Reapply r321781: [Modules] Allow modules specified by -fmodule-map-file to shadow implicitly found ones

When modules come from module map files explicitly specified by
-fmodule-map-file= arguments, allow those to override/shadow modules
with the same name that are found implicitly by header search. If such a
module is looked up by name (e.g. @import), we will always find the one
from -fmodule-map-file. If we try to use a shadowed module by including
one of its headers report an error.

This enables developers to force use of a specific copy of their module
to be used if there are multiple copies that would otherwise be visible,
for example if they develop modules that are installed in the default
search paths.

Patch originally by Ben Langmuir,
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20151116/143425.html

Based on cfe-dev discussion:
http://lists.llvm.org/pipermail/cfe-dev/2015-November/046164.html

Differential Revision: https://reviews.llvm.org/D31269

rdar://problem/23612102

llvm-svn: 321855

clang/include/clang/Basic/DiagnosticCommonKinds.td

@@ -94,6 +94,9 @@ def remark_module_lock_failure : Remark<
   "could not acquire lock file for module '%0': %1">, InGroup<ModuleBuild>;
 def remark_module_lock_timeout : Remark<
   "timed out waiting to acquire lock file for module '%0'">, InGroup<ModuleBuild>;
+def err_module_shadowed : Error<"import of shadowed module '%0'">, DefaultFatal;
+def err_module_build_shadowed_submodule : Error<
+  "build a shadowed submodule '%0'">, DefaultFatal;
 def err_module_cycle : Error<"cyclic dependency in module '%0': %1">, 
   DefaultFatal;
 def err_module_prebuilt : Error<

clang/include/clang/Basic/Module.h

@@ -197,6 +197,9 @@ public:
   /// will be false to indicate that this (sub)module is not available.
   SmallVector<Requirement, 2> Requirements;
 
+  /// \brief A module with the same name that shadows this module.
+  Module *ShadowingModule = nullptr;
+
   /// \brief Whether this module is missing a feature from \c Requirements.
   unsigned IsMissingRequirement : 1;
 
@@ -375,13 +378,20 @@ public:
   ///
   /// \param Target The target options used for the current translation unit.
   ///
-  /// \param Req If this module is unavailable, this parameter
-  /// will be set to one of the requirements that is not met for use of
-  /// this module.
+  /// \param Req If this module is unavailable because of a missing requirement,
+  /// this parameter will be set to one of the requirements that is not met for
+  /// use of this module.
+  ///
+  /// \param MissingHeader If this module is unavailable because of a missing
+  /// header, this parameter will be set to one of the missing headers.
+  ///
+  /// \param ShadowingModule If this module is unavailable because it is
+  /// shadowed, this parameter will be set to the shadowing module.
   bool isAvailable(const LangOptions &LangOpts, 
                    const TargetInfo &Target,
                    Requirement &Req,
-                   UnresolvedHeaderDirective &MissingHeader) const;
+                   UnresolvedHeaderDirective &MissingHeader,
+                   Module *&ShadowingModule) const;
 
   /// \brief Determine whether this module is a submodule.
   bool isSubModule() const { return Parent != nullptr; }

clang/include/clang/Lex/HeaderSearch.h

@@ -726,6 +726,7 @@ private:
   LoadModuleMapResult loadModuleMapFileImpl(const FileEntry *File,
                                             bool IsSystem,
                                             const DirectoryEntry *Dir,
+                                            bool IsExplicitlyProvided,
                                             FileID ID = FileID(),
                                             unsigned *Offset = nullptr);
 

clang/include/clang/Lex/ModuleMap.h

@@ -98,6 +98,9 @@ class ModuleMap {
   /// \brief The top-level modules that are known.
   llvm::StringMap<Module *> Modules;
 
+  /// Shadow modules created while building this module map.
+  llvm::SmallVector<Module*, 2> ShadowModules;
+
   /// \brief The number of modules we have created in total.
   unsigned NumCreatedModules = 0;
 
@@ -195,6 +198,17 @@ private:
   /// header.
   llvm::DenseMap<const DirectoryEntry *, Module *> UmbrellaDirs;
 
