Index: clang/lib/Basic/DiagnosticIDs.cpp
===================================================================
--- clang/lib/Basic/DiagnosticIDs.cpp
+++ clang/lib/Basic/DiagnosticIDs.cpp
@@ -457,12 +457,15 @@
   if (Result == diag::Severity::Ignored)
     return Result;
 
-  // Honor -w, which is lower in priority than pedantic-errors, but higher than
-  // -Werror.
-  // FIXME: Under GCC, this also suppresses warnings that have been mapped to
-  // errors by -W flags and #pragma diagnostic.
-  if (Result == diag::Severity::Warning && State->IgnoreAllWarnings)
-    return diag::Severity::Ignored;
+  // Honor -w: this disables all messages mapped to Warning severity, and *also*
+  // any diagnostics which are not Error/Fatal by default (that is, they were
+  // upgraded by any of the mechanisms available: -Werror, -pedantic, or #pragma
+  // diagnostic)
+  if (State->IgnoreAllWarnings) {
+    if (Result == diag::Severity::Warning ||
+        !isDefaultMappingAsError((diag::kind)DiagID))
+      return diag::Severity::Ignored;
+  }
 
   // If -Werror is enabled, map warnings to errors unless explicitly disabled.
   if (Result == diag::Severity::Warning) {
Index: clang/test/Frontend/warning-mapping-2.c
===================================================================
--- clang/test/Frontend/warning-mapping-2.c
+++ clang/test/Frontend/warning-mapping-2.c
@@ -1,5 +1,7 @@
-// Check that -w has lower priority than -pedantic-errors.
+// Check that -w takes precedence over -pedantic-errors.
 // RUN: %clang_cc1 -verify -pedantic-errors -w %s
 
-void f0() { f1(); } // expected-error {{implicit declaration of function}}
+// Expect *not* to see a diagnostic for "implicit declaration of function"
+// expected-no-diagnostics
 
+void f0() { f1(); }
Index: clang/test/Frontend/warning-mapping-4.c
===================================================================
--- clang/test/Frontend/warning-mapping-4.c
+++ clang/test/Frontend/warning-mapping-4.c
@@ -1,5 +1,9 @@
+// Verify that various combinations of flags properly keep the sign-compare
+// warning disabled.
+
 // RUN: %clang_cc1 -verify -Wno-error=sign-compare %s
 // RUN: %clang_cc1 -verify -Wsign-compare -w -Wno-error=sign-compare %s
+// RUN: %clang_cc1 -verify -w -Werror=sign-compare %s
 // expected-no-diagnostics
 
 int f0(int x, unsigned y) {
Index: clang/test/Frontend/warning-mapping-5.c
===================================================================
--- clang/test/Frontend/warning-mapping-5.c
+++ clang/test/Frontend/warning-mapping-5.c
@@ -1,6 +1,5 @@
-// Check that #pragma diagnostic warning overrides -Werror. This matches GCC's
-// original documentation, but not its earlier implementations.
-// 
+// Check that #pragma diagnostic warning overrides -Werror.
+//
 // RUN: %clang_cc1 -verify -Werror %s
 
 #pragma clang diagnostic warning "-Wsign-compare"
Index: clang/test/Frontend/warning-mapping-6.c
===================================================================
--- /dev/null
+++ clang/test/Frontend/warning-mapping-6.c
@@ -0,0 +1,9 @@
+// Check that "#pragma diagnostic error" is suppressed by -w.
+//
+// RUN: %clang_cc1 -verify -Werror -w %s
+
+// expected-no-diagnostics
+#pragma gcc diagnostic error "-Wsign-compare"
+int f0(int x, unsigned y) {
+  return x < y;
+}