[Modules] Don't search for modulemaps in the immediate sub-directories of search paths for recent Apple SDKs.

clang/include/clang/Driver/Options.td

@@ -3232,6 +3232,10 @@ def fimplicit_module_maps : Flag <["-"], "fimplicit-module-maps">, Group<f_Group
   Visibility<[ClangOption, CC1Option, CLOption]>,
   HelpText<"Implicitly search the file system for module map files.">,
   MarshallingInfoFlag<HeaderSearchOpts<"ImplicitModuleMaps">>;
+defm modulemap_allow_subdirectory_search : BoolFOption <"modulemap-allow-subdirectory-search",
+  HeaderSearchOpts<"AllowModuleMapSubdirectorySearch">, DefaultTrue,
+  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>,
   PosFlag<SetTrue, [], [ClangOption, CC1Option],

clang/include/clang/Lex/HeaderSearchOptions.h

@@ -270,6 +270,12 @@ class HeaderSearchOptions {
   LLVM_PREFERRED_TYPE(bool)
   unsigned ModulesIncludeVFSUsage : 1;
 
+  /// Whether we should look for a module in module maps only in provided
+  /// header search paths or if we are allowed to look for module maps in
+  /// subdirectories of provided paths too.
+  LLVM_PREFERRED_TYPE(bool)
+  unsigned AllowModuleMapSubdirectorySearch : 1;
+
   HeaderSearchOptions(StringRef _Sysroot = "/")
       : Sysroot(_Sysroot), ModuleFormat("raw"), DisableModuleHash(false),
         ImplicitModuleMaps(false), ModuleMapFileHomeIsCwd(false),
@@ -285,7 +291,8 @@ class HeaderSearchOptions {
         ModulesSkipHeaderSearchPaths(false),
         ModulesSkipPragmaDiagnosticMappings(false),
         ModulesPruneNonAffectingModuleMaps(true), ModulesHashContent(false),
-        ModulesStrictContextHash(false), ModulesIncludeVFSUsage(false) {}
+        ModulesStrictContextHash(false), ModulesIncludeVFSUsage(false),
+        AllowModuleMapSubdirectorySearch(true) {}
 
   /// AddPath - Add the \p Path path to the specified \p Group list.
   void AddPath(StringRef Path, frontend::IncludeDirGroup Group,

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3957,6 +3957,9 @@ static bool RenderModulesOptions(Compilation &C, const Driver &D,
                    options::OPT_fno_modules_strict_decluse, false))
     CmdArgs.push_back("-fmodules-strict-decluse");
 
+  Args.addOptOutFlag(CmdArgs, options::OPT_fmodulemap_allow_subdirectory_search,
+                     options::OPT_fno_modulemap_allow_subdirectory_search);
+
   // -fno-implicit-modules turns off implicitly compiling modules on demand.
   bool ImplicitModules = false;
   if (!Args.hasFlag(options::OPT_fimplicit_modules,

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -3036,6 +3036,35 @@ void Darwin::addClangTargetOptions(
   if (!DriverArgs.hasArgNoClaim(options::OPT_fdefine_target_os_macros,
                                 options::OPT_fno_define_target_os_macros))
     CC1Args.push_back("-fdefine-target-os-macros");
+
+  // Limit modulemap search on sufficiently recent SDKs.
+  if (SDKInfo &&
+      !DriverArgs.hasFlag(options::OPT_fmodulemap_allow_subdirectory_search,
+                          options::OPT_fno_modulemap_allow_subdirectory_search,
+                          false)) {
+    bool RequiresSubdirectorySearch;
+    VersionTuple SDKVersion = SDKInfo->getVersion();
+    switch (TargetPlatform) {
+    default:
+      RequiresSubdirectorySearch = true;
+      break;
+    case MacOS:
+      RequiresSubdirectorySearch = SDKVersion < VersionTuple(15, 0);
+      break;
+    case IPhoneOS:
+    case TvOS:
+      RequiresSubdirectorySearch = SDKVersion < VersionTuple(18, 0);
+      break;
+    case WatchOS:
+      RequiresSubdirectorySearch = SDKVersion < VersionTuple(11, 0);
+      break;
+    case XROS:
+      RequiresSubdirectorySearch = SDKVersion < VersionTuple(2, 0);
+      break;
+    }
+    if (!RequiresSubdirectorySearch)
+      CC1Args.push_back("-fno-modulemap-allow-subdirectory-search");
+  }
 }
 
