diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index d2e4c5077702..5312ad118d5b 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3765,6 +3765,13 @@ ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName,
       SourceMgr.getLoadedSLocEntryByID(Index);
     }
 
+    // Map the original source file ID into the ID space of the current
+    // compilation.
+    if (F.OriginalSourceFileID.isValid()) {
+      F.OriginalSourceFileID = FileID::get(
+          F.SLocEntryBaseID + F.OriginalSourceFileID.getOpaqueValue() - 1);
+    }
+
     // Preload all the pending interesting identifiers by marking them out of
     // date.
     for (auto Offset : F.PreloadIdentifierOffsets) {
@@ -3873,10 +3880,6 @@ ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName,
 
   ModuleFile &PrimaryModule = ModuleMgr.getPrimaryModule();
   if (PrimaryModule.OriginalSourceFileID.isValid()) {
-    PrimaryModule.OriginalSourceFileID
-      = FileID::get(PrimaryModule.SLocEntryBaseID
-                    + PrimaryModule.OriginalSourceFileID.getOpaqueValue() - 1);
-
     // If this AST file is a precompiled preamble, then set the
     // preamble file ID of the source manager to the file source file
     // from which the preamble was built.
@@ -5575,6 +5578,13 @@ void ASTReader::ReadPragmaDiagnosticMappings(DiagnosticsEngine &Diag) {
       FirstState = ReadDiagState(
           F.isModule() ? DiagState() : *Diag.DiagStatesByLoc.CurDiagState,
           SourceLocation(), F.isModule());
+
+      // For an explicit module, set up the root buffer of the module to start
+      // with the initial diagnostic state of the module itself, to cover files
+      // that contain no explicit transitions.
+      if (F.isModule())
+        Diag.DiagStatesByLoc.Files[F.OriginalSourceFileID]
+            .StateTransitions.push_back({FirstState, 0});
     }
 
     // Read the state transitions.
diff --git a/clang/test/Modules/Inputs/module.map b/clang/test/Modules/Inputs/module.map
index b95eec351e86..7416d7008b85 100644
--- a/clang/test/Modules/Inputs/module.map
+++ b/clang/test/Modules/Inputs/module.map
@@ -261,6 +261,10 @@ module config {
   config_macros [exhaustive] WANT_FOO, WANT_BAR
 }
 
+module diag_flags {
+  header "diag_flags.h"
+}
+
 module diag_pragma {
   header "diag_pragma.h"
 }
diff --git a/clang/test/Modules/diag-flags.cpp b/clang/test/Modules/diag-flags.cpp
new file mode 100644
index 000000000000..36222fb95575
--- /dev/null
+++ b/clang/test/Modules/diag-flags.cpp
@@ -0,0 +1,22 @@
+// RUN: rm -rf %t
+//
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -emit-module -fmodules-cache-path=%t -fmodule-name=diag_flags -x c++ %S/Inputs/module.map -fmodules-ts
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -verify -fmodules-cache-path=%t -I %S/Inputs %s -fmodules-ts
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -verify -fmodules-cache-path=%t -I %S/Inputs %s -fmodules-ts -DIMPLICIT_FLAG -Werror=padded
+//
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -emit-module -fmodule-name=diag_flags -x c++ %S/Inputs/module.map -fmodules-ts -o %t/explicit.pcm -Werror=string-plus-int -Wpadded
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -verify -fmodules-cache-path=%t -I %S/Inputs %s -fmodules-ts -DEXPLICIT_FLAG -fmodule-file=%t/explicit.pcm
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -verify -fmodules-cache-path=%t -I %S/Inputs %s -fmodules-ts -DEXPLICIT_FLAG -fmodule-file=%t/explicit.pcm -Werror=padded
+
+import diag_flags;
+
+// Diagnostic flags from the module user make no difference to diagnostics
+// emitted within the module when using an explicitly-loaded module.
+#ifdef IMPLICIT_FLAG
+// expected-error@diag_flags.h:14 {{padding struct}}
+#elif defined(EXPLICIT_FLAG)
+// expected-warning@diag_flags.h:14 {{padding struct}}
+#else
+// expected-no-diagnostics
+#endif
+unsigned n = sizeof(Padded);