1 files changed, 13 insertions, 5 deletions
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 84cae84dec7..c2d480c34ff 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -16176,9 +16176,19 @@ Sema::DeclGroupPtrTy Sema::ActOnModuleDecl(SourceLocation StartLoc,
     return nullptr;
   }
 
+  assert(ModuleScopes.size() == 1 && "expected to be at global module scope");
+
   // FIXME: Most of this work should be done by the preprocessor rather than
   // here, in order to support macro import.
 
+  // Only one module-declaration is permitted per source file.
+  if (ModuleScopes.back().Module->Kind == Module::ModuleInterfaceUnit) {
+    Diag(ModuleLoc, diag::err_module_redeclaration);
+    Diag(VisibleModules.getImportLoc(ModuleScopes.back().Module),
+         diag::note_prev_module_declaration);
+    return nullptr;
+  }
+
   // Flatten the dots in a module name. Unlike Clang's hierarchical module map
   // modules, the dots here are just another character that can appear in a
   // module name.
@@ -16189,8 +16199,6 @@ Sema::DeclGroupPtrTy Sema::ActOnModuleDecl(SourceLocation StartLoc,
     ModuleName += Piece.first->getName();
   }
 
-  // FIXME: If we've already seen a module-declaration, report an error.
-
   // If a module name was explicitly specified on the command line, it must be
   // correct.
   if (!getLangOpts().CurrentModule.empty() &&
@@ -16205,8 +16213,6 @@ Sema::DeclGroupPtrTy Sema::ActOnModuleDecl(SourceLocation StartLoc,
   auto &Map = PP.getHeaderSearchInfo().getModuleMap();
   Module *Mod;
 
-  assert(ModuleScopes.size() == 1 && "expected to be at global module scope");
-
   switch (MDK) {
   case ModuleDeclKind::Interface: {
     // We can't have parsed or imported a definition of this module or parsed a
@@ -16240,7 +16246,9 @@ Sema::DeclGroupPtrTy Sema::ActOnModuleDecl(SourceLocation StartLoc,
                                        /*IsIncludeDirective=*/false);
     if (!Mod) {
       Diag(ModuleLoc, diag::err_module_not_defined) << ModuleName;
-      return nullptr;
+      // Create an empty module interface unit for error recovery.
+      Mod = Map.createModuleForInterfaceUnit(ModuleLoc, ModuleName,
+                                             ModuleScopes.front().Module);
     }
     break;
   }