diff --git a/mlir/include/mlir/IR/BuiltinOps.td b/mlir/include/mlir/IR/BuiltinOps.td
--- a/mlir/include/mlir/IR/BuiltinOps.td
+++ b/mlir/include/mlir/IR/BuiltinOps.td
@@ -263,4 +263,10 @@
   let hasFolder = 1;
 }
 
+def PlaceholderOp : Builtin_Op<"placeholder"> {
+  let summary = "This op is internal to the parser and shouldn't exist "
+    "otherwise";
+}
+
+
 #endif // BUILTIN_OPS
diff --git a/mlir/lib/IR/Operation.cpp b/mlir/lib/IR/Operation.cpp
--- a/mlir/lib/IR/Operation.cpp
+++ b/mlir/lib/IR/Operation.cpp
@@ -176,6 +176,14 @@
       numRegions(numRegions), hasOperandStorage(hasOperandStorage), name(name),
       attrs(attributes) {
   assert(attributes && "unexpected null attribute dictionary");
+#ifndef NDEBUG
+  if (!getDialect() && !getContext()->allowsUnregisteredDialects())
+    llvm::report_fatal_error(
+        name.getStringRef() +
+        " created with unregistered dialect. If this is intended, please call "
+        "allowUnregisteredDialects() on the MLIRContext, or use "
+        "-allow-unregistered-dialect with mlir-opt");
+#endif
 }
 
 // Operations are deleted through the destroy() member because they are
diff --git a/mlir/test/CAPI/ir.c b/mlir/test/CAPI/ir.c
--- a/mlir/test/CAPI/ir.c
+++ b/mlir/test/CAPI/ir.c
@@ -479,6 +479,7 @@
 /// block/operation-relative API and their final order is checked.
 static void buildWithInsertionsAndPrint(MlirContext ctx) {
   MlirLocation loc = mlirLocationUnknownGet(ctx);
+  mlirContextSetAllowUnregisteredDialects(ctx, true);
 
   MlirRegion owningRegion = mlirRegionCreate();
   MlirBlock nullBlock = mlirRegionGetFirstBlock(owningRegion);
@@ -542,6 +543,7 @@
 
   mlirOperationDump(op);
   mlirOperationDestroy(op);
+  mlirContextSetAllowUnregisteredDialects(ctx, false);
   // clang-format off
   // CHECK-LABEL:  "insertion.order.test"
   // CHECK:      ^{{.*}}(%{{.*}}: i1
@@ -1561,6 +1563,8 @@
   // CHECK-LABEL: @testOperands
 
   MlirContext ctx = mlirContextCreate();
+  mlirRegisterAllDialects(ctx);
+  mlirContextGetOrLoadDialect(ctx, mlirStringRefCreateFromCString("test"));
   MlirLocation loc = mlirLocationUnknownGet(ctx);
   MlirType indexType = mlirIndexTypeGet(ctx);
 
@@ -1590,6 +1594,7 @@
   MlirValue constOneValue = mlirOperationGetResult(constOne, 0);
 
   // Create the operation under test.
+  mlirContextSetAllowUnregisteredDialects(ctx, true);
   MlirOperationState opState =
       mlirOperationStateGet(mlirStringRefCreateFromCString("dummy.op"), loc);
   MlirValue initialOperands[] = {constZeroValue};
@@ -1604,13 +1609,13 @@
   MlirValue opOperand = mlirOperationGetOperand(op, 0);
   fprintf(stderr, "Original operand: ");
   mlirValuePrint(opOperand, printToStderr, NULL);
-  // CHECK: Original operand: {{.+}} {value = 0 : index}
+  // CHECK: Original operand: {{.+}} constant 0 : index
 
   mlirOperationSetOperand(op, 0, constOneValue);
   opOperand = mlirOperationGetOperand(op, 0);
   fprintf(stderr, "Updated operand: ");
   mlirValuePrint(opOperand, printToStderr, NULL);
-  // CHECK: Updated operand: {{.+}} {value = 1 : index}
+  // CHECK: Updated operand: {{.+}} constant 1 : index
 
   mlirOperationDestroy(op);
   mlirOperationDestroy(constZero);
@@ -1626,6 +1631,8 @@
   // CHECK-LABEL: @testClone
 
