[PATCH] Fix PR c++/68978 (bogus error: lvalue required as left operand of assignment)

[PATCH] Fix PR c++/68978 (bogus error: lvalue required as left operand of assignment)

[Patrick Palka]

Before my patch for PR c++/21802, the function build_x_modify_expr() did
not call build_new_op() during template processing (and thus did not do
operator lookup); instead, it just built and returned a raw MODOP_EXPR.

After my patch, the function build_x_modify_expr() now calls
build_new_op() even when processing a template.  And when building a
MODOP_EXPR/MODIFY_EXPR, build_new_op() eventually calls
cp_build_modify_expr(), but that function apparently is not prepared to
handle a MODIFY_EXPR whose LHS is a MODOP_EXPR, ultimately because it
calls lvalue_kind() (via lvalue_or_else()) which returns clk_none for a
MODOP_EXPR.  And a non-lvalue can't be the LHS to a MODIFY_EXPR.

Again, we did not have this problem before the mentioned patch because
build_x_modify_expr() originally did not call build_new_op() at all
during template processing.  This is now necessary to in order to do
operator lookup at template definition time.

To fix this, I think lvalue_kind() should handle a MODOP_EXPR the same
as a MODIFY_EXPR, at least during template processing, so it should
return clk_ordinary instead of clk_none.  Though I'm not sure if the
added assert is entirely correct or appropriate..

Bootstrap and regtest in progress on x86_64-pc-linux-gnu, OK to commit
if testing is OK?  I will also try to build and test boost with this
change.

gcc/cp/ChangeLog:

	PR c++/68978
	* tree.c (lvalue_kind) [MODOP_EXPR]: New case.

gcc/testsuite/ChangeLog:

	PR c++/68978
	* g++.dg/template/pr68978.C: New test.
---
 gcc/cp/tree.c                           |  6 +++++
 gcc/testsuite/g++.dg/template/pr68978.C | 44 +++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/template/pr68978.C

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index ae176d0..250fe27 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -185,6 +185,12 @@ lvalue_kind (const_tree ref)
       op2_lvalue_kind = lvalue_kind (TREE_OPERAND (ref, 2));
       break;
 
+    case MODOP_EXPR:
+      /* We expect to see unlowered MODOP_EXPRs only during
+	 template processing.  */
+      gcc_assert (processing_template_decl);
+      return clk_ordinary;
+
     case MODIFY_EXPR:
     case TYPEID_EXPR:
       return clk_ordinary;
diff --git a/gcc/testsuite/g++.dg/template/pr68978.C b/gcc/testsuite/g++.dg/template/pr68978.C
new file mode 100644
index 0000000..b5946fc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/pr68978.C
@@ -0,0 +1,44 @@
+// PR c++/68978
+
+extern int x, y, z;
+
+template <typename T> int toi_1() {
+  int i = 0, c = 0, radix = 10, max = 0x7fffffff;
+  if (max < ((i *= radix) += c))
+    return 0;
+  return i;
+}
+
+template <typename T> int toi_2() {
+  int i = 0, c = 0, radix = 10, max = 0x7fffffff;
+  if (max < ((i = radix) = c))
+    return 0;
+  return i;
+}
+
+template <typename T> int toi_3() {
+  int i = 0, c = 0, radix = 10, max = 0x7fffffff;
+  if (max < ((i = radix) += c))
+    return 0;
+  return i;
+}
+
+template <typename T> int toi_4() {
+  int i = 0, c = 0, radix = 10, max = 0x7fffffff;
+  if (max < ((i += radix) = c))
+    return 0;
+  return i;
+}
+
+template <typename T> int toi_5() {
+  int i = 0, c = 0, radix = 10, max = 0x7fffffff;
+  if (max < (((i = radix) = (c += 5)) *= 30))
+    return 0;
+  return i;
+}
+
+int a = toi_1<int> ();
+int b = toi_2<int> ();
+int c = toi_3<int> ();
+int d = toi_4<int> ();
+int e = toi_5<int> ();
-- 
2.7.0.rc0.50.g1470d8f.dirty

Re: [PATCH] Fix PR c++/68978 (bogus error: lvalue required as left operand of assignment)

[Jason Merrill]

OK.

Jason