diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 9897a9f4d0d..2b794f05ea2 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2004-08-21  Joseph S. Myers  <jsm@polyomino.org.uk>
+
+	PR c++/17120
+	* pt.c (tsubst_copy_and_build): Avoid clearing TREE_NO_WARNING for
+	MODOP_EXPR.
+
 2004-08-20  Kriang Lerdsuwanakij  <lerdsuwa@users.sourceforge.net>
 
 	* pt.c (register_specialization): Check DECL_TEMPLATE_SPECIALIZATION
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index dd88b987fef..8ee4be9f804 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8353,7 +8353,14 @@ tsubst_copy_and_build (tree t,
 	  (RECUR (TREE_OPERAND (t, 0)),
 	   TREE_CODE (TREE_OPERAND (t, 1)),
 	   RECUR (TREE_OPERAND (t, 2)));
-	TREE_NO_WARNING (r) = TREE_NO_WARNING (t);
+	/* TREE_NO_WARNING must be set if either the expression was
+	   parenthesized or it uses an operator such as >>= rather
+	   than plain assignment.  In the former case, it was already
+	   set and must be copied.  In the latter case,
+	   build_x_modify_expr sets it and it must not be reset
+	   here.  */
+	if (TREE_NO_WARNING (t))
+	  TREE_NO_WARNING (r) = TREE_NO_WARNING (t);
 	return r;
       }
 
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 6e395b0ad32..d096a1b696d 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2004-08-21  Joseph S. Myers  <jsm@polyomino.org.uk>
+
+	PR c++/17120
+	* g++.dg/warn/Wparentheses-4.C: New test.
+
 2004-08-20  Mark Mitchell  <mark@codesourcery.com>
 
 	* gcc.dg/darwin-longlong.c: Include <stdlib.h>.
diff --git a/gcc/testsuite/g++.dg/warn/Wparentheses-4.C b/gcc/testsuite/g++.dg/warn/Wparentheses-4.C
new file mode 100644
index 00000000000..2048ed7c7f1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wparentheses-4.C
@@ -0,0 +1,19 @@
+// Test that -Wparentheses does not give bogus warnings in the
+// presence of templates for non-plain assignment.  Bug 17120.
+
+// { dg-do compile }
+// { dg-options "-Wparentheses" }
+
+template<typename _Tp>
+  inline _Tp
+  cmath_power(_Tp __x, unsigned int __n)
+  {
+    while (__n >>= 1)
+      ;
+    return __x;
+  }
+
+int main()
+{
+  cmath_power(1.0, 3);
+}