+  /// \brief The set of modules provided explicitly (e.g. by -fmodule-map-file),
+  /// which are allowed to shadow other implicitly discovered modules.
+  llvm::DenseSet<const Module *> ExplicitlyProvidedModules;
+
+  bool mayShadowModuleBeingParsed(Module *ExistingModule,
+                                  bool IsExplicitlyProvided) {
+    assert(!ExistingModule->Parent && "expected top-level module");
+    return !IsExplicitlyProvided &&
+           ExplicitlyProvidedModules.count(ExistingModule);
+  }
+
   /// \brief The set of attributes that can be attached to a module.
   struct Attributes {
     /// \brief Whether this is a system module.
@@ -475,9 +489,9 @@ public:
   ///
   /// \returns The found or newly-created module, along with a boolean value
   /// that will be true if the module is newly-created.
-  std::pair<Module *, bool> findOrCreateModule(StringRef Name, Module *Parent,
-                                               bool IsFramework,
-                                               bool IsExplicit);
+  std::pair<Module *, bool>
+  findOrCreateModule(StringRef Name, Module *Parent, bool IsFramework,
+                     bool IsExplicit, bool UsesExplicitModuleMapFile = false);
 
   /// \brief Create a 'global module' for a C++ Modules TS module interface
   /// unit.
@@ -502,6 +516,11 @@ public:
   Module *inferFrameworkModule(const DirectoryEntry *FrameworkDir,
                                bool IsSystem, Module *Parent);
 
+  /// \brief Create a new top-level module that is shadowed by
+  /// \p ShadowingModule.
+  Module *createShadowedModule(StringRef Name, bool IsFramework,
+                               Module *ShadowingModule);
+
   /// \brief Retrieve the module map file containing the definition of the given
   /// module.
   ///
@@ -587,6 +606,8 @@ public:
   /// \brief Marks this header as being excluded from the given module.
   void excludeHeader(Module *Mod, Module::Header Header);
 
+  void setExplicitlyProvided(Module *Mod);
+
   /// \brief Parse the given module map file, and record any modules we 
   /// encounter.
   ///
@@ -606,10 +627,15 @@ public:
   /// \param ExternModuleLoc The location of the "extern module" declaration
   ///        that caused us to load this module map file, if any.
   ///
+  /// \param IsExplicitlyProvided Whether this module map file was provided
+  /// explicitly by the user (e.g. -fmodule-map-file), rather than found
+  /// implicitly.
+  ///
   /// \returns true if an error occurred, false otherwise.
   bool parseModuleMapFile(const FileEntry *File, bool IsSystem,
-                          const DirectoryEntry *HomeDir, FileID ID = FileID(),
-                          unsigned *Offset = nullptr,
+                          const DirectoryEntry *HomeDir,
+                          bool IsExplicitlyProvided = false,
+                          FileID ID = FileID(), unsigned *Offset = nullptr,
                           SourceLocation ExternModuleLoc = SourceLocation());
 
   /// \brief Dump the contents of the module map, for debugging purposes.

clang/lib/Basic/Module.cpp

@@ -95,11 +95,16 @@ static bool hasFeature(StringRef Feature, const LangOptions &LangOpts,
 
 bool Module::isAvailable(const LangOptions &LangOpts, const TargetInfo &Target,
                          Requirement &Req,
-                         UnresolvedHeaderDirective &MissingHeader) const {
+                         UnresolvedHeaderDirective &MissingHeader,
+                         Module *&ShadowingModule) const {
   if (IsAvailable)
     return true;
 
   for (const Module *Current = this; Current; Current = Current->Parent) {
+    if (Current->ShadowingModule) {
+      ShadowingModule = Current->ShadowingModule;
+      return false;
+    }
     for (unsigned I = 0, N = Current->Requirements.size(); I != N; ++I) {
       if (hasFeature(Current->Requirements[I].first, LangOpts, Target) !=
               Current->Requirements[I].second) {

clang/lib/Lex/HeaderSearch.cpp

@@ -1367,7 +1367,8 @@ bool HeaderSearch::loadModuleMapFile(const FileEntry *File, bool IsSystem,
     }
   }
 
-  switch (loadModuleMapFileImpl(File, IsSystem, Dir, ID, Offset)) {
+  switch (loadModuleMapFileImpl(File, IsSystem, Dir,
+                                /*IsExplictlyProvided=*/true, ID, Offset)) {
   case LMM_AlreadyLoaded:
   case LMM_NewlyLoaded:
     return false;
@@ -1378,10 +1379,9 @@ bool HeaderSearch::loadModuleMapFile(const FileEntry *File, bool IsSystem,
   llvm_unreachable("Unknown load module map result");
 }
 