 void Darwin::addClangCC1ASTargetOptions(

clang/lib/Lex/HeaderSearch.cpp

@@ -378,20 +378,22 @@ Module *HeaderSearch::lookupModule(StringRef ModuleName, StringRef SearchName,
         break;
     }
 
-    // If we've already performed the exhaustive search for module maps in this
-    // search directory, don't do it again.
-    if (Dir.haveSearchedAllModuleMaps())
-      continue;
+    if (HSOpts->AllowModuleMapSubdirectorySearch) {
+      // If we've already performed the exhaustive search for module maps in
+      // this search directory, don't do it again.
+      if (Dir.haveSearchedAllModuleMaps())
+        continue;
 
-    // Load all module maps in the immediate subdirectories of this search
-    // directory if ModuleName was from @import.
-    if (AllowExtraModuleMapSearch)
-      loadSubdirectoryModuleMaps(Dir);
+      // Load all module maps in the immediate subdirectories of this search
+      // directory if ModuleName was from @import.
+      if (AllowExtraModuleMapSearch)
+        loadSubdirectoryModuleMaps(Dir);
 
-    // Look again for the module.
-    Module = ModMap.findModule(ModuleName);
-    if (Module)
-      break;
+      // Look again for the module.
+      Module = ModMap.findModule(ModuleName);
+      if (Module)
+        break;
+    }
   }
 
   return Module;

clang/test/Driver/modulemap-allow-subdirectory-search.c

@@ -0,0 +1,27 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// Check that with a sufficiently new SDK not searching for module maps in subdirectories.
+
+// New SDK.
+// RUN: %clang -target x86_64-apple-macos10.13 -isysroot %t/MacOSX15.0.sdk -fmodules %t/test.c -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=NO-SUBDIRECTORIES %t/test.c
+// Old SDK.
+// RUN: %clang -target x86_64-apple-macos10.13 -isysroot %t/MacOSX14.0.sdk -fmodules %t/test.c -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=SEARCH-SUBDIRECTORIES %t/test.c
+// Non-Darwin platform.
+// RUN: %clang -target i386-unknown-linux -isysroot %t/MacOSX15.0.sdk -fmodules %t/test.c -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=SEARCH-SUBDIRECTORIES %t/test.c
+// New SDK overriding the default.
+// RUN: %clang -target x86_64-apple-macos10.13 -isysroot %t/MacOSX15.0.sdk -fmodules %t/test.c -fmodulemap-allow-subdirectory-search -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=SEARCH-SUBDIRECTORIES %t/test.c
+
+//--- test.c
+// NO-SUBDIRECTORIES: "-fno-modulemap-allow-subdirectory-search"
+// SEARCH-SUBDIRECTORIES-NOT: "-fno-modulemap-allow-subdirectory-search"
+
+//--- MacOSX15.0.sdk/SDKSettings.json
+{"Version":"15.0", "MaximumDeploymentTarget": "15.0.99"}
+
+//--- MacOSX14.0.sdk/SDKSettings.json
+{"Version":"14.0", "MaximumDeploymentTarget": "14.0.99"}

clang/test/Modules/modulemap-allow-subdirectory-search.m

@@ -0,0 +1,18 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache -I %t/include %t/test.m
+// RUN: %clang_cc1 -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache -I %t/include %t/test.m -fmodulemap-allow-subdirectory-search
+// RUN: not %clang_cc1 -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache -I %t/include %t/test.m -fno-modulemap-allow-subdirectory-search
+
+//--- include/UnrelatedName/Header.h
+// empty
+
+//--- include/UnrelatedName/module.modulemap
+module UsefulCode {
+  header "Header.h"
+  export *
+}
+
+//--- test.m
+@import UsefulCode;