Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -1820,6 +1820,9 @@
 
 void Clang::AddWebAssemblyTargetArgs(const ArgList &Args,
                                      ArgStringList &CmdArgs) const {
+  // WebAssembly doesn't fully support code without prototypes.
+  CmdArgs.push_back("-Werror=strict-prototypes");
+
   // Default to "hidden" visibility.
   if (!Args.hasArg(options::OPT_fvisibility_EQ,
                    options::OPT_fvisibility_ms_compat)) {
Index: test/Driver/wasm-toolchain.c
===================================================================
--- test/Driver/wasm-toolchain.c
+++ test/Driver/wasm-toolchain.c
@@ -1,8 +1,8 @@
 // A basic clang -cc1 command-line. WebAssembly is somewhat special in
-// enabling -fvisibility=hidden by default.
+// enabling -fvisibility=hidden and -Werror-strict-prototypes by default.
 
 // RUN: %clang %s -### -no-canonical-prefixes -target wasm32-unknown-unknown 2>&1 | FileCheck -check-prefix=CC1 %s
-// CC1: clang{{.*}} "-cc1" "-triple" "wasm32-unknown-unknown" {{.*}} "-fvisibility" "hidden" {{.*}}
+// CC1: clang{{.*}} "-cc1" "-triple" "wasm32-unknown-unknown" {{.*}} "-Werror=strict-prototypes" "-fvisibility" "hidden" {{.*}}
 
 // Ditto, but ensure that a user -fvisibility=default disables the default
 // -fvisibility=hidden.
@@ -10,6 +10,12 @@
 // RUN: %clang %s -### -target wasm32-unknown-unknown -fvisibility=default 2>&1 | FileCheck -check-prefix=FVISIBILITY_DEFAULT %s
 // FVISIBILITY_DEFAULT-NOT: hidden
 
+// Ensure that a user -Wno-strict-prototypes disabled the default
+// -Werror=strict-prototypes.
+
+// RUN: %clang %s -### -target wasm32-unknown-unknown -Wno-strict-prototypes 2>&1 | FileCheck -check-prefix=WNO_STRICT_PROTOTYPES %s
+// WNO_STRICT_PROTOTYPES: "-Werror=strict-prototypes" {{.*}} "-Wno-strict-prototypes"
+
 // A basic C link command-line.
 
 // RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s 2>&1 | FileCheck -check-prefix=LINK %s