diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index 26d5d08979391..cde904ebaac1e 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -88,6 +88,24 @@ latest release, please see the [Clang Web Site](https://clang.llvm.org) or the
 
   are now ill-formed.
 
+- `__has_feature(modules)` is no longer true when just `-std=c++20` (or higher)
+  is passed. It's only true if `-fmodules` is passed, which enables Clang's
+  module map semantics. Objective-C++ of the form
+
+  ```objc
+  #if __has_feature(modules)
+  @import Foundation;
+  #else
+  #import <Foundation/Foundation.h>
+  #endif
+  ```
+
+  previously took the `@import` branch under `-std=c++20` even though no module
+  maps were in use, which would always fail.
+
+  `__cpp_modules` continues to be the standard macro to use to check if C++20
+  modules are available.
+
 ### C++ Specific Potentially Breaking Changes
 
 - Clang now more aggressively optimizes away stores to objects after they are
diff --git a/clang/include/clang/Basic/Features.def b/clang/include/clang/Basic/Features.def
index 2f7741ff74f9b..f0a9c4889ea2d 100644
--- a/clang/include/clang/Basic/Features.def
+++ b/clang/include/clang/Basic/Features.def
@@ -186,7 +186,7 @@ FEATURE(objc_default_synthesize_properties, LangOpts.ObjC)
 FEATURE(objc_fixed_enum, LangOpts.ObjC)
 FEATURE(objc_instancetype, LangOpts.ObjC)
 FEATURE(objc_kindof, LangOpts.ObjC)
-FEATURE(objc_modules, LangOpts.ObjC && LangOpts.Modules)
+FEATURE(objc_modules, LangOpts.ObjC && LangOpts.ClangModules)
 FEATURE(objc_nonfragile_abi, LangOpts.ObjCRuntime.isNonFragile())
 FEATURE(objc_property_explicit_atomic, true)
 FEATURE(objc_protocol_qualifier_mangling, true)
@@ -318,7 +318,7 @@ FEATURE(cfi_nvcall_sanitizer, LangOpts.Sanitize.has(SanitizerKind::CFINVCall))
 FEATURE(cfi_vcall_sanitizer, LangOpts.Sanitize.has(SanitizerKind::CFIVCall))
 FEATURE(kcfi, LangOpts.Sanitize.has(SanitizerKind::KCFI))
 FEATURE(kcfi_arity, LangOpts.Sanitize.has(SanitizerKind::KCFI))
-FEATURE(modules, LangOpts.Modules)
+FEATURE(modules, LangOpts.ClangModules)
 FEATURE(safe_stack, LangOpts.Sanitize.has(SanitizerKind::SafeStack))
 FEATURE(shadow_call_stack,
         LangOpts.Sanitize.has(SanitizerKind::ShadowCallStack))
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
index d68784b7efbcd..b4d03a1a3e0b5 100644
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -153,6 +153,7 @@ LANGOPT(Blocks            , 1, 0, NotCompatible, "blocks extension to C")
 LANGOPT(EmitAllDecls      , 1, 0, Benign, "emitting all declarations")
 LANGOPT(MathErrno         , 1, 1, NotCompatible, "errno in math functions")
 LANGOPT(Modules           , 1, 0, NotCompatible, "modules semantics")
+LANGOPT(ClangModules      , 1, 0, Compatible, "Clang header modules")
 LANGOPT(CPlusPlusModules  , 1, 0, Compatible, "C++ modules syntax")
 LANGOPT(SkipODRCheckInGMF , 1, 0, NotCompatible, "Skip ODR checks for decls in the global module fragment")
 LANGOPT(BuiltinHeadersInSystemModules, 1, 0, NotCompatible, "builtin headers belong to system modules, and _Builtin_ modules are ignored for cstdlib headers")
diff --git a/clang/include/clang/Options/Options.td b/clang/include/clang/Options/Options.td
index 3a3952a4397be..9f1f4a5a682ee 100644
--- a/clang/include/clang/Options/Options.td
+++ b/clang/include/clang/Options/Options.td
@@ -3770,7 +3770,7 @@ defm modulemap_allow_subdirectory_search : BoolFOption <"modulemap-allow-subdire
   PosFlag<SetTrue, [], [], "Allow to search for module maps in subdirectories of search paths">,
   NegFlag<SetFalse>, BothFlags<[NoXarchOption], [ClangOption, CC1Option]>>;
 defm modules : BoolFOption<"modules",
-  LangOpts<"Modules">, Default<fcxx_modules.KeyPath>,
+  LangOpts<"ClangModules">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption, CC1Option],
           "Enable the 'modules' language feature">,
   NegFlag<SetFalse>, BothFlags<
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 6f50defcfaf07..50a2359c0e986 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -4137,6 +4137,11 @@ bool CompilerInvocation::ParseLangArgs(LangOptions &Opts, ArgList &Args,
 #include "clang/Options/Options.inc"
 #undef LANG_OPTION_WITH_MARSHALLING
 
+  // "Modules semantics" (e.g. cross-translation-unit declaration merging) are
+  // needed for both Clang (header) modules and C++20 modules, so enable them
+  // for either.
+  Opts.Modules = Opts.ClangModules || Opts.CPlusPlusModules;
+
   if (const Arg *A = Args.getLastArg(OPT_fcf_protection_EQ)) {
     StringRef Name = A->getValue();
     if (Name == "full") {
diff --git a/clang/test/Lexer/has_feature_modules.m b/clang/test/Lexer/has_feature_modules.m
index 6cea3246892ad..e1f52aedd0793 100644
--- a/clang/test/Lexer/has_feature_modules.m
+++ b/clang/test/Lexer/has_feature_modules.m
@@ -6,6 +6,13 @@
 // RUN: %clang_cc1 -E %s -o - | FileCheck --check-prefix=CHECK-NO-MODULES %s
 // RUN: %clang_cc1 -E -x c -fmodules %s -o - | FileCheck --check-prefix=CHECK-HAS-MODULES %s
 
+// -std=c++20 enables C++ modules but not Clang modules, so __has_feature(modules)
+// must only be set when -fmodules is also passed.
+// RUN: %clang_cc1 -E -x c++ -std=c++20 %s -o - | FileCheck --check-prefix=CHECK-NO-MODULES %s
+// RUN: %clang_cc1 -E -x c++ -std=c++20 -fmodules %s -o - | FileCheck --check-prefix=CHECK-HAS-MODULES %s
+// RUN: %clang_cc1 -E -x objective-c++ -std=c++20 %s -o - | FileCheck --check-prefix=CHECK-NO-MODULES %s
+
+
 #if __has_feature(modules)
 int has_modules();
 #else