-HeaderSearch::LoadModuleMapResult
-HeaderSearch::loadModuleMapFileImpl(const FileEntry *File, bool IsSystem,
-                                    const DirectoryEntry *Dir, FileID ID,
-                                    unsigned *Offset) {
+HeaderSearch::LoadModuleMapResult HeaderSearch::loadModuleMapFileImpl(
+    const FileEntry *File, bool IsSystem, const DirectoryEntry *Dir,
+    bool IsExplicitlyProvided, FileID ID, unsigned *Offset) {
   assert(File && "expected FileEntry");
 
   // Check whether we've already loaded this module map, and mark it as being
@@ -1390,14 +1390,16 @@ HeaderSearch::loadModuleMapFileImpl(const FileEntry *File, bool IsSystem,
   if (!AddResult.second)
     return AddResult.first->second ? LMM_AlreadyLoaded : LMM_InvalidModuleMap;
 
-  if (ModMap.parseModuleMapFile(File, IsSystem, Dir, ID, Offset)) {
+  if (ModMap.parseModuleMapFile(File, IsSystem, Dir, IsExplicitlyProvided, ID,
+                                Offset)) {
     LoadedModuleMaps[File] = false;
     return LMM_InvalidModuleMap;
   }
 
   // Try to load a corresponding private module map.
   if (const FileEntry *PMMFile = getPrivateModuleMap(File, FileMgr)) {
-    if (ModMap.parseModuleMapFile(PMMFile, IsSystem, Dir)) {
+    if (ModMap.parseModuleMapFile(PMMFile, IsSystem, Dir,
+                                  IsExplicitlyProvided)) {
       LoadedModuleMaps[File] = false;
       return LMM_InvalidModuleMap;
     }
@@ -1468,8 +1470,8 @@ HeaderSearch::loadModuleMapFile(const DirectoryEntry *Dir, bool IsSystem,
     return KnownDir->second ? LMM_AlreadyLoaded : LMM_InvalidModuleMap;
 
   if (const FileEntry *ModuleMapFile = lookupModuleMapFile(Dir, IsFramework)) {
-    LoadModuleMapResult Result =
-        loadModuleMapFileImpl(ModuleMapFile, IsSystem, Dir);
+    LoadModuleMapResult Result = loadModuleMapFileImpl(
+        ModuleMapFile, IsSystem, Dir, /*IsExplicitlyProvided=*/false);
     // Add Dir explicitly in case ModuleMapFile is in a subdirectory.
     // E.g. Foo.framework/Modules/module.modulemap
     //      ^Dir                  ^ModuleMapFile

clang/lib/Lex/ModuleMap.cpp

@@ -281,6 +281,8 @@ ModuleMap::ModuleMap(SourceManager &SourceMgr, DiagnosticsEngine &Diags,
 ModuleMap::~ModuleMap() {
   for (auto &M : Modules)
     delete M.getValue();
+  for (auto *M : ShadowModules)
+    delete M;
 }
 
 void ModuleMap::setTarget(const TargetInfo &Target) {
@@ -744,14 +746,13 @@ Module *ModuleMap::lookupModuleQualified(StringRef Name, Module *Context) const{
   return Context->findSubmodule(Name);
 }
 
-std::pair<Module *, bool> ModuleMap::findOrCreateModule(StringRef Name,
-                                                        Module *Parent,
-                                                        bool IsFramework,
-                                                        bool IsExplicit) {
+std::pair<Module *, bool>
+ModuleMap::findOrCreateModule(StringRef Name, Module *Parent, bool IsFramework,
+                              bool IsExplicit, bool UsesExplicitModuleMapFile) {
   // Try to find an existing module with this name.
   if (Module *Sub = lookupModuleQualified(Name, Parent))
     return std::make_pair(Sub, false);
-  
+
   // Create a new module with this name.
   Module *Result = new Module(Name, SourceLocation(), Parent, IsFramework,
                               IsExplicit, NumCreatedModules++);
@@ -759,6 +760,8 @@ std::pair<Module *, bool> ModuleMap::findOrCreateModule(StringRef Name,
     if (LangOpts.CurrentModule == Name)
       SourceModule = Result;
     Modules[Name] = Result;
+    if (UsesExplicitModuleMapFile)
+      ExplicitlyProvidedModules.insert(Result);
   }
   return std::make_pair(Result, true);
 }
@@ -999,6 +1002,20 @@ Module *ModuleMap::inferFrameworkModule(const DirectoryEntry *FrameworkDir,
   return Result;
 }
 
