diff --git a/tslang/lib/TypeScript/MLIRGenCast.cpp b/tslang/lib/TypeScript/MLIRGenCast.cpp
index 2c6713628..0ae05704b 100644
--- a/tslang/lib/TypeScript/MLIRGenCast.cpp
+++ b/tslang/lib/TypeScript/MLIRGenCast.cpp
@@ -1603,6 +1603,18 @@ namespace mlirgen
             {
                 return castNullableToString(location, type, value, baseType, genContext);
             }
+
+            // A union of literals, as `c ? 1 : 0` is (`0 | 1`), is stored as its literals' type: cast
+            // it to that type first. Cast straight to another type (`index`, a tuple field's type),
+            // it reached the lowering as a union, which sign-extended the union itself.
+            if (!toAny)
+            {
+                if (auto literalBaseType = getLiteralUnionBaseType(location, valueType); literalBaseType && literalBaseType != type)
+                {
+                    CAST_A(baseValue, location, literalBaseType, value, genContext);
+                    return cast(location, type, baseValue, genContext);
+                }
+            }
         }
 
         // TODO: issue is with casting to Boolean type from union type for example, you need to cast optional type to boolean to check value
diff --git a/tslang/test/tester/CMakeLists.txt b/tslang/test/tester/CMakeLists.txt
index 891d8b56e..ddafdf95d 100644
--- a/tslang/test/tester/CMakeLists.txt
+++ b/tslang/test/tester/CMakeLists.txt
@@ -273,6 +273,7 @@ tslang_add_test(NAME test-compile-00-equals COMMAND test-runner "${PROJECT_SOURC
 tslang_add_test(NAME test-compile-00-bool-arith-ops COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00bool_arith_ops.ts")
 tslang_add_test(NAME test-compile-00-int64-shift-bitwise COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00int64_shift_bitwise.ts")
 tslang_add_test(NAME test-compile-00-literal-union-arith COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00literal_union_arith.ts")
+tslang_add_test(NAME test-compile-00-literal-union-cast COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00literal_union_cast.ts")
 tslang_add_test(NAME test-compile-00-bool-const-array COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00bool_const_array.ts")
 tslang_add_test(NAME test-compile-00-mixed-type-ops COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00mixed_type_ops.ts")
 tslang_add_test(NAME test-compile-00-generator-manual-next COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00generator_manual_next.ts")
@@ -808,6 +809,7 @@ tslang_add_test(NAME test-jit-00-equals COMMAND test-runner -jit "${PROJECT_SOUR
 tslang_add_test(NAME test-jit-00-bool-arith-ops COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00bool_arith_ops.ts")
 tslang_add_test(NAME test-jit-00-int64-shift-bitwise COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00int64_shift_bitwise.ts")
 tslang_add_test(NAME test-jit-00-literal-union-arith COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00literal_union_arith.ts")
+tslang_add_test(NAME test-jit-00-literal-union-cast COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00literal_union_cast.ts")
 tslang_add_test(NAME test-jit-00-bool-const-array COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00bool_const_array.ts")
 tslang_add_test(NAME test-jit-00-mixed-type-ops COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00mixed_type_ops.ts")
 tslang_add_test(NAME test-jit-00-generator-manual-next COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00generator_manual_next.ts")
@@ -1799,6 +1801,7 @@ set(TSLANG_CORPUS
     00bool_arith_ops.ts
     00int64_shift_bitwise.ts
     00literal_union_arith.ts
+    00literal_union_cast.ts
     00bool_const_array.ts
     00break_continue_scope_exit.ts
     00break_continue.ts
diff --git a/tslang/test/tester/tests/00literal_union_cast.ts b/tslang/test/tester/tests/00literal_union_cast.ts
new file mode 100644
index 000000000..93d4ea59c
--- /dev/null
+++ b/tslang/test/tester/tests/00literal_union_cast.ts
@@ -0,0 +1,39 @@
+// regression test: a union of literals, as a ternary of two literals is, converts to another
+// numeric type through its literals' type.
+//
+// bug found and fixed: `c ? 1 : 0` is the union `0 | 1`; cast straight to `index` (a variable, a
+// parameter, a tuple field) it reached the lowering as a union, and `llvm.sext` of the union failed
+// with no source location.
+
+type Vec = [items: [_0: Opaque | null], count: index];
+
+function make(item: Opaque | null): Vec {
+    return [[item], item ? 1 : 0];
+}
+
+function takeIndex(n: index): index {
+    return n;
+}
+
+function main() {
+    let c = true;
+    let off = false;
+
+    let x: index = c ? 1 : 0;
+    assert(x == 1);
+
+    let y: index = off ? 1 : 0;
+    assert(y == 0);
+
+    assert(takeIndex(c ? 7 : 3) == 7);
+
+    let n: number = c ? 1 : 0;
+    assert(n == 1);
+
+    let w: s64 = off ? 2 : 5;
+    assert(w == 5);
+
+    assert(make(null).count == 0);
+
+    print("done.");
+}