diff --git a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.h b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.h
--- a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.h
+++ b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.h
@@ -71,7 +71,7 @@
   /// Initializes \c ClangTidyContext instance.
   ClangTidyContext(std::unique_ptr<ClangTidyOptionsProvider> OptionsProvider,
                    bool AllowEnablingAnalyzerAlphaCheckers = false,
-                   bool DisableModuleHeadersParsing = false);
+                   bool EnableModuleHeadersParsing = false);
   /// Sets the DiagnosticsEngine that diag() will emit diagnostics to.
   // FIXME: this is required initialization, and should be a constructor param.
   // Fix the context -> diag engine -> consumer -> context initialization cycle.
@@ -200,7 +200,7 @@
   }
 
   bool canEnableModuleHeadersParsing() const {
-    return !DisableModuleHeadersParsing;
+    return EnableModuleHeadersParsing;
   }
 
   void setSelfContainedDiags(bool Value) { SelfContainedDiags = Value; }
@@ -250,7 +250,7 @@
   std::string ProfilePrefix;
 
   bool AllowEnablingAnalyzerAlphaCheckers;
-  bool DisableModuleHeadersParsing;
+  bool EnableModuleHeadersParsing;
 
   bool SelfContainedDiags;
 
diff --git a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
--- a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
+++ b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
@@ -161,11 +161,11 @@
 
 ClangTidyContext::ClangTidyContext(
     std::unique_ptr<ClangTidyOptionsProvider> OptionsProvider,
-    bool AllowEnablingAnalyzerAlphaCheckers, bool DisableModuleHeadersParsing)
+    bool AllowEnablingAnalyzerAlphaCheckers, bool EnableModuleHeadersParsing)
     : DiagEngine(nullptr), OptionsProvider(std::move(OptionsProvider)),
       Profile(false),
       AllowEnablingAnalyzerAlphaCheckers(AllowEnablingAnalyzerAlphaCheckers),
-      DisableModuleHeadersParsing(DisableModuleHeadersParsing),
+      EnableModuleHeadersParsing(EnableModuleHeadersParsing),
       SelfContainedDiags(false) {
   // Before the first translation unit we can get errors related to command-line
   // parsing, use empty string for the file name in this case.
diff --git a/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp b/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
--- a/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
+++ b/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
@@ -264,9 +264,9 @@
                                        cl::cat(ClangTidyCategory));
 
 static cl::opt<bool>
-    DisableModuleHeadersParsing("experimental-disable-module-headers-parsing",
-                                cl::init(false), cl::Hidden,
-                                cl::cat(ClangTidyCategory));
+    EnableModuleHeadersParsing("experimental-enable-module-headers-parsing",
+                               cl::init(false), cl::Hidden,
+                               cl::cat(ClangTidyCategory));
 
 static cl::opt<std::string> ExportFixes("export-fixes", desc(R"(
 YAML file to store suggested fixes in. The
@@ -665,7 +665,7 @@
 
   ClangTidyContext Context(std::move(OwningOptionsProvider),
                            AllowEnablingAnalyzerAlphaCheckers,
-                           DisableModuleHeadersParsing);
+                           EnableModuleHeadersParsing);
   std::vector<ClangTidyError> Errors =
       runClangTidy(Context, OptionsParser->getCompilations(), PathList, BaseFS,
                    FixNotes, EnableCheckProfile, ProfilePrefix);
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -111,6 +111,20 @@
   be promoted to errors. For custom error promotion, use `-Werror=<warning>`
   on the compiler command-line, irrespective of `Checks` (`--checks=`) settings.
 
+- Preprocessor-level module header parsing is now disabled by default due to
+  the problems it caused in C++20 and above, leading to performance and code
+  parsing issues regardless of whether modules were used or not. This change
+  will impact only the following checks:
+  :doc:`modernize-replace-disallow-copy-and-assign-macro
+  <clang-tidy/checks/modernize/replace-disallow-copy-and-assign-macro>`,
+  :doc:`bugprone-reserved-identifier
+  <clang-tidy/checks/bugprone/reserved-identifier>`, and
+  :doc:`readability-identifier-naming
+  <clang-tidy/checks/readability/identifier-naming>`. Those checks will no
+  longer see macros defined in modules. Users can still enable this
+  functionality using the hidden command line option
+  `--experimental-enable-module-headers-parsing`.
+
 New checks
 ^^^^^^^^^^
 
diff --git a/clang-tools-extra/test/clang-tidy/infrastructure/expand-modular-headers-ppcallbacks.cpp b/clang-tools-extra/test/clang-tidy/infrastructure/expand-modular-headers-ppcallbacks.cpp
--- a/clang-tools-extra/test/clang-tidy/infrastructure/expand-modular-headers-ppcallbacks.cpp
+++ b/clang-tools-extra/test/clang-tidy/infrastructure/expand-modular-headers-ppcallbacks.cpp
@@ -5,7 +5,7 @@
 // RUN: %check_clang_tidy -std=c++11 %s readability-identifier-naming %t/without-modules -- \
 // RUN:   -config="CheckOptions: [{ \
 // RUN:      key: readability-identifier-naming.MacroDefinitionCase, value: UPPER_CASE }]" \
-// RUN:   -header-filter=.* \
+// RUN:   -header-filter=.* --experimental-enable-module-headers-parsing \
 // RUN:   -- -I %t/
 //
 // RUN: rm -rf %t
@@ -14,7 +14,7 @@
 // RUN: %check_clang_tidy -std=c++17 %s readability-identifier-naming %t/without-modules -- \
 // RUN:   -config="CheckOptions: [{ \
 // RUN:      key: readability-identifier-naming.MacroDefinitionCase, value: UPPER_CASE }]" \
-// RUN:   -header-filter=.* \
+// RUN:   -header-filter=.* --experimental-enable-module-headers-parsing \
 // RUN:   -- -I %t/
 //
 // Run clang-tidy on a file with modular includes:
@@ -25,7 +25,7 @@
 // RUN: %check_clang_tidy -std=c++11 %s readability-identifier-naming %t/with-modules -- \
 // RUN:   -config="CheckOptions: [{ \
 // RUN:      key: readability-identifier-naming.MacroDefinitionCase, value: UPPER_CASE }]" \
-// RUN:   -header-filter=.* \
+// RUN:   -header-filter=.* --experimental-enable-module-headers-parsing \
 // RUN:   -- -I %t/ \
 // RUN:   -fmodules -fimplicit-modules -fno-implicit-module-maps \
 // RUN:   -fmodule-map-file=%t/module.modulemap \
@@ -37,7 +37,7 @@
 // RUN: %check_clang_tidy -std=c++17 %s readability-identifier-naming %t/with-modules -- \
 // RUN:   -config="CheckOptions: [{ \
 // RUN:      key: readability-identifier-naming.MacroDefinitionCase, value: UPPER_CASE }]" \
-// RUN:   -header-filter=.* \
+// RUN:   -header-filter=.* --experimental-enable-module-headers-parsing \
 // RUN:   -- -I %t/ \
 // RUN:   -fmodules -fimplicit-modules -fno-implicit-module-maps \
 // RUN:   -fmodule-map-file=%t/module.modulemap \