c++: Handle COMPOUND_EXPRs in get_narrower and warnings_for_convert_and_check [PR91993]

As the testcases shows, the -Wconversion warning behaves quite differently
when -fsanitize=undefined vs. when not sanitizing, but in the end it is
not something specific to sanitizing, if a user uses
  return static_cast<uc>(static_cast<uc>((d++, a) << 1U) | b) | c;
instead of
  return static_cast<uc>(static_cast<uc>(a << 1U) | b) | c;
and thus there is some COMPOUND_EXPR involved, cp_build_binary_op behaves
significantly different, e.g. shorten_binary_op will have different result
(uc for the case without COMPOUND_EXPR, int with it), but it isn't limited
to that.

> How about improving get_narrower to handle COMPOUND_EXPR?  I'd think that
> would do the trick without affecting evaluation order.

Not completely, had to change also warnings_for_convert_and_check, but with
that it works.  The float-cast-overflow* changes are needed because now with
-O1+ we emit lots of -Woverflow warnings on the testcase, but we do emit
those warnings on the testcase even when compiling just with -O1 and without
-fsanitize=float-cast-overflow, so it seems to me a change in the right
direction, having -fsanitize= or explicit comma expressions smaller effect
on the warnings that are emitted.

2020-03-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/91993
	* tree.c (get_narrower): Handle COMPOUND_EXPR by recursing on
	ultimate rhs and if returned something different, reconstructing
	the COMPOUND_EXPRs.

	* c-warn.c (warnings_for_convert_and_check): For expr and/or
	result being COMPOUND_EXPRs, skip to ultimate rhs.

	* g++.dg/warn/Wconversion-pr91993.C: New test.
	* g++.dg/ubsan/pr91993.C: New test.
	* c-c++-common/ubsan/float-cast-overflow-1.c: Add -Wno-overflow
	to dg-options.
	* c-c++-common/ubsan/float-cast-overflow-2.c: Likewise.
	* c-c++-common/ubsan/float-cast-overflow-4.c: Likewise.

2 files changed, 11 insertions, 0 deletions

diff --git a/gcc/c-family/ChangeLog b/gcc/c-family/ChangeLog
index 3f195ebcc5d..0d805c64bde 100644
--- a/gcc/c-family/ChangeLog
+++ b/gcc/c-family/ChangeLog
@@ -1,3 +1,9 @@
+2020-03-23  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/91993
+	* c-warn.c (warnings_for_convert_and_check): For expr and/or
+	result being COMPOUND_EXPRs, skip to ultimate rhs.
+
 2020-03-20  Richard Sandiford  <richard.sandiford@arm.com>
 
 	PR middle-end/94072
diff --git a/gcc/c-family/c-warn.c b/gcc/c-family/c-warn.c
index 9315cac683e..f6b3afc727c 100644
--- a/gcc/c-family/c-warn.c
+++ b/gcc/c-family/c-warn.c
@@ -1359,6 +1359,11 @@ warnings_for_convert_and_check (location_t loc, tree type, tree expr,
 {
   loc = expansion_point_location_if_in_system_header (loc);
 
+  while (TREE_CODE (expr) == COMPOUND_EXPR)
+    expr = TREE_OPERAND (expr, 1);
+  while (TREE_CODE (result) == COMPOUND_EXPR)
+    result = TREE_OPERAND (result, 1);
+
   bool cst = TREE_CODE_CLASS (TREE_CODE (result)) == tcc_constant;
 
   tree exprtype = TREE_TYPE (expr);