c++: capture folding and lvalueness checking [PR125408]

Here after the recursive mark_rvalue_use we end up with a VAR_DECL in r, so
the assert fails.  But in that case r is still an lvalue, so we don't need
to do any adjustment.

Fixed by adjusting the condition to detect the case we actually need to fix
up, i.e. when ref is an lvalue and r is not.  The previous !rvalue_p
condition doesn't make much sense; rvalue_p indicates whether we want to do
an lvalue-rvalue conversion on expr, not whether expr was already an rvalue.

	PR c++/125408

gcc/cp/ChangeLog:

	* expr.cc (mark_use): Fix condition for changing rvalue to lvalue.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ34.C: New test.

Author: jicama

gcc/cp/expr.cc

@@ -186,7 +186,7 @@ mark_use (tree expr, bool rvalue_p, bool read_p,
 	    return error_mark_node;
 	  if (r != ref)
 	    {
-	      if (!rvalue_p)
+	      if (DECL_P (ref) && !lvalue_p (r))
 		{
 		  /* Make sure we still return an lvalue.  */
 		  gcc_assert (TREE_CODE (r) == NOP_EXPR);

gcc/testsuite/g++.dg/cpp2a/lambda-targ34.C

@@ -0,0 +1,31 @@
+// PR c++/125408
+// { dg-do compile { target c++20 } }
+
+struct Role {
+  long id;
+};
+
+template <typename T>
+struct StaticObj {
+  inline static T obj;
+};
+
+template<int... I> struct A { };
+
+template <auto Ptr, typename Table>
+constexpr int find_member_index() {
+  constexpr auto& t = StaticObj<Table>::obj;
+
+  [&]<int... Idx>(A<Idx...>) {
+    ([&]{
+      using T1 = decltype(&(t.*Ptr));
+      auto p = &(t.*Ptr);
+      return Idx;
+    }(), ...);
+  }(A<0,1>{});
+  return 42;
+}
+
+auto p = find_member_index<&Role::id, Role>();
+
+int main() {}