c++: lifetime extension with .* expression [PR53288]

This PR points out a case where we are not extending the lifetime of a
temporary when the subobject is denoted by a pointer-to-member operation.
These rules were clarified in C++20 by CWG1299.

There are other cases that also need to be handled under CWG1299, but are
not fixed by this patch.

	PR c++/53288
	DR 1299

gcc/cp/ChangeLog:

	* call.cc (extend_ref_init_temps_1): Handle ptrmem expression.

gcc/testsuite/ChangeLog:

	* g++.dg/init/lifetime4.C: New test.

1 files changed, 38 insertions, 0 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 9917307..a7de0e8 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13952,6 +13952,34 @@ static tree
 extend_ref_init_temps_1 (tree decl, tree init, vec<tree, va_gc> **cleanups,
 			 tree *cond_guard)
 {
+  /* CWG1299 (C++20): The temporary object to which the reference is bound or
+     the temporary object that is the complete object of a subobject to which
+     the reference is bound persists for the lifetime of the reference if the
+     glvalue to which the reference is bound was obtained through one of the
+     following:
+     - a temporary materialization conversion ([conv.rval]),
+     - ( expression ), where expression is one of these expressions,
+     - subscripting ([expr.sub]) of an array operand, where that operand is one
+       of these expressions,
+     - a class member access ([expr.ref]) using the . operator where the left
+       operand is one of these expressions and the right operand designates a
+       non-static data member of non-reference type,
+     - a pointer-to-member operation ([expr.mptr.oper]) using the .* operator
+       where the left operand is one of these expressions and the right operand
+       is a pointer to data member of non-reference type,
+     - a const_cast ([expr.const.cast]), static_cast ([expr.static.cast]),
+       dynamic_cast ([expr.dynamic.cast]), or reinterpret_cast
+       ([expr.reinterpret.cast]) converting, without a user-defined conversion,
+       a glvalue operand that is one of these expressions to a glvalue that
+       refers to the object designated by the operand, or to its complete
+       object or a subobject thereof,
+     - a conditional expression ([expr.cond]) that is a glvalue where the
+       second or third operand is one of these expressions, or
+     - a comma expression ([expr.comma]) that is a glvalue where the right
+       operand is one of these expressions.  */
+
+  /* FIXME several cases are still handled wrong (101572, 81420).  */
+
   tree sub = init;
   tree *p;
   STRIP_NOPS (sub);
@@ -13962,6 +13990,16 @@ extend_ref_init_temps_1 (tree decl, tree init, vec<tree, va_gc> **cleanups,
 				   cond_guard);
       return init;
     }
+  if (TREE_CODE (sub) == POINTER_PLUS_EXPR
+      && TYPE_PTRDATAMEM_P (TREE_TYPE (tree_strip_nop_conversions
+				       (TREE_OPERAND (sub, 1)))))
+    {
+      /* A pointer-to-member operation.  */
+      TREE_OPERAND (sub, 0)
+	= extend_ref_init_temps_1 (decl, TREE_OPERAND (sub, 0), cleanups,
+				   cond_guard);
+      return init;
+    }
   if (TREE_CODE (sub) == COND_EXPR)
     {
       tree cur_cond_guard = NULL_TREE;