+Module *ModuleMap::createShadowedModule(StringRef Name, bool IsFramework,
+                                        Module *ShadowingModule) {
+
+  // Create a new module with this name.
+  Module *Result =
+      new Module(Name, SourceLocation(), /*Parent=*/nullptr, IsFramework,
+                 /*IsExplicit=*/false, NumCreatedModules++);
+  Result->ShadowingModule = ShadowingModule;
+  Result->IsAvailable = false;
+  ShadowModules.push_back(Result);
+
+  return Result;
+}
+
 void ModuleMap::setUmbrellaHeader(Module *Mod, const FileEntry *UmbrellaHeader,
                                   Twine NameAsWritten) {
   Headers[UmbrellaHeader].push_back(KnownHeader(Mod, NormalHeader));
@@ -1116,6 +1133,11 @@ void ModuleMap::excludeHeader(Module *Mod, Module::Header Header) {
   Mod->Headers[Module::HK_Excluded].push_back(std::move(Header));
 }
 
+void ModuleMap::setExplicitlyProvided(Module *Mod) {
+  assert(Modules[Mod->Name] == Mod && "explicitly provided module is shadowed");
+  ExplicitlyProvidedModules.insert(Mod);
+}
+
 const FileEntry *
 ModuleMap::getContainingModuleMapFile(const Module *Module) const {
   if (Module->DefinitionLoc.isInvalid())
@@ -1319,7 +1341,9 @@ namespace clang {
 
     /// \brief Consume the current token and return its location.
     SourceLocation consumeToken();
-    
+
+    bool UsesExplicitModuleMapFile = false;
+
     /// \brief Skip tokens until we reach the a token with the given kind
     /// (or the end of the file).
     void skipUntil(MMToken::TokenKind K);
@@ -1345,20 +1369,19 @@ namespace clang {
     bool parseOptionalAttributes(Attributes &Attrs);
     
   public:
-    explicit ModuleMapParser(Lexer &L, SourceManager &SourceMgr, 
-                             const TargetInfo *Target,
-                             DiagnosticsEngine &Diags,
-                             ModuleMap &Map,
-                             const FileEntry *ModuleMapFile,
-                             const DirectoryEntry *Directory,
-                             bool IsSystem)
+    explicit ModuleMapParser(Lexer &L, SourceManager &SourceMgr,
+                             const TargetInfo *Target, DiagnosticsEngine &Diags,
+                             ModuleMap &Map, const FileEntry *ModuleMapFile,
+                             const DirectoryEntry *Directory, bool IsSystem,
+                             bool UsesExplicitModuleMapFile)
         : L(L), SourceMgr(SourceMgr), Target(Target), Diags(Diags), Map(Map),
           ModuleMapFile(ModuleMapFile), Directory(Directory),
-          IsSystem(IsSystem) {
+          IsSystem(IsSystem),
+          UsesExplicitModuleMapFile(UsesExplicitModuleMapFile) {
       Tok.clear();
       consumeToken();
     }
-    
+
     bool parseModuleMapFile();
 
     bool terminatedByDirective() { return false; }
@@ -1787,6 +1810,7 @@ void ModuleMapParser::parseModuleDecl() {
   SourceLocation LBraceLoc = consumeToken();
   
   // Determine whether this (sub)module has already been defined.
+  Module *ShadowingModule = nullptr;
   if (Module *Existing = Map.lookupModuleQualified(ModuleName, ActiveModule)) {
     // We might see a (re)definition of a module that we already have a
     // definition for in two cases:
@@ -1812,23 +1836,36 @@ void ModuleMapParser::parseModuleDecl() {
       }
       return;
     }
-    
-    Diags.Report(ModuleNameLoc, diag::err_mmap_module_redefinition)
-      << ModuleName;
-    Diags.Report(Existing->DefinitionLoc, diag::note_mmap_prev_definition);
-    
-    // Skip the module definition.
-    skipUntil(MMToken::RBrace);
-    if (Tok.is(MMToken::RBrace))
-      consumeToken();
-    
-    HadError = true;
-    return;
+
+    if (!Existing->Parent &&
+        Map.mayShadowModuleBeingParsed(Existing, UsesExplicitModuleMapFile)) {
+      ShadowingModule = Existing;
+    } else {
+      // This is not a shawdowed module decl, it is an illegal redefinition.
+      Diags.Report(ModuleNameLoc, diag::err_mmap_module_redefinition)
+          << ModuleName;
+      Diags.Report(Existing->DefinitionLoc, diag::note_mmap_prev_definition);
+
+      // Skip the module definition.
+      skipUntil(MMToken::RBrace);
+      if (Tok.is(MMToken::RBrace))
+        consumeToken();
+
+      HadError = true;
+      return;
+    }
   }
 
