diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index ef3f59f4f2263..8aa3489a2a62b 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -631,6 +631,7 @@ def MissingFieldInitializers : DiagGroup<"missing-field-initializers",
 def ModuleLock : DiagGroup<"module-lock">;
 def ModuleBuild : DiagGroup<"module-build">;
 def ModuleImport : DiagGroup<"module-import">;
+def ModuleValidation : DiagGroup<"module-validation">;
 def ModuleConflict : DiagGroup<"module-conflict">;
 def ModuleFileExtension : DiagGroup<"module-file-extension">;
 def ModuleIncludeDirectiveTranslation : DiagGroup<"module-include-translation">;
diff --git a/clang/include/clang/Basic/DiagnosticSerializationKinds.td b/clang/include/clang/Basic/DiagnosticSerializationKinds.td
index fc3585ffe8549..b80aff385e01f 100644
--- a/clang/include/clang/Basic/DiagnosticSerializationKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSerializationKinds.td
@@ -86,6 +86,10 @@ def remark_module_import : Remark<
   "importing module '%0'%select{| into '%3'}2 from '%1'">,
   ShowInSystemHeader,
   InGroup<ModuleImport>;
+def remark_module_validation : Remark<
+  "validating %0 input files in module '%1' from '%2'">,
+  ShowInSystemHeader,
+  InGroup<ModuleValidation>;
 
 def err_imported_module_not_found : Error<
     "module '%0' in precompiled file '%1' %select{(imported by precompiled file '%2') |}4"
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 868f0cc8b1da7..c9104f5bc2ed0 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3190,6 +3190,10 @@ ASTReader::ReadControlBlock(ModuleFile &F,
             F.Kind == MK_ImplicitModule)
           N = ForceValidateUserInputs ? NumUserInputs : 0;
 
+        if (N != 0)
+          Diag(diag::remark_module_validation)
+              << N << F.ModuleName << F.FileName;
+
         for (unsigned I = 0; I < N; ++I) {
           InputFile IF = getInputFile(F, I+1, Complain);
           if (!IF.getFile() || IF.isOutOfDate())
diff --git a/clang/lib/Serialization/ModuleCache.cpp b/clang/lib/Serialization/ModuleCache.cpp
index 96687277ebafd..9850956380423 100644
--- a/clang/lib/Serialization/ModuleCache.cpp
+++ b/clang/lib/Serialization/ModuleCache.cpp
@@ -115,8 +115,10 @@ class CrossProcessModuleCache : public ModuleCache {
   }
 
   std::time_t getModuleTimestamp(StringRef ModuleFilename) override {
+    std::string TimestampFilename =
+        serialization::ModuleFile::getTimestampFilename(ModuleFilename);
     llvm::sys::fs::file_status Status;
-    if (llvm::sys::fs::status(ModuleFilename, Status) != std::error_code{})
+    if (llvm::sys::fs::status(TimestampFilename, Status) != std::error_code{})
       return 0;
     return llvm::sys::toTimeT(Status.getLastModificationTime());
   }
diff --git a/clang/test/Modules/fmodules-validate-once-per-build-session.c b/clang/test/Modules/fmodules-validate-once-per-build-session.c
index d9d79b001e30c..2348ca1381e8a 100644
--- a/clang/test/Modules/fmodules-validate-once-per-build-session.c
+++ b/clang/test/Modules/fmodules-validate-once-per-build-session.c
@@ -1,119 +1,134 @@
-#include "foo.h"
-#include "bar.h"
-
-// Clear the module cache.
-// RUN: rm -rf %t
-// RUN: mkdir -p %t/Inputs
-// RUN: mkdir -p %t/modules-to-compare
+// This tests the behavior of -fmodules-validate-once-per-build-session with
+// different combinations of flags and states of the module cache.
 
