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

[Jason Merrill via Gcc-patches]

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.

Tested x86_64-pc-linux-gnu, applying to trunk.

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.
---
 gcc/cp/cp-gimplify.c                               | 12 ++++++++++++
 gcc/testsuite/g++.dg/cpp1y/lambda-generic-empty1.C |  9 +++++++++
 2 files changed, 21 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/lambda-generic-empty1.C

diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 4d0e92c3321..1c5e15b957f 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 00000000000..ffb0cf149ce
--- /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){};

base-commit: 2da7ce23cfd81b67f77dc102d6f97dd19363b5f4
prerequisite-patch-id: 67efffa865b1469c2b63fe7a6ae9fec692c63079
-- 
2.27.0