c++: generic lambda, fn* conv, empty class [PR98326]

Here, in the thunk returned from the captureless lambda conversion to
pointer-to-function, we try to pass through invisible reference parameters
by reference, without doing a copy.  The empty class copy optimization was
messing that up.

gcc/cp/ChangeLog:

	PR c++/98326
	PR c++/20408
	* cp-gimplify.c (simple_empty_class_p): Don't touch an invisiref
	parm.

gcc/testsuite/ChangeLog:

	PR c++/98326
	* g++.dg/cpp1y/lambda-generic-empty1.C: New test.

2 files changed, 21 insertions, 0 deletions

diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 4d0e92c..1c5e15b 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -324,6 +324,18 @@ simple_empty_class_p (tree type, tree op, tree_code code)
       && TYPE_HAS_TRIVIAL_DESTRUCTOR (type))
     /* The TARGET_EXPR is itself a simple copy, look through it.  */
     return simple_empty_class_p (type, TARGET_EXPR_INITIAL (op), code);
+
+  if (TREE_CODE (op) == PARM_DECL
+      && TREE_ADDRESSABLE (TREE_TYPE (op)))
+    {
+      tree fn = DECL_CONTEXT (op);
+      if (DECL_THUNK_P (fn)
+	  || lambda_static_thunk_p (fn))
+	/* In a thunk, we pass through invisible reference parms, so this isn't
+	   actually a copy.  */
+	return false;
+    }
+
   return
     (TREE_CODE (op) == EMPTY_CLASS_EXPR
      || code == MODIFY_EXPR
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-empty1.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-empty1.C
new file mode 100644
index 0000000..ffb0cf1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-empty1.C
@@ -0,0 +1,9 @@
+// PR c++/98326
+// { dg-do compile { target c++14 } }
+
+struct A {
+    A() = default;
+    A(const A&) {}
+};
+
+void (*fptr)(A) = [](auto){};