[gcc r10-11307] c-family: -Wsequence-point and COMPONENT_REF [PR107163]

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:e81e393cd864abcd2de02602bd51e435dc28f418

commit r10-11307-ge81e393cd864abcd2de02602bd51e435dc28f418
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Mar 23 15:57:39 2023 -0400

    c-family: -Wsequence-point and COMPONENT_REF [PR107163]
    
    The patch for PR91415 fixed -Wsequence-point to treat shifts and ARRAY_REF
    as sequenced in C++17, and COMPONENT_REF as well.  But this is unnecessary
    for COMPONENT_REF, since the RHS is just a FIELD_DECL with no actual
    evaluation, and in this testcase handling COMPONENT_REF as sequenced blows
    up fast in a deep inheritance tree.  Instead, look through it.
    
            PR c++/107163
    
    gcc/c-family/ChangeLog:
    
            * c-common.c (verify_tree): Don't use sequenced handling
            for COMPONENT_REF.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/warn/Wsequence-point-5.C: New test.

Diff:
---
 gcc/c-family/c-common.c                       |  7 ++++-
 gcc/testsuite/g++.dg/warn/Wsequence-point-5.C | 37 +++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index 8105a27ab56..409dbfeec09 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -2006,12 +2006,17 @@ verify_tree (tree x, struct tlist **pbefore_sp, struct tlist **pno_sp,
 
     case LSHIFT_EXPR:
     case RSHIFT_EXPR:
-    case COMPONENT_REF:
     case ARRAY_REF:
       if (cxx_dialect >= cxx17)
 	goto sequenced_binary;
       goto do_default;
 
+    case COMPONENT_REF:
+      /* Treat as unary, the other operands aren't evaluated.  */
+      x = TREE_OPERAND (x, 0);
+      writer = 0;
+      goto restart;
+
     default:
     do_default:
       /* For other expressions, simply recurse on their operands.
diff --git a/gcc/testsuite/g++.dg/warn/Wsequence-point-5.C b/gcc/testsuite/g++.dg/warn/Wsequence-point-5.C
new file mode 100644
index 00000000000..0354ab09f53
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wsequence-point-5.C
@@ -0,0 +1,37 @@
+// PR c++/107163
+// { dg-additional-options "-Wsequence-point" }
+
+struct BaseType  {
+  int i;
+};
+
+template< int Seq >
+class DerivedType : public DerivedType< Seq - 1 > { };
+
+template<>
+class DerivedType< -1 > : public BaseType { };
+
+int main() {
+  DerivedType< 400 > d;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  d.i = 42;
+  return d.i;
+}