[Windows] TimeZone.current returns GMT despite system settings

[jmschonfeld]

Running TimeZone.current on Windows always returns GMT despite any present system settings.

This appears to be because on Windows we check TZDEFAULT:

swift-foundation/Sources/FoundationEssentials/TimeZone/TimeZone_Cache.swift

Lines 116 to 131 in 5141cf6

	#if os(Windows)
	let hFile = TZDEFAULT.withCString(encodedAs: UTF16.self) {
	CreateFileW($0, GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, nil, OPEN_EXISTING, 0, nil)
	}
	defer { CloseHandle(hFile) }
	let dwSize = GetFinalPathNameByHandleW(hFile, nil, 0, VOLUME_NAME_DOS)
	let path = withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(dwSize)) {
	_ = GetFinalPathNameByHandleW(hFile, $0.baseAddress, dwSize, VOLUME_NAME_DOS)
	return String(decodingCString: $0.baseAddress!, as: UTF16.self)
	}
	if let rangeOfZoneInfo = path._range(of: "\(TZDIR)\\", anchored: false, backwards: false) {
	let name = path[rangeOfZoneInfo.upperBound...]
	if let result = fixed(String(name)) {
	return TimeZone(inner: result)
	}
	}

However, TZDEFAULT does not appear to be set in the WinSDK/ucrt modules so we instead pickup the default value from our CShims:

swift-foundation/Sources/_FoundationCShims/include/_CStdlib.h

Lines 151 to 157 in 5141cf6

	#ifndef TZDIR
	#define TZDIR "/usr/share/zoneinfo/" /* Time zone object file directory */
	#endif /* !defined TZDIR */
	#ifndef TZDEFAULT
	#define TZDEFAULT "/etc/localtime"
	#endif /* !defined TZDEFAULT */

These default paths seem unsuitable for Windows and result in us failing to detect the current timezone and falling back to GMT

[jmschonfeld]

@compnerd do you happen to know if we'd expect TZDEFAULT to be defined for Windows somewhere, or do we need to instead do something like what CoreFoundation has historically done: https://github.com/swiftlang/swift-corelibs-foundation/blob/e55e1d88001997be830fbc01086564431d405dad/Sources/CoreFoundation/CFTimeZone.c#L723-L734

[compnerd]

I don't expect TZDEFAULT to be set on Windows, we should be doing what CF did historically here.