Re: [PATCH] c++: value category of compound object expr [PR104173]

[Jason Merrill via Gcc-patches]

On 1/22/22 16:24, Patrick Palka wrote:

Here the call to the &&-qualified toLower() is incorrectly rejected
because the object expression is deemed to be an lvalue, even though it's
really a prvalue.  The object expression, instance()->applicationName(),
is expressed as an INDIRECT_REF of a COMPOUND_EXPR

   *(*instance ();, &TARGET_EXPR <D.2383, QCoreApplication::applicationName 
()>;);

which lvalue_kind categorizes as an lvalue.

This issue seems similar to PR88103 except that here the compound object
expression is a prvalue rather than an xvalue.  The fix there was to
adjust the result of unary_complex_lvalue in build_class_member_access_expr
so that xvalue-ness is preserved.  This patch extends the fix so that
rvalue-ness is preserved more generally.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

OK.

        PR c++/104173

gcc/cp/ChangeLog:

        * typeck.cc (build_class_member_access_expr): Extend
        unary_complex_lvalue result adjustment to preserve all
        rvalues, not just xvalues.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/ref-qual21.C: New test.
---
  gcc/cp/typeck.cc                        | 19 ++++++++-----------
  gcc/testsuite/g++.dg/cpp0x/ref-qual21.C | 16 ++++++++++++++++
  2 files changed, 24 insertions(+), 11 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/ref-qual21.C

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 3a28d639cb1..59668203573 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -2726,17 +2726,14 @@ build_class_member_access_expr (cp_expr object, tree 
member,
    /* Transform `(a, b).x' into `(*(a, &b)).x', `(a ? b : c).x' into
       `(*(a ?  &b : &c)).x', and so on.  A COND_EXPR is only an lvalue
       in the front end; only _DECLs and _REFs are lvalues in the back end.  */
-  {
-    tree temp = unary_complex_lvalue (ADDR_EXPR, object);
-    if (temp)
-      {
-       temp = cp_build_fold_indirect_ref (temp);
-       if (xvalue_p (object) && !xvalue_p (temp))
-         /* Preserve xvalue kind.  */
-         temp = move (temp);
-       object = temp;
-      }
-  }
+  if (tree temp = unary_complex_lvalue (ADDR_EXPR, object))
+    {
+      temp = cp_build_fold_indirect_ref (temp);
+      if (!lvalue_p (object) && lvalue_p (temp))
+       /* Preserve rvalue-ness.  */
+       temp = move (temp);
+      object = temp;
+    }

/* In [expr.ref], there is an explicit list of the valid choices for

       MEMBER.  We check for each of those cases here.  */
diff --git a/gcc/testsuite/g++.dg/cpp0x/ref-qual21.C 
b/gcc/testsuite/g++.dg/cpp0x/ref-qual21.C
new file mode 100644
index 00000000000..e2c43c345ab
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/ref-qual21.C
@@ -0,0 +1,16 @@
+// PR c++/104173
+// { dg-do compile { target c++11 } }
+
+struct QString {
+  QString toLower() &&;
+};
+
+struct QCoreApplication {
+  static QString applicationName();
+};
+
+QCoreApplication *instance();
+
+int main() {
+  instance()->applicationName().toLower();
+}