diff --git a/src/main/kotlin/ts2kt/TypeScriptToKotlinBase.kt b/src/main/kotlin/ts2kt/TypeScriptToKotlinBase.kt
index 164a2d4..9a3788b 100644
--- a/src/main/kotlin/ts2kt/TypeScriptToKotlinBase.kt
+++ b/src/main/kotlin/ts2kt/TypeScriptToKotlinBase.kt
@@ -1,5 +1,6 @@
 package ts2kt
 
+import converter.replaceTypeParameters
 import ts2kt.kotlin.ast.*
 import typescriptServices.ts.ImportEqualsDeclaration
 import typescriptServices.ts.Symbol
@@ -14,8 +15,13 @@ abstract class TypeScriptToKotlinBase(
     open val defaultAnnotations: List<KtAnnotation> = listOf()
 
     open fun addVariable(symbol: Symbol?, name: String, type: KtType, extendsType: KtType? = null, typeParams: List<KtTypeParam>? = null, isVar: Boolean = true, needsNoImpl: Boolean = true, additionalAnnotations: List<KtAnnotation> = listOf(), isOverride: Boolean = false) {
+        val (typeParamsToKeep, typeParamsToReplace) = (typeParams ?: emptyList()).partition {
+            typeParam -> extendsType?.typeArgs?.any { it.qualifiedName.name == typeParam.name } ?: false
+        }
+        val substitution = typeParamsToReplace.map { it.name.value to (it.upperBound ?: KtType(ANY)).copy(comment = it.name.value) }.toMap()
+
         val annotations = defaultAnnotations + additionalAnnotations
-        addDeclaration(symbol, KtVariable(KtName(name), KtTypeAnnotation(type), extendsType?.let { KtHeritageType(it) }, annotations, typeParams, isVar = isVar, needsNoImpl = needsNoImpl, isInInterface = isInterface, isOverride = isOverride, hasOpenModifier = hasMembersOpenModifier))
+        addDeclaration(symbol, KtVariable(KtName(name), KtTypeAnnotation(type.replaceTypeParameters(substitution)), extendsType?.let { KtHeritageType(it) }, annotations, typeParamsToKeep, isVar = isVar, needsNoImpl = needsNoImpl, isInInterface = isInterface, isOverride = isOverride, hasOpenModifier = hasMembersOpenModifier))
     }
 
     open fun addFunction(symbol: Symbol?, name: String, callSignature: KtCallSignature, extendsType: KtType? = null, needsNoImpl: Boolean = true, additionalAnnotations: List<KtAnnotation> = listOf(), isOverride: Boolean = false, isOperator: Boolean = false) {
diff --git a/testData/interface/generics/genericOptionalMethods.d.kt b/testData/interface/generics/genericOptionalMethods.d.kt
index 1d05f56..7b9ea91 100644
--- a/testData/interface/generics/genericOptionalMethods.d.kt
+++ b/testData/interface/generics/genericOptionalMethods.d.kt
@@ -2,6 +2,7 @@ package genericOptionalMethods
 
 external interface Foo<T> {
     val methodWithOutArgs: (() -> Unit)? get() = definedExternally
-    val <A> methodWithString: ((s: A) -> T)? get() = definedExternally
-    val <A : T, B> methodWithManyArgs: ((n: A, settings: Bar) -> B)? get() = definedExternally
+    val methodWithString: ((s: Any /* A */) -> T)? get() = definedExternally
+    val methodWithManyArgs: ((n: T /* A */, settings: Bar) -> Any /* B */)? get() = definedExternally
+    val methodWithInOutArg: ((n: T /* A */) -> T /* A */)? get() = definedExternally
 }
diff --git a/testData/interface/generics/genericOptionalMethods.d.ts b/testData/interface/generics/genericOptionalMethods.d.ts
index 8b0da2c..8f69c54 100644
--- a/testData/interface/generics/genericOptionalMethods.d.ts
+++ b/testData/interface/generics/genericOptionalMethods.d.ts
@@ -2,4 +2,5 @@ interface Foo<T> {
     methodWithOutArgs?();
     methodWithString?<A>(s: A): T;
     methodWithManyArgs?<A extends T, B>(n: A, settings: Bar): B;
+    methodWithInOutArg?<A extends T>(n: A): A;
 }