-// ===
-// Create a module.  We will use -I or -isystem to determine whether to treat
-// foo.h as a system header.
-// RUN: echo 'void meow(void);' > %t/Inputs/foo.h
-// RUN: echo 'void woof(void);' > %t/Inputs/bar.h
-// RUN: echo 'module Foo { header "foo.h" }' > %t/Inputs/module.modulemap
-// RUN: echo 'extern module Bar "bar.modulemap"' >> %t/Inputs/module.modulemap
-// RUN: echo 'module Bar { header "bar.h" }' > %t/Inputs/bar.modulemap
+// Note: The `sleep 1` commands sprinkled throughout this test make the strict
+//       comparisons of epoch mtimes work as expected. Some may be unnecessary,
+//       but make the intent clearer.
 
-// ===
-// Compile the module.
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user-no-force -fsyntax-only -I %t/Inputs -fno-modules-force-validate-user-headers -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user | grep Bar.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user-no-force | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user-no-force | grep Bar.pcm.timestamp
-// RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-before.pcm
-// RUN: cp %t/modules-cache/Bar.pcm %t/modules-to-compare/Bar-before.pcm
-// RUN: cp %t/modules-cache-user/Foo.pcm %t/modules-to-compare/Foo-before-user.pcm
-// RUN: cp %t/modules-cache-user/Bar.pcm %t/modules-to-compare/Bar-before-user.pcm
-// RUN: cp %t/modules-cache-user-no-force/Foo.pcm %t/modules-to-compare/Foo-before-user-no-force.pcm
-// RUN: cp %t/modules-cache-user-no-force/Bar.pcm %t/modules-to-compare/Bar-before-user-no-force.pcm
-
-// ===
-// Use it, and make sure that we did not recompile it.
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-use-no-force -fsyntax-only -I %t/Inputs -fno-modules-force-validate-user-headers -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user | grep Bar.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user-no-force | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user-no-force | grep Bar.pcm.timestamp
-// RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-after.pcm
-// RUN: cp %t/modules-cache/Bar.pcm %t/modules-to-compare/Bar-after.pcm
-// RUN: cp %t/modules-cache-user/Foo.pcm %t/modules-to-compare/Foo-after-user.pcm
-// RUN: cp %t/modules-cache-user/Bar.pcm %t/modules-to-compare/Bar-after-user.pcm
-// RUN: cp %t/modules-cache-user-no-force/Foo.pcm %t/modules-to-compare/Foo-after-user-no-force.pcm
-// RUN: cp %t/modules-cache-user-no-force/Bar.pcm %t/modules-to-compare/Bar-after-user-no-force.pcm
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: echo "-fsyntax-only -fmodules -fmodules-cache-path=%/t/module-cache" > %t/ctx.rsp
+// RUN: echo "-fbuild-session-file=%/t/module-cache/session.timestamp"      >> %t/ctx.rsp
+// RUN: echo "-fmodules-validate-once-per-build-session"                    >> %t/ctx.rsp
+// RUN: echo "-Rmodule-build -Rmodule-validation"                           >> %t/ctx.rsp
 
-// RUN: diff %t/modules-to-compare/Foo-before.pcm %t/modules-to-compare/Foo-after.pcm
-// RUN: diff %t/modules-to-compare/Bar-before.pcm %t/modules-to-compare/Bar-after.pcm
-// RUN: diff %t/modules-to-compare/Foo-before-user.pcm %t/modules-to-compare/Foo-after-user.pcm
-// RUN: diff %t/modules-to-compare/Bar-before-user.pcm %t/modules-to-compare/Bar-after-user.pcm
-// RUN: diff %t/modules-to-compare/Foo-before-user-no-force.pcm %t/modules-to-compare/Foo-after-user-no-force.pcm
-// RUN: diff %t/modules-to-compare/Bar-before-user-no-force.pcm %t/modules-to-compare/Bar-after-user-no-force.pcm
+//--- include/foo.h
+//--- include/module.modulemap
+module Foo { header "foo.h" }
 
