Kotlin Library Compatibility Issues and Proposed Fixes

[dangrima90]

I have been going through a library and was creating typings for it's various different .aar files. I noticed some issues that have already been highlighted here: #83 (@NathanWalker) and #75 (@farfromrefug).

Since I had a build issue with the generated typings I set about investigating the issues. I am proficient in TypeScript, however I'm no expert in terms of Kotlin which meant I could see what the issues were in the typing files, but wasn't sure how to solve them. Therefore, I investigated the issues via Claude Code and the solutions that were generated have solved my issues. The following description was also generated by Claude Code, I did so to make sure that everything that was found is listed accurately:

Summary

While generating TypeScript definitions for Kotlin-based Android libraries (specifically MiSnap SDK v5.8.1), three critical issues were discovered that prevent proper type generation. This document outlines each issue, provides examples, and proposes fixes.

Environment

• DTS Generator Version: 4.0.0
• Test Library: MiSnap SDK v5.8.1 (Kotlin-based Android library)
• Kotlin Version: 1.8.20 (as seen in generated code)
• Java Target: 17

---

Issue 1: Kotlin Serializer Classes Generate Invalid TypeScript

Problem

Kotlin's kotlinx.serialization compiler generates classes with double dollar signs ($$serializer) in their bytecode names. The DTS generator processes these, creating invalid TypeScript with empty module declarations.

Example from Bytecode

com/miteksystems/misnap/core/Barcode$$serializer.class
com/miteksystems/misnap/core/MiSnapSettings$$serializer.class

Generated TypeScript (Invalid)

export module  {  // ❌ Empty module name!
    export module Barcode {
        export class serializer extends kotlinx.serialization.internal.GeneratedSerializer<com.miteksystems.misnap.core.Barcode> {
            public static class: java.lang.Class<com.miteksystems.misnap.core.Barcode..serializer>;  // ❌ Double dots
            public static INSTANCE: com.miteksystems.misnap.core.Barcode..serializer;  // ❌ Double dots
        }
    }
}

Root Cause

In DtsApi.java line 541, the code replaces $ with .:

String[] currParts = currClass.getClassName().replace('$', '.').split("\\.");

For Barcode$$serializer:

1. Replace $ → .: Barcode..serializer
2. Split on .: ["Barcode", "", "serializer"]
3. Empty string in array creates module { (invalid!)

Impact

• 63 empty module declarations in test file
• 126 invalid double-dot type references
• TypeScript compilation fails

Proposed Fix

Add $$serializer to the filtered class list in DtsApi.java around line 143:

if (currentFileClassname.startsWith("java.util.function") ||
        currentFileClassname.startsWith("android.support.v4.media.routing.MediaRouterJellybeanMr1") ||
        currentFileClassname.startsWith("android.support.v4.media.routing.MediaRouterJellybeanMr2") ||
        currentFileClassname.contains(".debugger.") ||
        currentFileClassname.endsWith("package-info") ||
        currentFileClassname.endsWith("module-info") ||
        currentFileClassname.endsWith("Kt") ||
        currentFileClassname.contains("$$serializer")) {  // ← NEW LINE
    continue;
}

Why This Fix is Safe

• $$serializer classes are Kotlin compiler internals for serialization
• They're never directly used by application code
• Filtering them is similar to filtering other compiler-generated classes (already done for Kt suffix)

---

Issue 2: Kotlin Synthetic Parameter Names Are Invalid TypeScript

Problem

Kotlin compiler generates synthetic setter methods with the parameter name <set-?> in the LocalVariableTable. The DTS generator reads this directly, creating invalid TypeScript identifiers.

Example from Bytecode (javap output)

public final void setInitialDelay(java.lang.Integer);
  LocalVariableTable:
    Start  Length  Slot  Name   Signature
        0       6     0  this   Lcom/miteksystems/misnap/core/MiSnapSettings$Analysis;
        0       6     1  <set-?>  Ljava/lang/Integer;

Generated TypeScript (Invalid)

public setInitialDelay(<set-?>: java.lang.Integer): void;  // ❌ Invalid identifier
public setEnableAiBasedRts(<set-?>: java.lang.Boolean): void;  // ❌ Invalid identifier
public setMotionDetectorSensitivity(<set-?>: com.miteksystems.misnap.core.MiSnapSettings.Analysis.MotionDetectorSensitivity): void;  // ❌ Invalid identifier

