call.c (build_over_call): Mark COMPOUND_EXPRs generated for empty class assignment as having side-effects...

* call.c (build_over_call): Mark COMPOUND_EXPRs generated for
	empty class assignment as having side-effects to avoid
	spurious warnings.

From-SVN: r44868

gcc/cp/ChangeLog

@@ -1,3 +1,9 @@
+2001-08-13  Mark Mitchell  <mark@codesourcery.com>
+
+	* call.c (build_over_call): Mark COMPOUND_EXPRs generated for
+	empty class assignment as having side-effects to avoid
+	spurious warnings.
+
 2001-08-13  Zack Weinberg  <zackw@panix.com>
 
 	* Make-lang.in (cp/except.o): Add libfuncs.h to dependencies.

gcc/cp/call.c

@@ -4292,6 +4292,15 @@ build_over_call (cand, args, flags)
 	  TREE_USED (arg) = 1;
 
 	  val = build (COMPOUND_EXPR, DECL_CONTEXT (fn), arg, to);
+	  /* Even though the assignment may not actually result in any
+	     code being generated, we do not want to warn about the
+	     assignment having no effect.  That would be confusing to
+	     users who may be performing the assignment as part of a
+	     generic algorithm, for example.
+	     
+	     Ideally, the notions of having side-effects and of being
+	     useless would be orthogonal.  */
+	  TREE_SIDE_EFFECTS (val) = 1;
 	}
       else
 	val = build (MODIFY_EXPR, TREE_TYPE (to), to, arg);

gcc/testsuite/g++.old-deja/g++.other/empty2.C

@@ -0,0 +1,10 @@
+// Build don't link:
+// Origin: Mark Mitchell <mark@codesourcery.com>
+
+struct E {};
+
+void f () {
+  E e1, e2;
+  e1 = e2; // We should not warn about this statement, even though no
+           // code is generated for it.
+}