c++: -Wunused-value on ternary indexed by non-constant

c++: -Wunused-value on ternary indexed by non-constant

On this expression:

	(true ? "a" : "b")[index()]

"g++ -Wunused-value" incorrectly produces

	warning: left operand of comma operator has no effect [-Wunused-value]

From the -fdump-tree-original output:

	if ((void) SAVE_EXPR <index ()>, 1)
	  {
	    (void) "a"[SAVE_EXPR <index ()>];
	  }
	else
	  {
	    (void) "b"[SAVE_EXPR <index ()>];
	  }

Observe that we evaluate index() (and save it) before evaluating the
ternary expression.  Since "(void) SAVE_EXPR <index ()>" is ostensibly
side-effect free, we get this warning.  Since SAVE_EXPR is not useless,
this is a false positive. Also the comma operator compiler-generated,
so warning about it is wrong.

Suppress this warning for this implicit expression. Test that the
warning is gone for "$ternary[index()]" but we still warn on cases like
"$ternary[(1, 0)]".

gcc/cp/ChangeLog:

	* typeck.cc (cp_build_array_ref): Suppress unused-value
	warning for implicit comma expression.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wunused-value-2.C: New test.

Signed-off-by: Johannes Altmanninger <aclopte@gmail.com>
Co-authored-by: Jason Merrill <jason@redhat.com>