Suppress spurious clang -Wbraced-scalar-init

As reported in <llvm/llvm-project#54702>,
clang warns about use of braces in contexts where they are useful to
explicitly disallow narrowing conversions.  Such braces should not be
removed, as it would allow a future change to accidentally introduce a
narrowing conversion and not receive a diagnostic.  As of this commit,
the LLVM issue has seen no activity and there is no indication the
spurious warning will be fixed.

Add an SConf test to detect if the compiler warns for these cases, and
suppress it if so.

Author: vLKp

SConstruct

@@ -2621,6 +2621,42 @@ constexpr literal_as_type<T, v...> operator""_literal_as_type();
 ''', msg='whether compiler accepts string literal operator templates'):
 			self.successful_flags['CXXFLAGS'].append('-Wno-gnu-string-literal-operator-template')
 
+	@_custom_test
+	def check_compiler_overzealous_braced_scalar_init(self,context,_text='''
+/* clang-16 issues a spurious -Wbraced-scalar-init warning if the result
+ * is returned anonymously.
+
+```
+error: braces around scalar initializer [-Werror,-Wbraced-scalar-init]
+```
+
+ * Removing the braces allows an implicit narrowing of the result, which would
+ * be a trap for a future maintenance programmer.
+ *
+ * gcc accepts the anonymous brace-initialized result.
+ */
+unsigned f(unsigned i);
+unsigned f(unsigned i)
+{
+	return {i + 1};
+}
+
+unsigned g(unsigned i);
+unsigned g(unsigned i)
+{
+	++i;
+	return {i};
+}
+''',_main='''
+	f({5});
+	g({6});
+''',successflags={'CXXFLAGS' : ['-Wno-braced-scalar-init']}):
+		if self.Compile(context, text=_text, main=_main, msg='whether compiler accepts braced variables as function arguments and return values'):
+			return
+		if self.Compile(context, text=_text, main=_main, msg='whether compiler accepts -Wno-braced-scalar-init', successflags=successflags):
+			return
+		raise SCons.Errors.StopError("C++ compiler rejects braced scalar initialization, even with `-Wno-braced-scalar-init`.")
+
 	@_custom_test
 	def check_have_std_ranges(self,context,_testflags={'CPPDEFINES' : ['_LIBCPP_ENABLE_EXPERIMENTAL']}):
 		text = '''