From e32234536f361796e7cad8ed69a1c0bb46ee55de Mon Sep 17 00:00:00 2001
From: Marek Polacek <polacek@redhat.com>
Date: Thu, 15 Jul 2021 18:45:49 -0400
Subject: c++: Don't hide narrowing errors in system headers

Jonathan pointed me at this issue where

  constexpr unsigned f() { constexpr int n = -1; return unsigned{n}; }

is accepted in system headers, despite the narrowing conversion from
a constant.  I suspect that whereas narrowing warnings should be
disabled, ill-formed narrowing of constants should be a hard error
(which can still be disabled by -Wno-narrowing).

gcc/cp/ChangeLog:

	* typeck2.c (check_narrowing): Don't suppress the pedantic error
	in system headers.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/ratio/operations/ops_overflow_neg.cc: Add
	dg-error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/Wnarrowing2.C: New test.
	* g++.dg/cpp1y/Wnarrowing2.h: New test.
---
 gcc/cp/typeck2.c                         | 1 +
 gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.C | 4 ++++
 gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.h | 2 ++
 3 files changed, 7 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.h

(limited to 'gcc')

diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index 6679e24..dcfdff2 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -986,6 +986,7 @@ check_narrowing (tree type, tree init, tsubst_flags_t complain,
 	{
 	  int savederrorcount = errorcount;
 	  global_dc->pedantic_errors = 1;
+	  auto s = make_temp_override (global_dc->dc_warn_system_headers, true);
 	  pedwarn (loc, OPT_Wnarrowing,
 		   "narrowing conversion of %qE from %qH to %qI",
 		   init, ftype, type);
diff --git a/gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.C b/gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.C
new file mode 100644
index 0000000..048d484
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.C
@@ -0,0 +1,4 @@
+// { dg-do compile { target c++14 } }
+
+#include "Wnarrowing2.h"
+// { dg-error "narrowing conversion" "" { target *-*-* } 0 }
diff --git a/gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.h b/gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.h
new file mode 100644
index 0000000..7dafa51
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/Wnarrowing2.h
@@ -0,0 +1,2 @@
+#pragma GCC system_header
+constexpr unsigned f() { constexpr int n = -1; return unsigned{n}; }
-- 
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