Add an STL-wide suppression for C5246 (#4527)

Co-authored-by: Stephan T. Lavavej <[email protected]>

stl/inc/yvals_core.h

@@ -805,14 +805,15 @@
 // warning C5045: Compiler will insert Spectre mitigation for memory load if /Qspectre switch specified (/Wall)
 // warning C5220: a non-static data member with a volatile qualified type no longer implies that compiler generated
 //                copy/move constructors and copy/move assignment operators are not trivial (/Wall)
+// warning C5246: 'member': the initialization of a subobject should be wrapped in braces (/Wall)
 // warning C6294: Ill-defined for-loop: initial condition does not satisfy test. Loop body not executed
 
 #ifndef _STL_DISABLED_WARNINGS
 // clang-format off
 #define _STL_DISABLED_WARNINGS                        \
     4180 4324 4412 4455 4494 4514 4574 4582 4583 4587 \
     4588 4619 4623 4625 4626 4643 4648 4702 4793 4820 \
-    4868 4988 5026 5027 5045 5220 6294                \
+    4868 4988 5026 5027 5045 5220 5246 6294           \
     _STL_DISABLED_WARNING_C4577                       \
     _STL_DISABLED_WARNING_C4984                       \
     _STL_DISABLED_WARNING_C5053                       \

tests/std/test.lst

@@ -241,6 +241,7 @@ tests\GH_004129_conversion_in_new_numeric_algorithms
 tests\GH_004201_chrono_formatter
 tests\GH_004275_seeking_fancy_iterators
 tests\GH_004388_unordered_meow_operator_equal
+tests\GH_004477_mdspan_warning_5246
 tests\LWG2381_num_get_floating_point
 tests\LWG2597_complex_branch_cut
 tests\LWG3018_shared_ptr_function

tests/std/tests/GH_004477_mdspan_warning_5246/env.lst

@@ -0,0 +1,6 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst
+RUNALL_CROSSLIST
+*	PM_CL="/w15246"

tests/std/tests/GH_004477_mdspan_warning_5246/test.compile.pass.cpp

@@ -0,0 +1,15 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <array>
+#include <cstddef>
+#include <mdspan>
+
+using namespace std;
+
+// Test GH-4477 "Using std::mdspan results in multiple warnings (C5246)"
+void test_gh_4477() {
+    array test_data{0, 1, 2, 3, 4, 5};
+    mdspan<int, dextents<size_t, 2>> test_span(test_data.data(), 2, 3);
+    (void) test_span;
+}