Enable -Wextra-semi on Windows.

Requires suppressing the warning for a file generated by mc.exe,
and for a system header (which is arguably a clang bug, see comment
in the .h).

Bug: 926235
Change-Id: Iee8ecd6eab476207c1507719ba5102403f381cae
Reviewed-on: https://chromium-review.googlesource.com/c/1489102Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635689}

base/trace_event/trace_event_etw_export_win.cc

@@ -30,8 +30,19 @@
 // file. Note that this includes evntprov.h which requires a Vista+ Windows SDK.
 //
 // In SHARED_INTERMEDIATE_DIR.
+
+// Headers generated by mc.exe have a ';' at the end of extern "C" {} blocks.
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wextra-semi"
+#endif
+
 #include "base/trace_event/etw_manifest/chrome_events_win.h"  // NOLINT
 
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
 namespace {
 
 // |kFilteredEventGroupNames| contains the event categories that can be

build/config/compiler/BUILD.gn

@@ -1590,7 +1590,7 @@ config("chromium_code") {
 
     # TODO(thakis): Enable this for more platforms, https://crbug.com/926235
     has_dchecks = is_debug || dcheck_always_on
-    if (has_dchecks && (is_android || is_ios || is_linux || is_mac)) {
+    if (has_dchecks && (is_android || is_ios || is_linux || is_mac || is_win)) {
       cflags += [ "-Wextra-semi" ]
     }
   }

chrome/credential_provider/gaiacp/gaia_credential_provider.h

@@ -100,8 +100,20 @@ class ATL_NO_VTABLE CGaiaCredentialProvider
   size_t index_ = std::numeric_limits<size_t>::max();
 };
 
+// OBJECT_ENTRY_AUTO() contains an extra semicolon.
+// TODO(thakis): Make -Wextra-semi not warn on semicolons that are from a
+// macro in a system header, then remove the pragma, https://llvm.org/PR40874
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wextra-semi"
+#endif
+
 OBJECT_ENTRY_AUTO(__uuidof(GaiaCredentialProvider), CGaiaCredentialProvider)
 
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
 }  // namespace credential_provider
 
 #endif  // CHROME_CREDENTIAL_PROVIDER_GAIACP_GAIA_CREDENTIAL_PROVIDER_H_