Root Cause

In DtsApi.java lines 962-969, the code reads parameter names from bytecode but doesn't sanitize Kotlin synthetic names:

if (localVariable != null) {
    String name = localVariable.getName();
    // No sanitization for <set-?> or other synthetic names!
    if (reservedJsKeywords.contains(name)) {
        sb.append(name + "_");
    } else {
        sb.append(name);  // Writes <set-?> directly
    }
}

Impact

• 68 occurrences in test file
• All Kotlin property setter methods affected
• TypeScript parser fails on angle bracket identifiers

Proposed Fix

Sanitize synthetic parameter names by deriving meaningful names from method names:

if (localVariable != null) {
    String name = localVariable.getName();

    // Sanitize Kotlin synthetic parameter names like <set-?>
    if (name.startsWith("<") && name.endsWith(">")) {
        // For setter methods, derive parameter name from method name
        String methodName = m.getName();
        if (methodName.startsWith("set") && methodName.length() > 3) {
            // setInitialDelay -> initialDelay
            name = Character.toLowerCase(methodName.charAt(3)) + methodName.substring(4);
        } else {
            // Fallback to "value" for other synthetic names
            name = "value";
        }
    }

    if (reservedJsKeywords.contains(name)) {
        System.out.println(String.format("Appending _ to reserved JS keyword %s", name));
        sb.append(name + "_");
    } else {
        sb.append(name);
    }
}

Result After Fix

public setInitialDelay(initialDelay: java.lang.Integer): void;  // ✅ Valid and semantic!
public setEnableAiBasedRts(enableAiBasedRts: java.lang.Boolean): void;  // ✅ Valid and semantic!
public setMotionDetectorSensitivity(motionDetectorSensitivity: com.miteksystems.misnap.core.MiSnapSettings.Analysis.MotionDetectorSensitivity): void;  // ✅ Valid and semantic!

Why This Fix is Safe

• Only affects synthetic Kotlin parameter names (those with < and >)
• Generates semantically meaningful names matching the property being set
• Follows common naming conventions (same pattern as Kotlin itself uses in Kotlin files)

---

Issue 3: Kotlin Type Generics Create Invalid any<...> Syntax

Problem

Kotlin standard library types (kotlin.jvm.functions.*, kotlin.properties.*) are in the "ignored namespaces" list and get replaced with any. However, the regex replacement preserves generic type parameters, creating invalid TypeScript syntax (any is not a generic type).

Example from Bytecode

public final class FragmentViewBindingDelegate<T extends ViewBinding>
    implements kotlin.properties.ReadOnlyProperty<androidx.fragment.app.Fragment, T>

Generated TypeScript (Invalid)

export class FragmentViewBindingDelegate<T> extends any<androidx.fragment.app.Fragment,any> {  // ❌ any is not generic!
    public getViewBindingFactory(): any<globalAndroid.view.View,any>;  // ❌ Invalid syntax
    public constructor(fragment: androidx.fragment.app.Fragment, viewBindingFactory: any<any,any>);  // ❌ Invalid syntax
}

Root Cause

In DtsApi.java line 248, the regex pattern doesn't include commas and spaces in the character class for matching generic parameters:

String regexFormat = "(?<Replace>%s(?:(?:\\.[a-zA-Z\\d]*)|<[a-zA-Z\\d\\.<>]*>)*)(?<Suffix>[^a-zA-Z\\d]+)";
//                                                           ^^^^^^^^^^^^^^ Missing comma and space!

When the regex encounters kotlin.properties.ReadOnlyProperty<Fragment, T>:

1. Matches up to: kotlin.properties.ReadOnlyProperty
2. Encounters < which is [^a-zA-Z\d], so it becomes the Suffix
3. Replacement: any< + remaining text
4. Result: any<Fragment, T> (invalid!)

