1;

and we shouldn't warn, because simple assignments like x = 24; should
not provoke the warning when they are a discarded-value expression.

We warn here because when ?: is used as an lvalue, we transform it in
cp_build_modify_expr/COND_EXPR from (a ? b : c) = rhs to

  (a ? (b = rhs) : (c = rhs))

and build_conditional_expr then calls mark_lvalue_use for the new
artificial assignments, which then evokes the warning.  The calls
to mark_lvalue_use were added in r160289 to suppress warnings in
Wunused-var-10.c, but looks like they're no longer needed.

To warn on

    (b ? (x = 2) : y) = 1;
    (b ? x : (y = 5)) = 1;

I've tweaked a check in mark_use/MODIFY_EXPR.

I'd argue this is a regression because GCC 9 doesn't warn.

gcc/cp/ChangeLog:

	PR c++/98947
	* call.c (build_conditional_expr_1): Don't call mark_lvalue_use
	on arg2/arg3.
	* expr.c (mark_use) <case MODIFY_EXPR>: Don't check read_p when
	issuing the -Wvolatile warning.  Only set TREE_THIS_VOLATILE if
	a warning was emitted.

gcc/testsuite/ChangeLog:

	PR c++/98947
	* g++.dg/cpp2a/volatile5.C: New test.

1 files changed, 7 insertions, 7 deletions

diff --git a/gcc/cp/expr.c b/gcc/cp/expr.c
index 480e740..d16d189 100644
--- a/gcc/cp/expr.c
+++ b/gcc/cp/expr.c
@@ -224,17 +224,17 @@ mark_use (tree expr, bool rvalue_p, bool read_p,
 	     a volatile-qualified type is deprecated unless the assignment
 	     is either a discarded-value expression or appears in an
 	     unevaluated context."  */
-	  if (read_p
-	      && !cp_unevaluated_operand
+	  if (!cp_unevaluated_operand
 	      && (TREE_THIS_VOLATILE (lhs)
 		  || CP_TYPE_VOLATILE_P (TREE_TYPE (lhs)))
 	      && !TREE_THIS_VOLATILE (expr))
 	    {
-	      warning_at (location_of (expr), OPT_Wvolatile,
-			  "using value of simple assignment with %<volatile%>-"
-			  "qualified left operand is deprecated");
-	      /* Make sure not to warn about this assignment again.  */
-	      TREE_THIS_VOLATILE (expr) = true;
+	      if (warning_at (location_of (expr), OPT_Wvolatile,
+			      "using value of simple assignment with "
+			      "%<volatile%>-qualified left operand is "
+			      "deprecated"))
+		/* Make sure not to warn about this assignment again.  */
+		TREE_THIS_VOLATILE (expr) = true;
 	    }
 	  break;
 	}