diff --git a/tslang/include/TypeScript/MLIRLogic/MLIRTypeHelper.h b/tslang/include/TypeScript/MLIRLogic/MLIRTypeHelper.h
index aa59fce11..a06688a7f 100644
--- a/tslang/include/TypeScript/MLIRLogic/MLIRTypeHelper.h
+++ b/tslang/include/TypeScript/MLIRLogic/MLIRTypeHelper.h
@@ -2894,24 +2894,36 @@ class MLIRTypeHelper
 
                             if (auto genericClass = getGenericClassInfoByFullName(srcClassInfo->originClassType.getName().getValue()))
                             {
+                                // every type argument has to extend, not only the first:
+                                // Pair<number, number> is not a Pair<number, string>
+                                auto result = ExtendsResult::True;
                                 for (auto &typeParam : genericClass->typeParams)
                                 {
                                     auto name = typeParam->getName();
                                     auto srcFound = srcClassInfo->typeParamsWithArgs.find(name);
                                     auto extFound = extClassInfo->typeParamsWithArgs.find(name);
-                                    if (srcFound != srcClassInfo->typeParamsWithArgs.end() && 
-                                        extFound != extClassInfo->typeParamsWithArgs.end())
+                                    if (srcFound == srcClassInfo->typeParamsWithArgs.end() ||
+                                        extFound == extClassInfo->typeParamsWithArgs.end())
                                     {
-                                        auto srcType = srcFound->getValue().second;
-                                        auto extType = extFound->getValue().second;
+                                        return ExtendsResult::False;
+                                    }
 
-                                        return extendsType(location, srcType, extType, typeParamsWithArgs);
+                                    auto srcType = srcFound->getValue().second;
+                                    auto extType = extFound->getValue().second;
+
+                                    auto argResult = extendsType(location, srcType, extType, typeParamsWithArgs);
+                                    if (!isTrue(argResult))
+                                    {
+                                        return argResult;
                                     }
-                                    else
+
+                                    if (argResult == ExtendsResult::Any)
                                     {
-                                        return ExtendsResult::False;
+                                        result = argResult;
                                     }
                                 }
+
+                                return result;
                             }
 
                             // default behavior - false, because something is different
diff --git a/tslang/lib/TypeScript/MLIRGenCast.cpp b/tslang/lib/TypeScript/MLIRGenCast.cpp
index 7375426e9..aeb601759 100644
--- a/tslang/lib/TypeScript/MLIRGenCast.cpp
+++ b/tslang/lib/TypeScript/MLIRGenCast.cpp
@@ -1515,6 +1515,30 @@ namespace mlirgen
             }
         }
 
+        // Two specializations of one generic class keep fields of different types in the same
+        // place: `const b: Box<string> = new Box<number>(1)` read a number as a string and crashed.
+        // One's type arguments have to extend the other's - Box<Derived> as Box<Base>, or back as
+        // a downcast - which is TypeScript's rule for a type assertion as well.
+        if (auto classType = dyn_cast<mlir_ts::ClassType>(type))
+        {
+            if (auto valueClassType = dyn_cast<mlir_ts::ClassType>(valueType))
+            {
+                auto classInfo = getClassInfoByFullName(classType.getName().getValue());
+                auto valueClassInfo = getClassInfoByFullName(valueClassType.getName().getValue());
+                if (classInfo && valueClassInfo && classInfo->originClassType
+                    && classInfo->originClassType == valueClassInfo->originClassType)
+                {
+                    llvm::StringMap<std::pair<ts::TypeParameterDOM::TypePtr,mlir::Type>> typeParamsWithArgs;
+                    if (!isTrue(mth.extendsType(location, valueType, type, typeParamsWithArgs))
+                        && !isTrue(mth.extendsType(location, type, valueType, typeParamsWithArgs)))
+                    {
+                        emitError(location, "type ") << to_print(valueType) << " is not assignable to type " << to_print(type);
+                        return mlir::failure();
+                    }
+                }
+            }
+        }
+
         if (isa<mlir_ts::ClassType>(type) || isa<mlir_ts::InterfaceType>(type))
         {
             if (isa<mlir_ts::NumberType>(valueType)
diff --git a/tslang/test/tester/CMakeLists.txt b/tslang/test/tester/CMakeLists.txt
index c23b97214..30492ad5b 100644
--- a/tslang/test/tester/CMakeLists.txt
+++ b/tslang/test/tester/CMakeLists.txt
@@ -387,6 +387,7 @@ tslang_add_test(NAME test-compile-01-sizeof COMMAND test-runner "${PROJECT_SOURC
 tslang_add_test(NAME test-compile-02-sizeof COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/02sizeof.ts")
 tslang_add_test(NAME test-compile-00-new-delete COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00new_delete.ts")
 tslang_add_test(NAME test-compile-00-new-generic-from-receiver COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00new_generic_from_receiver.ts")
+tslang_add_test(NAME test-compile-00-generic-specialization-cast COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00generic_specialization_cast.ts")
 tslang_add_test(NAME test-compile-00-owned-locals COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00owned_locals.ts")
 # The JIT variants are the ones that matter here: this shape was a JIT-only miscompile until
 # the catchable-type size was fixed (see the file header). Kept in all three models, since the
@@ -838,6 +839,7 @@ tslang_add_test(NAME test-jit-01-sizeof COMMAND test-runner -jit "${PROJECT_SOUR
 tslang_add_test(NAME test-jit-02-sizeof COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/02sizeof.ts")
 tslang_add_test(NAME test-jit-00-new-delete COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00new_delete.ts")
 tslang_add_test(NAME test-jit-00-new-generic-from-receiver COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00new_generic_from_receiver.ts")
+tslang_add_test(NAME test-jit-00-generic-specialization-cast COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00generic_specialization_cast.ts")
 tslang_add_test(NAME test-jit-00-owned-locals COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00owned_locals.ts")
 tslang_add_test(NAME test-jit-00-throw-in-catch COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00throw_in_catch.ts")
 tslang_add_test(NAME test-jit-00-throw-inlined COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00throw_inlined.ts")
@@ -1711,6 +1713,7 @@ set(TSLANG_CORPUS
     00try_mismatch_in_catch_clause_using.ts
     00new_delete.ts
     00new_generic_from_receiver.ts
+    00generic_specialization_cast.ts
     00ns_bug.ts
     00ns.ts
     00ns2.ts
@@ -2501,6 +2504,24 @@ set_tests_properties(test-compile-declare-overloads-error
                      PROPERTIES PASS_REGULAR_EXPRESSION "'abs' is declared again with another signature"
                                 FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed|redefinition of symbol")
 