   // Start defining this module.
-  ActiveModule = Map.findOrCreateModule(ModuleName, ActiveModule, Framework,
-                                        Explicit).first;
+  if (ShadowingModule) {
+    ActiveModule =
+        Map.createShadowedModule(ModuleName, Framework, ShadowingModule);
+  } else {
+    ActiveModule = Map.findOrCreateModule(ModuleName, ActiveModule, Framework,
+                                          Explicit, UsesExplicitModuleMapFile)
+                       .first;
+  }
+
   ActiveModule->DefinitionLoc = ModuleNameLoc;
   if (Attrs.IsSystem || IsSystem)
     ActiveModule->IsSystem = true;
@@ -2004,7 +2041,7 @@ void ModuleMapParser::parseExternModuleDecl() {
         Map.HeaderInfo.getHeaderSearchOpts().ModuleMapFileHomeIsCwd
             ? Directory
             : File->getDir(),
-        FileID(), nullptr, ExternLoc);
+        false /*IsExplicitlyProvided*/, FileID(), nullptr, ExternLoc);
 }
 
 /// Whether to add the requirement \p Feature to the module \p M.
@@ -2811,7 +2848,8 @@ bool ModuleMapParser::parseModuleMapFile() {
 }
 
 bool ModuleMap::parseModuleMapFile(const FileEntry *File, bool IsSystem,
-                                   const DirectoryEntry *Dir, FileID ID,
+                                   const DirectoryEntry *Dir,
+                                   bool IsExplicitlyProvided, FileID ID,
                                    unsigned *Offset,
                                    SourceLocation ExternModuleLoc) {
   assert(Target && "Missing target information");
@@ -2841,7 +2879,7 @@ bool ModuleMap::parseModuleMapFile(const FileEntry *File, bool IsSystem,
           Buffer->getBufferEnd());
   SourceLocation Start = L.getSourceLocation();
   ModuleMapParser Parser(L, SourceMgr, Target, Diags, *this, File, Dir,
-                         IsSystem);
+                         IsSystem, IsExplicitlyProvided);
   bool Result = Parser.parseModuleMapFile();
   ParsedModuleMap[File] = Result;
 
@@ -2854,5 +2892,6 @@ bool ModuleMap::parseModuleMapFile(const FileEntry *File, bool IsSystem,
   // Notify callbacks that we parsed it.
   for (const auto &Cb : Callbacks)
     Cb->moduleMapFileRead(Start, *File, IsSystem);
+
   return Result;
 }

clang/lib/Lex/PPDirectives.cpp

@@ -1655,12 +1655,18 @@ bool Preprocessor::checkModuleIsAvailable(const LangOptions &LangOpts,
                                           DiagnosticsEngine &Diags, Module *M) {
   Module::Requirement Requirement;
   Module::UnresolvedHeaderDirective MissingHeader;
-  if (M->isAvailable(LangOpts, TargetInfo, Requirement, MissingHeader))
+  Module *ShadowingModule = nullptr;
+  if (M->isAvailable(LangOpts, TargetInfo, Requirement, MissingHeader,
+                     ShadowingModule))
     return false;
 
   if (MissingHeader.FileNameLoc.isValid()) {
     Diags.Report(MissingHeader.FileNameLoc, diag::err_module_header_missing)
         << MissingHeader.IsUmbrella << MissingHeader.FileName;
+  } else if (ShadowingModule) {
+    Diags.Report(M->DefinitionLoc, diag::err_module_shadowed) << M->Name;
+    Diags.Report(ShadowingModule->DefinitionLoc,
+                 diag::note_previous_definition);
   } else {
     // FIXME: Track the location at which the requirement was specified, and
     // use it here.
@@ -2024,6 +2030,15 @@ void Preprocessor::HandleIncludeDirective(SourceLocation HashLoc,
 
   // Determine if we're switching to building a new submodule, and which one.
   if (auto *M = SuggestedModule.getModule()) {
+    if (M->getTopLevelModule()->ShadowingModule) {
+      // We are building a submodule that belongs to a shadowed module. This
+      // means we find header files in the shadowed module.
+      Diag(M->DefinitionLoc, diag::err_module_build_shadowed_submodule)
+        << M->getFullModuleName();
+      Diag(M->getTopLevelModule()->ShadowingModule->DefinitionLoc,
+           diag::note_previous_definition);
+      return;
+    }
     // When building a pch, -fmodule-name tells the compiler to textually
     // include headers in the specified module. We are not building the
     // specified module.