   MlirContext ctx = mlirContextCreate();
+  mlirRegisterAllDialects(ctx);
+  mlirContextGetOrLoadDialect(ctx, mlirStringRefCreateFromCString("std"));
   MlirLocation loc = mlirLocationUnknownGet(ctx);
   MlirType indexType = mlirIndexTypeGet(ctx);
   MlirStringRef valueStringRef =  mlirStringRefCreateFromCString("value");
@@ -1646,8 +1653,8 @@
 
   mlirOperationPrint(constZero, printToStderr, NULL);
   mlirOperationPrint(constOne, printToStderr, NULL);
-  // CHECK: %0 = "std.constant"() {value = 0 : index} : () -> index
-  // CHECK: %0 = "std.constant"() {value = 1 : index} : () -> index
+  // CHECK: constant 0 : index
+  // CHECK: constant 1 : index
 
   return 0;
 }
diff --git a/mlir/test/Dialect/PDL/invalid.mlir b/mlir/test/Dialect/PDL/invalid.mlir
--- a/mlir/test/Dialect/PDL/invalid.mlir
+++ b/mlir/test/Dialect/PDL/invalid.mlir
@@ -167,7 +167,7 @@
 // expected-error@below {{expected only `pdl` operations within the pattern body}}
 pdl.pattern : benefit(1) {
   // expected-note@below {{see non-`pdl` operation defined here}}
-  "foo.other_op"() : () -> ()
+  "test.foo.other_op"() : () -> ()
 
   %root = pdl.operation "foo.op"
   pdl.rewrite %root with "foo"
diff --git a/mlir/test/IR/invalid-module-op.mlir b/mlir/test/IR/invalid-module-op.mlir
--- a/mlir/test/IR/invalid-module-op.mlir
+++ b/mlir/test/IR/invalid-module-op.mlir
@@ -6,9 +6,9 @@
   // expected-error@+1 {{Operations with a 'SymbolTable' must have exactly one block}}
   module {
   ^bb1:
-    "module_terminator"() : () -> ()
+    "test.dummy"() : () -> ()
   ^bb2:
-    "module_terminator"() : () -> ()
+    "test.dummy"() : () -> ()
   }
   return
 }
diff --git a/mlir/test/IR/invalid-unregistered.mlir b/mlir/test/IR/invalid-unregistered.mlir
--- a/mlir/test/IR/invalid-unregistered.mlir
+++ b/mlir/test/IR/invalid-unregistered.mlir
@@ -1,5 +1,7 @@
 // RUN: mlir-opt %s -split-input-file -verify-diagnostics
 
+// REQUIRES: noasserts
+
 // expected-error @below {{op created with unregistered dialect}}
 "unregistered_dialect.op"() : () -> ()
 
diff --git a/mlir/test/lit.cfg.py b/mlir/test/lit.cfg.py
--- a/mlir/test/lit.cfg.py
+++ b/mlir/test/lit.cfg.py
@@ -111,3 +111,8 @@
         # lib/Bindings/Python/CMakeLists.txt for where this is set up.
         os.path.join(config.llvm_obj_root, 'python'),
     ], append_path=True)
+
+if config.enable_assertions:
+    config.available_features.add('asserts')
+else:
+    config.available_features.add('noasserts')
diff --git a/mlir/unittests/Pass/PassManagerTest.cpp b/mlir/unittests/Pass/PassManagerTest.cpp
--- a/mlir/unittests/Pass/PassManagerTest.cpp
+++ b/mlir/unittests/Pass/PassManagerTest.cpp
@@ -91,6 +91,7 @@
 
 TEST(PassManagerTest, InvalidPass) {
   MLIRContext context;
+  context.allowUnregisteredDialects();
 
   // Create a module
   OwningModuleRef module(ModuleOp::create(UnknownLoc::get(&context)));