+# A cast between two specializations of one generic class whose type arguments don't extend each
+# other is an error - it compiled, and then read one field type as another.
+add_test(NAME test-compile-specialization-cast-box-error
+         COMMAND $<TARGET_FILE:tslang> --emit=obj --no-default-lib -mm=none
+                 "${PROJECT_SOURCE_DIR}/test/tester/specialization-cast/box.ts"
+                 -o "${CMAKE_CURRENT_BINARY_DIR}/specialization-cast-box.obj")
+set_tests_properties(test-compile-specialization-cast-box-error
+                     PROPERTIES PASS_REGULAR_EXPRESSION "Box<!ts.number> is not assignable to type Box<!ts.string>"
+                                FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed")
+
+add_test(NAME test-compile-specialization-cast-pair-error
+         COMMAND $<TARGET_FILE:tslang> --emit=obj --no-default-lib -mm=none
+                 "${PROJECT_SOURCE_DIR}/test/tester/specialization-cast/pair.ts"
+                 -o "${CMAKE_CURRENT_BINARY_DIR}/specialization-cast-pair.obj")
+set_tests_properties(test-compile-specialization-cast-pair-error
+                     PROPERTIES PASS_REGULAR_EXPRESSION "is not assignable to type Pair<!ts.number,!ts.string>"
+                                FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed")
+
 # The shared-component tier under the other two models. A shared library records the model
 # it was built under, so both halves of a pair are built with the same flag - which is what
 # these run. The file pairs are the default model's, verbatim.
diff --git a/tslang/test/tester/specialization-cast/box.ts b/tslang/test/tester/specialization-cast/box.ts
new file mode 100644
index 000000000..39d88f972
--- /dev/null
+++ b/tslang/test/tester/specialization-cast/box.ts
@@ -0,0 +1,10 @@
+// Box<number> and Box<string> keep a different type in `v`; reading one as the other crashed.
+class Box<T> {
+    constructor(public v: T) {}
+}
+
+function main() {
+    const a = new Box<number>(1);
+    const b: Box<string> = a;
+    print(b.v);
+}
diff --git a/tslang/test/tester/specialization-cast/pair.ts b/tslang/test/tester/specialization-cast/pair.ts
new file mode 100644
index 000000000..e3fdf19b5
--- /dev/null
+++ b/tslang/test/tester/specialization-cast/pair.ts
@@ -0,0 +1,10 @@
+// Every type argument has to match, not only the first.
+class Pair<A, B> {
+    constructor(public a: A, public b: B) {}
+}
+
+function main() {
+    const p = new Pair<number, number>(1, 2);
+    const q: Pair<number, string> = p;
+    print(q.b);
+}
diff --git a/tslang/test/tester/tests/00generic_specialization_cast.ts b/tslang/test/tester/tests/00generic_specialization_cast.ts
new file mode 100644
index 000000000..61809f2ac
--- /dev/null
+++ b/tslang/test/tester/tests/00generic_specialization_cast.ts
@@ -0,0 +1,34 @@
+// A cast between two specializations of one generic class is allowed when one's type arguments
+// extend the other's: up (Box<Dog> as Box<Animal>) and back down again.
+class Animal {
+    name = "animal";
+}
+
+class Dog extends Animal {
+    bark() {
+        return 1;
+    }
+}
+
+class Box<T> {
+    constructor(public v: T) {}
+}
+
+class Pair<A, B> {
+    constructor(public a: A, public b: B) {}
+}
+
+function main() {
+    const d = new Box<Dog>(new Dog());
+    const up: Box<Animal> = d;
+    assert(up.v.name == "animal", "up");
+
+    const down = <Box<Dog>>up;
+    assert(down.v.bark() == 1, "down");
+
+    const p = new Pair<Dog, number>(new Dog(), 2);
+    const q: Pair<Animal, number> = p;
+    assert(q.b == 2, "pair");
+
+    print("done.");
+}