c, c++: Fix up -Wunused-value on COMPLEX_EXPRs [PR97748]

The -Wunused-value warning in both C and C++ FEs (implemented
significantly differently between the two) sees the COMPLEX_EXPRs created
e.g. for complex pre/post increment and many other expressions as useless
and warns about it.

For the C warning implementation, on e.g.
COMPLEX_EXPR < ++REALPART_EXPR <x>, IMAGPART_EXPR <x>>;
would warn even on the IMAGPART_EXPR <x> there alone etc., so what works
is check if we'd warn about both operands of COMPLEX_EXPR and if yes,
warn on the whole COMPLEX_EXPR, otherwise don't warn.

The C++ warning implementation is significantly different and for that one
the only warn if both would be warned about doesn't really work,
we then miss warnings e.g. about
COMPLEX_EXPR <REALPART_EXPR <SAVE_EXPR <x>> + 1.0e+0, IMAGPART_EXPR <SAVE_EXPR <x>>> >>>>>
The patch replaces the warning_at call with call to the c-family
warn_if_unused_value function.

On the testcase which after the initial new tests contains pretty much
everything from gcc.dg/Wunused-value-1.c both approaches seem to work
nicely.

2020-11-10  Jakub Jelinek  <jakub@redhat.com>

	PR c/97748
gcc/c-family/
	* c-common.h (warn_if_unused_value): Add quiet argument defaulted
	to false.
	* c-warn.c (warn_if_unused_value): Likewise.  Pass it down
	recursively and just return true instead of warning if it is true.
	Handle COMPLEX_EXPR.
gcc/cp/
	* cvt.c (convert_to_void): Check (complain & tf_warning) in the outer
	if rather than twice times in the inner one.  Use warn_if_unused_value.
	Formatting fix.
gcc/testsuite/
	* c-c++-common/Wunused-value-1.c: New test.

1 files changed, 14 insertions, 24 deletions

diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index c9e7b1f..bcd7c5a 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -1568,12 +1568,13 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 	  && warn_unused_value
 	  && !TREE_NO_WARNING (expr)
 	  && !processing_template_decl
-	  && !cp_unevaluated_operand)
+	  && !cp_unevaluated_operand
+	  && (complain & tf_warning))
 	{
 	  /* The middle end does not warn about expressions that have
 	     been explicitly cast to void, so we must do so here.  */
-	  if (!TREE_SIDE_EFFECTS (expr)) {
-            if (complain & tf_warning)
+	  if (!TREE_SIDE_EFFECTS (expr))
+	    {
 	      switch (implicit)
 		{
 		  case ICV_SECOND_OF_COND:
@@ -1605,14 +1606,10 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 		  default:
 		    gcc_unreachable ();
 		}
-          }
+	    }
 	  else
 	    {
-	      tree e;
-	      enum tree_code code;
-	      enum tree_code_class tclass;
-
-	      e = expr;
+	      tree e = expr;
 	      /* We might like to warn about (say) "(int) f()", as the
 		 cast has no effect, but the compiler itself will
 		 generate implicit conversions under some
@@ -1626,21 +1623,14 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 	      while (TREE_CODE (e) == NOP_EXPR)
 		e = TREE_OPERAND (e, 0);
 
-	      code = TREE_CODE (e);
-	      tclass = TREE_CODE_CLASS (code);
-	      if ((tclass == tcc_comparison
-		   || tclass == tcc_unary
-		   || (tclass == tcc_binary
-		       && !(code == MODIFY_EXPR
-			    || code == INIT_EXPR
-			    || code == PREDECREMENT_EXPR
-			    || code == PREINCREMENT_EXPR
-			    || code == POSTDECREMENT_EXPR
-			    || code == POSTINCREMENT_EXPR))
-		   || code == VEC_PERM_EXPR
-		   || code == VEC_COND_EXPR)
-                  && (complain & tf_warning))
-		warning_at (loc, OPT_Wunused_value, "value computed is not used");
+	      enum tree_code code = TREE_CODE (e);
+	      enum tree_code_class tclass = TREE_CODE_CLASS (code);
+	      if (tclass == tcc_comparison
+		  || tclass == tcc_unary
+		  || tclass == tcc_binary
+		  || code == VEC_PERM_EXPR
+		  || code == VEC_COND_EXPR)
+		warn_if_unused_value (e, loc);
 	    }
 	}
       expr = build1 (CONVERT_EXPR, void_type_node, expr);