clang/test/Modules/Inputs/shadow/A1/A.h

@@ -0,0 +1 @@
+#define A1_A_h

clang/test/Modules/Inputs/shadow/A1/module.modulemap

@@ -0,0 +1,5 @@
+module A {
+  header "A.h"
+}
+
+module A1 {}

clang/test/Modules/Inputs/shadow/A2/A.h

@@ -0,0 +1 @@
+#define A2_A_h

clang/test/Modules/Inputs/shadow/A2/module.modulemap

@@ -0,0 +1,5 @@
+module A {
+  header "A.h"
+}
+
+module A2 {}

clang/test/Modules/Inputs/shadowed-submodule/A1/Foo.h

@@ -0,0 +1 @@
+#include <stdarg.h> // expected-error {{could not build module 'A'}}

clang/test/Modules/Inputs/shadowed-submodule/A1/module.modulemap

@@ -0,0 +1,14 @@
+module A [system] { // expected-note {{previous definition is here}}
+  module sub {
+    header "sys/A.h"
+  }
+  module sub2 {
+    header "sys/A2.h"
+  }
+  module stdarg {
+    header "stdarg.h"
+    export *
+  }
+}
+
+module A2 {}

clang/test/Modules/Inputs/shadowed-submodule/A1/sys/A.h

@@ -0,0 +1 @@
+#include <sys/A2.h>

clang/test/Modules/Inputs/shadowed-submodule/A1/sys/A2.h

@@ -0,0 +1 @@
+// nothing

clang/test/Modules/Inputs/shadowed-submodule/A2/Foo.h

@@ -0,0 +1 @@
+#include <stdarg.h>

clang/test/Modules/Inputs/shadowed-submodule/A2/module.modulemap

@@ -0,0 +1,14 @@
+module A [system] {
+  module sub {
+    header "sys/A.h"
+  }
+  module sub2 { // expected-error {{build a shadowed submodule 'A.sub2'}}
+    header "sys/A2.h"
+  }
+  module stdarg {
+    header "stdarg.h"
+    export *
+  }
+}
+
+module A2 {}

clang/test/Modules/Inputs/shadowed-submodule/A2/sys/A.h

@@ -0,0 +1 @@
+#include <sys/A2.h>

clang/test/Modules/Inputs/shadowed-submodule/A2/sys/A2.h

@@ -0,0 +1 @@
+// nothing

clang/test/Modules/Inputs/shadowed-submodule/Foo/module.modulemap

@@ -0,0 +1,3 @@
+module Foo {
+  header "../A1/Foo.h"
+}

clang/test/Modules/shadow.m

@@ -0,0 +1,21 @@
+// RUN: rm -rf %t
+// RUN: not %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/shadow/A1 -I %S/Inputs/shadow/A2 %s -fsyntax-only 2>&1 | FileCheck %s -check-prefix=REDEFINITION
+// RUN: not %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -fmodule-map-file=%S/Inputs/shadow/A1/module.modulemap -fmodule-map-file=%S/Inputs/shadow/A2/module.modulemap %s -fsyntax-only 2>&1 | FileCheck %s -check-prefix=REDEFINITION
+// REDEFINITION: error: redefinition of module 'A'
+// REDEFINITION: note: previously defined
+
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -fmodule-map-file=%S/Inputs/shadow/A1/module.modulemap -I %S/Inputs/shadow %s -verify
+
+@import A1;
+@import A2;
+@import A;
+
+#import "A2/A.h" // expected-note {{implicitly imported}}
+// expected-error@A2/module.modulemap:1 {{import of shadowed module 'A'}}
+// expected-note@A1/module.modulemap:1 {{previous definition}}
+
+#if defined(A2_A_h)
+#error got the wrong definition of module A
+#elif !defined(A1_A_h)
+#error missing definition from A1
+#endif

clang/test/Modules/shadowed-submodule.m

@@ -0,0 +1,5 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/shadowed-submodule/Foo -I %S/Inputs/shadowed-submodule/A2 %s -verify
+
+@import Foo; // expected-error {{module 'A' was built in directory}}
+             // expected-note@shadowed-submodule.m:4 {{imported by module 'Foo'}}