diff --git a/src/compiler.ts b/src/compiler.ts
index 9d2c3ca6f7..192f0dc8a1 100644
--- a/src/compiler.ts
+++ b/src/compiler.ts
@@ -195,7 +195,7 @@ import {
   writeF32,
   writeF64,
   writeV128,
-  uniqueMap,
+  cloneMap,
   isPowerOf2,
   v128_zero,
   readI32,
@@ -2848,7 +2848,7 @@ export class Compiler extends DiagnosticEmitter {
         type = resolver.resolveType( // reports
           typeNode,
           flow.actualFunction,
-          uniqueMap(flow.contextualTypeArguments)
+          cloneMap(flow.contextualTypeArguments)
         );
         if (!type) continue;
         this.checkTypeSupported(type, typeNode);
@@ -3626,7 +3626,7 @@ export class Compiler extends DiagnosticEmitter {
         let toType = this.resolver.resolveType( // reports
           assert(expression.toType),
           flow.actualFunction,
-          uniqueMap(flow.contextualTypeArguments)
+          cloneMap(flow.contextualTypeArguments)
         );
         if (!toType) return this.module.unreachable();
         return this.compileExpression(expression.expression, toType, inheritedConstraints | Constraints.CONV_EXPLICIT);
@@ -6419,7 +6419,7 @@ export class Compiler extends DiagnosticEmitter {
         assert(typeParameterNodes),
         typeArgumentNodes,
         this.currentFlow.actualFunction.parent,
-        uniqueMap<string,Type>(this.currentFlow.contextualTypeArguments), // don't update
+        cloneMap(this.currentFlow.contextualTypeArguments), // don't update
         expression
       );
     }
@@ -7333,7 +7333,7 @@ export class Compiler extends DiagnosticEmitter {
       DecoratorFlags.NONE
     );
     var instance: Function | null;
-    var contextualTypeArguments = uniqueMap(flow.contextualTypeArguments);
+    var contextualTypeArguments = cloneMap(flow.contextualTypeArguments);
     var module = this.module;
 
     // compile according to context. this differs from a normal function in that omitted parameter
@@ -7715,7 +7715,7 @@ export class Compiler extends DiagnosticEmitter {
         let functionInstance = this.resolver.resolveFunction(
           functionPrototype,
           null,
-          uniqueMap<string,Type>(flow.contextualTypeArguments)
+          cloneMap(flow.contextualTypeArguments)
         );
         if (!functionInstance || !this.compileFunction(functionInstance)) return module.unreachable();
         if (functionInstance.hasDecorator(DecoratorFlags.BUILTIN)) {
@@ -7770,7 +7770,7 @@ export class Compiler extends DiagnosticEmitter {
     var expectedType = this.resolver.resolveType(
       expression.isType,
       flow.actualFunction,
-      uniqueMap(flow.contextualTypeArguments)
+      cloneMap(flow.contextualTypeArguments)
     );
     if (!expectedType) {
       this.currentType = Type.bool;
@@ -8148,7 +8148,7 @@ export class Compiler extends DiagnosticEmitter {
           let instance = this.resolver.resolveFunction(
             <FunctionPrototype>target,
             null,
-            uniqueMap<string,Type>(),
+            new Map(),
             ReportMode.SWALLOW
           );
           if (!instance) break;
@@ -8734,14 +8734,14 @@ export class Compiler extends DiagnosticEmitter {
       classInstance = this.resolver.resolveClass(
         classPrototype,
         classReference.typeArguments,
-        uniqueMap<string,Type>(flow.contextualTypeArguments)
+        cloneMap(flow.contextualTypeArguments)
       );
     } else {
       classInstance = this.resolver.resolveClassInclTypeArguments(
         classPrototype,
         typeArguments,
         flow.actualFunction.parent, // relative to caller
-        uniqueMap<string,Type>(flow.contextualTypeArguments),
+        cloneMap(flow.contextualTypeArguments),
         expression
       );
     }
@@ -8773,7 +8773,7 @@ export class Compiler extends DiagnosticEmitter {
       // clone base constructor if a derived class. note that we cannot just
       // call the base ctor since the derived class may have additional fields.
       let baseClass = classInstance.base;
-      let contextualTypeArguments = uniqueMap(classInstance.contextualTypeArguments);
+      let contextualTypeArguments = cloneMap(classInstance.contextualTypeArguments);
       if (baseClass) {
         let baseCtor = this.ensureConstructor(baseClass, reportNode);
         this.checkFieldInitialization(baseClass, reportNode);
diff --git a/src/flow.ts b/src/flow.ts
index 2ab9e4b4dc..6214f026c4 100644
--- a/src/flow.ts
+++ b/src/flow.ts
@@ -85,7 +85,7 @@ import {
 
 import {
   BitSet,
-  uniqueMap
+  cloneMap
 } from "./util";
 