Impact

• ~30 occurrences across various Kotlin function and property types
• All Kotlin lambda and property delegate types affected
• TypeScript compilation fails

Proposed Fix

Update the regex to include commas, spaces, and match end-of-string:

private String replaceIgnoredNamespaces(String content) {
    // Updated regex to properly capture generics with commas, spaces, and nested angle brackets
    // Also matches end of string or line as valid suffix
    String regexFormat = "(?<Replace>%s(?:(?:\\.[a-zA-Z\\d]*)|<[a-zA-Z\\d\\.<>, ]*>)*)(?<Suffix>[^a-zA-Z\\d]+|$)";
    //                                                                     ^^^^^ Added comma and space
    //                                                                                        ^^^ Added end-of-string

    for (String ignoredNamespace : this.getIgnoredNamespaces()) {
        String regexString = String.format(regexFormat, ignoredNamespace.replace(".", "\\."));
        content = content.replaceAll(regexString, "any$2");
        regexString = String.format(regexFormat, getGlobalAliasedClassName(ignoredNamespace).replace(".", "\\."));
        content = content.replaceAll(regexString, "any$2");
    }

    // replace "extends any" with "extends java.lang.Object"
    content = content.replace(" extends any ", String.format(" extends %s ", DtsApi.JavaLangObject));

    return content;
}

Result After Fix

export class FragmentViewBindingDelegate<T> extends java.lang.Object {  // ✅ Valid TypeScript!
    public getViewBindingFactory(): any;  // ✅ Valid TypeScript!
    public constructor(fragment: androidx.fragment.app.Fragment, viewBindingFactory: any);  // ✅ Valid TypeScript!
}

Why This Fix is Safe

• Only affects types in the ignored namespaces list (already being replaced with any)
• Properly strips all generic parameters when replacing, creating valid TypeScript
• The regex already existed; this just makes it work correctly for types with commas in generics

---

Testing Results

Test Environment

• Library: MiSnap SDK v5.8.1 (11 modules)
• Project: NativeScript mobile app
• TypeScript Version: As configured by NativeScript

Before Fixes

• ❌ Empty module declarations: 63
• ❌ Double-dot references: 126
• ❌ Invalid <set-?> parameters: 68
• ❌ Invalid any<...> syntax: ~30
• ❌ Total TypeScript errors: 287+

After Fixes

• ✅ Empty module declarations: 0
• ✅ Double-dot references: 0
• ✅ Invalid parameters: 0
• ✅ Invalid any<...> syntax: 0
• ✅ All type definitions compile successfully
• ✅ NativeScript project runs without issues

Generated File Sizes (Example: misnap-core.d.ts)

• Before: 3,789 lines with errors
• After: 3,096 lines (693 lines of invalid code removed)
• Reduction: 18.3% smaller, 100% valid

---

Compatibility

These fixes specifically target Kotlin-generated bytecode patterns that are:

• Standard Kotlin compiler output (not library-specific)
• Present in any modern Kotlin Android library
• Not handled by the current generator logic

The fixes are backward compatible - they only affect Kotlin libraries and don't change behavior for Java-only libraries.

---

Proposed Changes Summary

File: dts-generator/src/main/java/com/telerik/dts/DtsApi.java

Change 1 (around line 143):

// Add to filtered classes list
currentFileClassname.contains("$$serializer")

Change 2 (lines 965-976, in getMethodParamSignature method):

// Sanitize Kotlin synthetic parameter names
if (name.startsWith("<") && name.endsWith(">")) {
    String methodName = m.getName();
    if (methodName.startsWith("set") && methodName.length() > 3) {
        name = Character.toLowerCase(methodName.charAt(3)) + methodName.substring(4);
    } else {
        name = "value";
    }
}

Change 3 (line 249, in replaceIgnoredNamespaces method):

// Update regex pattern
String regexFormat = "(?<Replace>%s(?:(?:\\.[a-zA-Z\\d]*)|<[a-zA-Z\\d\\.<>, ]*>)*)(?<Suffix>[^a-zA-Z\\d]+|$)";

---

References

• Kotlin Serialization Docs: https://kotlinlang.org/docs/serialization.html
• Kotlin Bytecode Conventions: https://kotlinlang.org/spec/
• Related Issue: Similar to the 2016 fix for interface static initializers (commit 26a0870)

---

Tested With:

• MiSnap SDK v5.8.1 (Mitek Systems)
• NativeScript 8.x
• Multiple Kotlin-based AndroidX libraries

Let me know if I should commit the changes so that they can be tested :)

[farfromrefug]

@dangrima90 all that seems pretty awesome. pretty impressed by Claude there.
you should definitely create a PR and we would test it.

[dangrima90]

@farfromrefug updates committed. Feel free to make edits or let me know about any potential changes that might be needed. Thank you.