diff --git a/clang/test/Driver/modules-cache-path.m b/clang/test/Driver/modules-cache-path.m
--- a/clang/test/Driver/modules-cache-path.m
+++ b/clang/test/Driver/modules-cache-path.m
@@ -1,4 +1,4 @@
-// RUN: %clang -fmodules -### %s 2>&1 | FileCheck %s -check-prefix=CHECK-DEFAULT
+// RUN: env -u CLANG_MODULE_CACHE_PATH %clang -fmodules -### %s 2>&1 | FileCheck %s -check-prefix=CHECK-DEFAULT
 // CHECK-DEFAULT: -fmodules-cache-path={{.*}}clang{{[/\\]+}}ModuleCache
 
 // RUN: env CLANG_MODULE_CACHE_PATH=/dev/null \
diff --git a/clang/test/Modules/driver.c b/clang/test/Modules/driver.c
--- a/clang/test/Modules/driver.c
+++ b/clang/test/Modules/driver.c
@@ -1,4 +1,4 @@
-// RUN: %clang -fmodules -fimplicit-module-maps %s -### 2>&1 | FileCheck -check-prefix CHECK-NO_MODULE_CACHE %s
+// RUN: env -u CLANG_MODULE_CACHE_PATH %clang -fmodules -fimplicit-module-maps %s -### 2>&1 | FileCheck -check-prefix CHECK-NO_MODULE_CACHE %s
 // RUN: %clang -fmodules -fimplicit-module-maps -fmodules-cache-path=blarg %s -### 2>&1 | FileCheck -check-prefix CHECK-WITH_MODULE_CACHE %s
 
 // CHECK-NO_MODULE_CACHE: {{clang.*"-fmodules-cache-path=.*ModuleCache"}}
diff --git a/llvm/utils/lit/lit/llvm/config.py b/llvm/utils/lit/lit/llvm/config.py
--- a/llvm/utils/lit/lit/llvm/config.py
+++ b/llvm/utils/lit/lit/llvm/config.py
@@ -495,6 +495,10 @@
 
         self.clear_environment(possibly_dangerous_env_vars)
 
+        # Make the default module cache path invalid so that tests are forced to
+        # provide a cache path if they use implicit modules.
+        self.with_environment('CLANG_MODULE_CACHE_PATH', '/dev/null')
+
         # Tweak the PATH to include the tools dir and the scripts dir.
         # Put Clang first to avoid LLVM from overriding out-of-tree clang
         # builds.