diff --git a/include/swift/AST/IRGenOptions.h b/include/swift/AST/IRGenOptions.h
index 2f11a629ef2..8c53ec0f49c 100644
--- a/include/swift/AST/IRGenOptions.h
+++ b/include/swift/AST/IRGenOptions.h
@@ -86,6 +86,9 @@ public:
   /// \brief Whether we're generating IR for the JIT.
   unsigned UseJIT : 1;
   
+  /// \brief Whether we allow dynamic value type layout.
+  unsigned EnableDynamicValueTypeLayout : 1;
+
   /// \brief Whether we should run LLVM optimizations after IRGen.
   unsigned DisableLLVMOptzns : 1;
 
@@ -119,8 +122,9 @@ public:
 
   IRGenOptions() : OutputKind(IRGenOutputKind::LLVMAssembly), Verify(true),
                    Optimize(false), DebugInfoKind(IRGenDebugInfoKind::None),
-                   UseJIT(false), DisableLLVMOptzns(false),
-                   DisableLLVMARCOpts(false), DisableLLVMSLPVectorizer(false),
+                   UseJIT(false), EnableDynamicValueTypeLayout(false),
+                   DisableLLVMOptzns(false), DisableLLVMARCOpts(false),
+                   DisableLLVMSLPVectorizer(false),
                    DisableFPElim(true), HasUnderlyingModule(false),
                    Playground(false), GenerateProfile(false),
                    EmbedMode(IRGenEmbedMode::None) {}
diff --git a/include/swift/Option/FrontendOptions.td b/include/swift/Option/FrontendOptions.td
index 1ba6b02b8af..d5d0287b04b 100644
--- a/include/swift/Option/FrontendOptions.td
+++ b/include/swift/Option/FrontendOptions.td
@@ -174,6 +174,10 @@ def dump_clang_diagnostics : Flag<["-"], "dump-clang-diagnostics">,
 def emit_verbose_sil : Flag<["-"], "emit-verbose-sil">,
   HelpText<"Emit locations during SIL emission">;
 
+def enable_dynamic_value_type_layout :
+  Flag<["-"], "enable-dynamic-value-type-layout">,
+  HelpText<"Enable experimental dynamic generic struct/enum/class type layout">;
+
 def enable_experimental_patterns : Flag<["-"], "enable-experimental-patterns">,
   HelpText<"Enable experimental 'switch' pattern matching features">;
 
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 734aaa1351c..d3b05138c4a 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1028,6 +1028,8 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_disable_llvm_verify))
     Opts.Verify = false;
 
+  Opts.EnableDynamicValueTypeLayout |=
+    Args.hasArg(OPT_enable_dynamic_value_type_layout);
   Opts.HasUnderlyingModule |= Args.hasArg(OPT_import_underlying_module);
 
   if (Args.hasArg(OPT_autolink_force_load))
diff --git a/lib/IRGen/GenClass.cpp b/lib/IRGen/GenClass.cpp
index f5d47ae9ba6..4c6b3084000 100644
--- a/lib/IRGen/GenClass.cpp
+++ b/lib/IRGen/GenClass.cpp
@@ -367,9 +367,23 @@ namespace {
 
     void addDirectFieldsFromClass(ClassDecl *theClass,
                                   SILType classType) {
+      bool complainedAboutUnimplementedLayout = false;
       for (VarDecl *var : theClass->getStoredProperties()) {
         SILType type = classType.getFieldType(var, *IGM.SILMod);
         auto &eltType = IGM.getTypeInfo(type);
+
+        // FIXME: Type-parameter-dependent field layout isn't fully
+        // implemented yet for ObjC-derived classes.
+        if (theClass->isObjC()
+            && !eltType.isFixedSize()
+            && !IGM.Opts.EnableDynamicValueTypeLayout) {
+          if (!complainedAboutUnimplementedLayout) {
+            IGM.unimplemented(var->getLoc(),
+                              "non-fixed class layout in ObjC-derived classes");
+            complainedAboutUnimplementedLayout = true;
+          }
+        }
+        
         Elements.push_back(ElementLayout::getIncomplete(eltType));
         AllStoredProperties.push_back(var);
       }
diff --git a/test/IRGen/generic_classes_objc.sil b/test/IRGen/generic_classes_objc.sil
index 1233f158ae3..b023d753bba 100644
--- a/test/IRGen/generic_classes_objc.sil
+++ b/test/IRGen/generic_classes_objc.sil
@@ -1,6 +1,6 @@
 // RUN: rm -rf %t && mkdir %t
 // RUN: %build-irgen-test-overlays
-// RUN: %target-swift-frontend(mock-sdk: -sdk %S/Inputs -I %t) %s -emit-ir | FileCheck %s
+// RUN: %target-swift-frontend(mock-sdk: -sdk %S/Inputs -I %t) -enable-dynamic-value-type-layout %s -emit-ir | FileCheck %s
 
 // REQUIRES: CPU=i386_or_x86_64
 // REQUIRES: objc_interop
diff --git a/test/IRGen/unimplemented_objc_generic_class.swift b/test/IRGen/unimplemented_objc_generic_class.swift
new file mode 100644
index 00000000000..933b065d6f7
--- /dev/null
+++ b/test/IRGen/unimplemented_objc_generic_class.swift
@@ -0,0 +1,11 @@
+// RUN: %target-swift-frontend %s -emit-ir -verify -disable-objc-attr-requires-foundation-module
+
+// REQUIRES: objc_interop
+
+@objc class ObjCBox<T> {
+  var x: T // expected-error{{unimplemented}}
+
+  init(x: T) {
+    self.x = x
+  }
+}
diff --git a/test/Interpreter/generic_objc_subclass.swift b/test/Interpreter/generic_objc_subclass.swift
index 43433e0db81..e8159af02e9 100644
--- a/test/Interpreter/generic_objc_subclass.swift
+++ b/test/Interpreter/generic_objc_subclass.swift
@@ -2,7 +2,7 @@
 // RUN: mkdir -p %t
 //
 // RUN: %target-clang -fobjc-arc %S/Inputs/ObjCClasses/ObjCClasses.m -c -o %t/ObjCClasses.o
-// RUN: %target-build-swift -I %S/Inputs/ObjCClasses/ -Xlinker %t/ObjCClasses.o %s -o %t/a.out
+// RUN: %target-build-swift -Xfrontend -enable-dynamic-value-type-layout -I %S/Inputs/ObjCClasses/ -Xlinker %t/ObjCClasses.o %s -o %t/a.out
 // RUN: %target-run %t/a.out | FileCheck %s
 
 // XFAIL: linux
diff --git a/validation-test/compiler_crashers_fixed/0054-swift-substitutedtype-get.swift b/validation-test/compiler_crashers/0054-swift-substitutedtype-get.swift
similarity index 84%
rename from validation-test/compiler_crashers_fixed/0054-swift-substitutedtype-get.swift
rename to validation-test/compiler_crashers/0054-swift-substitutedtype-get.swift
index 6abc8505a71..2ccaebb92ca 100644
--- a/validation-test/compiler_crashers_fixed/0054-swift-substitutedtype-get.swift
+++ b/validation-test/compiler_crashers/0054-swift-substitutedtype-get.swift
@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend %s -emit-ir
+// RUN: not --crash %target-swift-frontend %s -emit-ir
 
 // REQUIRES: objc_interop