-// ===
-// Change the sources.
+//--- clean.c
+// Clean module cache. Modules will get compiled regardless of validation settings.
+// RUN: mkdir %t/module-cache
 // RUN: sleep 1
-// RUN: echo 'void meow2(void);' > %t/Inputs/foo.h
-// RUN: echo 'module Bar { header "bar.h" export * }' > %t/Inputs/bar.modulemap
+// RUN: touch %t/module-cache/session.timestamp
+// RUN: sleep 1
+// RUN: %clang @%t/ctx.rsp %t/clean.c -DCTX=1 \
+// RUN:   -isystem %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/clean.c
+// RUN: %clang @%t/ctx.rsp %t/clean.c -DCTX=2 \
+// RUN:   -I %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/clean.c
+// RUN: %clang @%t/ctx.rsp %t/clean.c -DCTX=3 \
+// RUN:   -I %t/include -fmodules-validate-system-headers -Xclang -fno-modules-force-validate-user-headers \
+// RUN:     2>&1 | FileCheck %t/clean.c
+#include "foo.h"
+// CHECK: building module 'Foo'
 
-// ===
-// Use the module, and make sure that we did not recompile it if foo.h or
-// module.modulemap are system files or user files with force validation disabled,
-// even though the sources changed.
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user-no-force -fsyntax-only -I %t/Inputs -fno-modules-force-validate-user-headers -fmodules-validate-system-headers -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
-// RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user | grep Bar.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user-no-force | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user-no-force | grep Bar.pcm.timestamp
-// RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-after.pcm
-// RUN: cp %t/modules-cache/Bar.pcm %t/modules-to-compare/Bar-after.pcm
-// RUN: cp %t/modules-cache-user/Foo.pcm %t/modules-to-compare/Foo-after-user.pcm
-// RUN: cp %t/modules-cache-user/Bar.pcm %t/modules-to-compare/Bar-after-user.pcm
-// RUN: cp %t/modules-cache-user-no-force/Foo.pcm %t/modules-to-compare/Foo-after-user-no-force.pcm
-// RUN: cp %t/modules-cache-user-no-force/Bar.pcm %t/modules-to-compare/Bar-after-user-no-force.pcm
+//--- no-change-same-session.c
+// Populated module cache in the same build session with unchanged inputs.
+// Validation only happens when it's forced for user headers. No compiles.
+// RUN: sleep 1
+// RUN: %clang @%t/ctx.rsp %t/no-change-same-session.c -DCTX=1 \
+// RUN:   -isystem %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-same-session.c --check-prefix=CHECK-NO-VALIDATION-OR-BUILD --allow-empty
+// RUN: %clang @%t/ctx.rsp %t/no-change-same-session.c -DCTX=2 \
+// RUN:   -I %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-same-session.c  --check-prefix=CHECK-VALIDATION-ONLY
+// RUN: %clang @%t/ctx.rsp %t/no-change-same-session.c -DCTX=3 \
+// RUN:   -I %t/include -fmodules-validate-system-headers -Xclang -fno-modules-force-validate-user-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-same-session.c --check-prefix=CHECK-NO-VALIDATION-OR-BUILD --allow-empty
+#include "foo.h"
+// CHECK-NO-VALIDATION-OR-BUILD-NOT: validating {{[0-9]+}} input files in module 'Foo'
+// CHECK-NO-VALIDATION-OR-BUILD-NOT: building module 'Foo'
+// CHECK-VALIDATION-ONLY: validating {{[0-9]+}} input files in module 'Foo'
+// CHECK-VALIDATION-ONLY-NOT: building module 'Foo'
 
