diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -97,6 +97,12 @@
 Windows Support
 ---------------
 
+- An MSVC compatibility workaround for C++ operator names was removed. As a
+  result, the ``<query.h>`` Windows SDK header may not compile out of the box.
+  Users should use a recent SDK and pass ``-DQUERY_H_RESTRICTION_PERMISSIVE``
+  or pass ``/permissive`` to disable C++ operator names altogether. See
+  `PR42427 <https://llvm.org/pr42427>` for more info.
+
 C Language Changes in Clang
 ---------------------------
 
diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -723,11 +723,7 @@
   // is cleaned to tok::identifier "B". After cleaning the token's length is
   // still 3 and the SourceLocation refers to the location of the backslash.
   Identifier.setIdentifierInfo(II);
-  if (getLangOpts().MSVCCompat && II->isCPlusPlusOperatorKeyword() &&
-      getSourceManager().isInSystemHeader(Identifier.getLocation()))
-    Identifier.setKind(tok::identifier);
-  else
-    Identifier.setKind(II->getTokenID());
+  Identifier.setKind(II->getTokenID());
 
   return II;
 }
diff --git a/clang/test/Headers/ms-cppoperkey.cpp b/clang/test/Headers/ms-cppoperkey.cpp
deleted file mode 100644
--- a/clang/test/Headers/ms-cppoperkey.cpp
+++ /dev/null
@@ -1,16 +0,0 @@
-// RUN: %clang_cc1 \
-// RUN:     -fms-compatibility -x c++-cpp-output \
-// RUN:     -ffreestanding -fsyntax-only -Werror \
-// RUN:     %s -verify
-// expected-no-diagnostics
-# 1 "t.cpp"
-# 1 "query.h" 1 3
-// MS header <query.h> uses operator keyword as field name.  
-// Compile without syntax errors.
-struct tagRESTRICTION
-  {
-   union _URes 
-     {
-       int or; // Note use of cpp operator token
-     } res;
-  };
diff --git a/clang/test/Headers/ms-cppoperkey1.cpp b/clang/test/Headers/ms-cppoperkey1.cpp
deleted file mode 100644
--- a/clang/test/Headers/ms-cppoperkey1.cpp
+++ /dev/null
@@ -1,27 +0,0 @@
-// RUN: %clang_cc1 \
-// RUN:     -fms-compatibility -x c++-cpp-output \
-// RUN:     -ffreestanding -fsyntax-only -Werror \
-// RUN:     %s -verify
-
-
-# 1 "t.cpp"
-# 1 "query.h" 1 3 4
-// MS header <query.h> uses operator keyword as field name.  
-// Compile without syntax errors.
-struct tagRESTRICTION
-  {
-   union _URes 
-     {
-       int or; // Note use of cpp operator token
-     } res;
-  };
-   ;
-
-int aa ( int x)
-{
-  // In system header code, treat operator keyword as identifier.
-  if ( // expected-note{{to match this '('}}
-    x>1 or x<0) return 1; // expected-error{{expected ')'}}
-  else return 0;  
-}
-
diff --git a/clang/test/Headers/ms-cppoperkey2.cpp b/clang/test/Headers/ms-cppoperkey2.cpp
deleted file mode 100644
--- a/clang/test/Headers/ms-cppoperkey2.cpp
+++ /dev/null
@@ -1,11 +0,0 @@
-// RUN: %clang_cc1  -triple x86_64-pc-win32 -fms-compatibility \
-// RUN:     -ffreestanding -fsyntax-only -Werror  %s -verify
-// RUN: %clang_cc1 \
-// RUN:     -ffreestanding -fsyntax-only -Werror  %s -verify
-// expected-no-diagnostics
-int bb ( int x)
-{
-  // In user code, treat operator keyword as operator keyword.
-  if ( x>1 or x<0) return 1;
-  else return 0;  
-}