set loc on call even if result is discarded

[Alexandre Oliva <oliva@adacore.com>]

This patch fixes a divergence in line numbers in diagnostics and,
presumably, debug information, between targets whose cdtors return
this and those that don't.

The problem was visible in g++.dg/cpp2a/constexpr-dtor3.C: while the
dtor call in the cleanup for f4 was expected at the closing brace, on
returning-this targets it came up at the assignment.

The reason is convoluted: statements in cleanups have their location
information removed, to avoid bumpy debugger behavior, and then set to
the location of the end of the scope.

The cleanup dtor call has its locus cleared in both kinds of targets,
but the end-of-scope locus doesn't make it on returning-this targets.
The calls are wrapped with a cast-to-void to discard the unused return
value, and the existing logic only attached the locus to the
conversion NOP_EXPR.

The call thus remains locus-less.  When constexpr logic copies and
evals the body, it sets unset locations; while copying cleanups, the
locus is taken from the cleanup expression, rather than matching the
end-of-scope locus set by the parser.  So we end up with different
locations.

This patch sets the locus of the call even when it's wrapped by a
convert-to-void NOP_EXPR, so it won't diverge any more.

Regstrapped on x86_64-linux-gnu, also verified the testcase fix on
arm-eabi.  Ok to install?


for  gcc/ChangeLog

	* tree.cc (protected_set_expr_location): Propagate locus to
	call wrapped in cast-to-void.
---
 gcc/tree.cc |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/gcc/tree.cc b/gcc/tree.cc
index ec200e9a7eb43..228d279ab0aa1 100644
--- a/gcc/tree.cc
+++ b/gcc/tree.cc
@@ -5372,7 +5372,18 @@ void
 protected_set_expr_location (tree t, location_t loc)
 {
   if (CAN_HAVE_LOCATION_P (t))
-    SET_EXPR_LOCATION (t, loc);
+    {
+      SET_EXPR_LOCATION (t, loc);
+      /* Avoid locus differences for C++ cdtor calls depending on whether
+	 cdtor_returns_this: a conversion to void is added to discard the return
+	 value, and this conversion ends up carrying the location, and when it
+	 gets discarded, the location is lost.  So hold it in the call as
+	 well.  */
+      if (TREE_CODE (t) == NOP_EXPR
+	  && TREE_TYPE (t) == void_type_node
+	  && TREE_CODE (TREE_OPERAND (t, 0)) == CALL_EXPR)
+	SET_EXPR_LOCATION (TREE_OPERAND (t, 0), loc);
+    }
   else if (t && TREE_CODE (t) == STATEMENT_LIST)
     {
       t = expr_single (t);


-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>