-// RUN: diff %t/modules-to-compare/Foo-before.pcm %t/modules-to-compare/Foo-after.pcm
-// RUN: diff %t/modules-to-compare/Bar-before.pcm %t/modules-to-compare/Bar-after.pcm
-// When foo.h is an user header, we will validate it by default.
-// RUN: not diff %t/modules-to-compare/Foo-before-user.pcm %t/modules-to-compare/Foo-after-user.pcm
-// RUN: not diff %t/modules-to-compare/Bar-before-user.pcm %t/modules-to-compare/Bar-after-user.pcm
-// When foo.h is an user header, we will not validate it if force validation is turned off.
-// RUN: diff %t/modules-to-compare/Foo-before-user-no-force.pcm %t/modules-to-compare/Foo-after-user-no-force.pcm
-// RUN: diff %t/modules-to-compare/Bar-before-user-no-force.pcm %t/modules-to-compare/Bar-after-user-no-force.pcm
+//--- change-same-session.c
+// Populated module cache in the same build session with changed inputs.
+// Validation only happens when it's forced for user headers and results in compilation.
+// RUN: sleep 1
+// RUN: touch %t/include/foo.h
+// RUN: sleep 1
+// RUN: %clang @%t/ctx.rsp %t/change-same-session.c -DCTX=1 \
+// RUN:   -isystem %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/change-same-session.c --check-prefix=CHECK-NO-VALIDATION-OR-BUILD --allow-empty
+// RUN: %clang @%t/ctx.rsp %t/change-same-session.c -DCTX=2 \
+// RUN:   -I %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/change-same-session.c --check-prefix=CHECK-VALIDATION-AND-BUILD
+// RUN: %clang @%t/ctx.rsp %t/change-same-session.c -DCTX=3 \
+// RUN:   -I %t/include -fmodules-validate-system-headers -Xclang -fno-modules-force-validate-user-headers \
+// RUN:     2>&1 | FileCheck %t/change-same-session.c --check-prefix=CHECK-NO-VALIDATION-OR-BUILD --allow-empty
+#include "foo.h"
+// CHECK-NO-VALIDATION-OR-BUILD-NOT: validating {{[0-9]+}} input files in module 'Foo'
+// CHECK-NO-VALIDATION-OR-BUILD-NOT: building module 'Foo'
+// CHECK-VALIDATION-AND-BUILD: validating {{[0-9]+}} input files in module 'Foo'
+// CHECK-VALIDATION-AND-BUILD: building module 'Foo'
 
-// ===
-// Recompile the module if the today's date is before 01 January 2100.
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=4102441200 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fmodules-validate-system-headers -fbuild-session-timestamp=4102441200 -fmodules-validate-once-per-build-session %s
-// RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user-no-force -fsyntax-only -I %t/Inputs -fno-modules-force-validate-user-headers -fmodules-validate-system-headers -fbuild-session-timestamp=4102441200 -fmodules-validate-once-per-build-session %s
-// RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user | grep Bar.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user-no-force | grep Foo.pcm.timestamp
-// RUN: ls -R %t/modules-cache-user-no-force | grep Bar.pcm.timestamp
-// RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-after.pcm
-// RUN: cp %t/modules-cache/Bar.pcm %t/modules-to-compare/Bar-after.pcm
-// RUN: cp %t/modules-cache-user/Foo.pcm %t/modules-to-compare/Foo-after-user.pcm
-// RUN: cp %t/modules-cache-user/Bar.pcm %t/modules-to-compare/Bar-after-user.pcm
-// RUN: cp %t/modules-cache-user-no-force/Foo.pcm %t/modules-to-compare/Foo-after-user-no-force.pcm
-// RUN: cp %t/modules-cache-user-no-force/Bar.pcm %t/modules-to-compare/Bar-after-user-no-force.pcm
+//--- change-new-session.c
+// Populated module cache in a new build session with changed inputs.
+// All configurations validate and recompile.
+// RUN: sleep 1
+// RUN: touch %t/include/foo.h
+// RUN: sleep 1
+// RUN: touch %t/module-cache/session.timestamp
+// RUN: sleep 1
+// RUN: %clang @%t/ctx.rsp %t/change-new-session.c -DCTX=1 \
+// RUN:   -isystem %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/change-new-session.c --check-prefixes=CHECK,CHECK-VALIDATE-ONCE
+// NOTE: Forced user headers validation causes redundant validation of the just-built module.
+// RUN: %clang @%t/ctx.rsp %t/change-new-session.c -DCTX=2 \
+// RUN:   -I %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/change-new-session.c --check-prefixes=CHECK,CHECK-FORCE-VALIDATE-TWICE
+// RUN: %clang @%t/ctx.rsp %t/change-new-session.c -DCTX=3 \
+// RUN:   -I %t/include -fmodules-validate-system-headers -Xclang -fno-modules-force-validate-user-headers \
+// RUN:     2>&1 | FileCheck %t/change-new-session.c --check-prefixes=CHECK,CHECK-VALIDATE-ONCE
+#include "foo.h"
+// CHECK: validating {{[0-9]+}} input files in module 'Foo'
+// CHECK: building module 'Foo'
+// CHECK-VALIDATE-ONCE-NOT: validating {{[0-9]+}} input files in module 'Foo'
+// CHECK-FORCE-VALIDATE-TWICE: validating {{[0-9]+}} input files in module 'Foo'
 
-// RUN: not diff %t/modules-to-compare/Foo-before.pcm %t/modules-to-compare/Foo-after.pcm
-// RUN: not diff %t/modules-to-compare/Bar-before.pcm %t/modules-to-compare/Bar-after.pcm
-// RUN: not diff %t/modules-to-compare/Foo-before-user.pcm %t/modules-to-compare/Foo-after-user.pcm
-// RUN: not diff %t/modules-to-compare/Bar-before-user.pcm %t/modules-to-compare/Bar-after-user.pcm
-// RUN: not diff %t/modules-to-compare/Foo-before-user-no-force.pcm %t/modules-to-compare/Foo-after-user-no-force.pcm
-// RUN: not diff %t/modules-to-compare/Bar-before-user-no-force.pcm %t/modules-to-compare/Bar-after-user-no-force.pcm
+//--- no-change-new-session-twice.c
+// Populated module cache in a new build session with unchanged inputs.
+// At first, all configurations validate but don't recompile.
+// RUN: sleep 1
+// RUN: touch %t/module-cache/session.timestamp
+// RUN: sleep 1
+// RUN: %clang @%t/ctx.rsp %t/no-change-new-session-twice.c -DCTX=1 \
+// RUN:   -isystem %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-new-session-twice.c --check-prefix=CHECK-ONCE
+// RUN: %clang @%t/ctx.rsp %t/no-change-new-session-twice.c -DCTX=2 \
+// RUN:   -I %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-new-session-twice.c --check-prefix=CHECK-ONCE
+// RUN: %clang @%t/ctx.rsp %t/no-change-new-session-twice.c -DCTX=3 \
+// RUN:   -I %t/include -fmodules-validate-system-headers -Xclang -fno-modules-force-validate-user-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-new-session-twice.c --check-prefix=CHECK-ONCE
+//
+// Then, only the forced user header validation performs redundant validation (but no compilation).
+// All other configurations do not validate and do not compile.
+// RUN: sleep 1
+// RUN: %clang @%t/ctx.rsp %t/no-change-new-session-twice.c -DCTX=1 \
+// RUN:   -isystem %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-new-session-twice.c --check-prefix=CHECK-NOT-TWICE --allow-empty
+// NOTE: Forced user headers validation causes redundant validation of the just-validated module.
+// RUN: %clang @%t/ctx.rsp %t/no-change-new-session-twice.c -DCTX=2 \
+// RUN:   -I %t/include -fmodules-validate-system-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-new-session-twice.c --check-prefix=CHECK-ONCE
+// RUN: %clang @%t/ctx.rsp %t/no-change-new-session-twice.c -DCTX=3 \
+// RUN:   -I %t/include -fmodules-validate-system-headers -Xclang -fno-modules-force-validate-user-headers \
+// RUN:     2>&1 | FileCheck %t/no-change-new-session-twice.c --check-prefix=CHECK-NOT-TWICE --allow-empty
+#include "foo.h"
+// CHECK-ONCE: validating {{[0-9]+}} input files in module 'Foo'
+// CHECK-ONCE-NOT: building module 'Foo'
+// CHECK-NOT-TWICE-NOT: validating {{[0-9]+}} input files in module 'Foo'
+// CHECK-NOT-TWICE-NOT: building module 'Foo'