 import {
@@ -314,7 +314,7 @@ export class Flow {
     branch.localFlags = this.localFlags.slice();
     if (this.actualFunction.is(CommonFlags.CONSTRUCTOR)) {
       let thisFieldFlags = assert(this.thisFieldFlags);
-      branch.thisFieldFlags = uniqueMap<Field,FieldFlags>(thisFieldFlags);
+      branch.thisFieldFlags = cloneMap(thisFieldFlags);
     } else {
       assert(!this.thisFieldFlags);
     }
diff --git a/src/resolver.ts b/src/resolver.ts
index 7c7aac2fa9..46d5d16570 100644
--- a/src/resolver.ts
+++ b/src/resolver.ts
@@ -95,7 +95,7 @@ import {
 } from "./common";
 
 import {
-  uniqueMap,
+  cloneMap,
   isPowerOf2
 } from "./util";
 
@@ -255,7 +255,7 @@ export class Resolver extends DiagnosticEmitter {
           <ClassPrototype>element,
           typeArgumentNodes,
           ctxElement,
-          uniqueMap<string,Type>(ctxTypes), // don't inherit
+          cloneMap(ctxTypes), // don't inherit
           node,
           reportMode
         );
@@ -309,7 +309,7 @@ export class Resolver extends DiagnosticEmitter {
           typeParameterNodes,
           typeArgumentNodes,
           ctxElement,
-          ctxTypes = uniqueMap(ctxTypes), // update
+          ctxTypes = cloneMap(ctxTypes), // update
           node,
           reportMode
         );
@@ -626,7 +626,7 @@ export class Resolver extends DiagnosticEmitter {
     /** Contextual element. */
     ctxElement: Element,
     /** Contextual types, i.e. `T`. Updated in place with the new set of contextual types. */
-    ctxTypes: Map<string,Type> = uniqueMap<string,Type>(),
+    ctxTypes: Map<string,Type> = new Map(),
     /** Alternative report node in case of empty type arguments. */
     alternativeReportNode: Node | null = null,
     /** How to proceed with eventual diagnostics. */
@@ -656,7 +656,7 @@ export class Resolver extends DiagnosticEmitter {
       return null;
     }
     var typeArguments = new Array<Type>(maxParameterCount);
-    var oldCtxTypes = uniqueMap<string,Type>(ctxTypes);
+    var oldCtxTypes = cloneMap(ctxTypes);
     ctxTypes.clear();
     for (let i = 0; i < maxParameterCount; ++i) {
       let type = i < argumentCount
@@ -669,7 +669,7 @@ export class Resolver extends DiagnosticEmitter {
         : this.resolveType( // reports
             assert(typeParameters[i].defaultType),
             ctxElement,
-            uniqueMap<string,Type>(ctxTypes), // don't update
+            cloneMap(ctxTypes), // don't update
             reportMode
           );
       if (!type) return null;
@@ -704,7 +704,7 @@ export class Resolver extends DiagnosticEmitter {
         prototype,
         typeArguments,
         ctxFlow.actualFunction,
-        uniqueMap(ctxFlow.contextualTypeArguments), // don't inherit
+        cloneMap(ctxFlow.contextualTypeArguments), // don't inherit
         node,
         reportMode
       );
@@ -712,7 +712,7 @@ export class Resolver extends DiagnosticEmitter {
 
     // infer generic call if type arguments have been omitted
     if (prototype.is(CommonFlags.GENERIC)) {
-      let contextualTypeArguments = uniqueMap<string,Type>(ctxFlow.contextualTypeArguments);
+      let contextualTypeArguments = cloneMap(ctxFlow.contextualTypeArguments);
 
       // fill up contextual types with auto for each generic component
       let typeParameterNodes = assert(prototype.typeParameterNodes);
@@ -780,13 +780,13 @@ export class Resolver extends DiagnosticEmitter {
       return this.resolveFunction(
         prototype,
         resolvedTypeArguments,
-        uniqueMap<string,Type>(ctxFlow.contextualTypeArguments),
+        cloneMap(ctxFlow.contextualTypeArguments),
         reportMode
       );
     }
 
     // otherwise resolve the non-generic call as usual
-    return this.resolveFunction(prototype, null, uniqueMap<string,Type>(), reportMode);
+    return this.resolveFunction(prototype, null, new Map(), reportMode);
   }
 
   /** Updates contextual types with a possibly encapsulated inferred type. */
@@ -1220,7 +1220,7 @@ export class Resolver extends DiagnosticEmitter {
     var element = this.lookupIdentifierExpression(node, ctxFlow, ctxElement, reportMode);
     if (!element) return null;
     if (element.kind == ElementKind.FUNCTION_PROTOTYPE) {
-      let instance = this.resolveFunction(<FunctionPrototype>element, null, uniqueMap<string,Type>(), reportMode);
+      let instance = this.resolveFunction(<FunctionPrototype>element, null, new Map(), reportMode);
       if (!instance) return null;
       element = instance;
     }
@@ -1357,7 +1357,7 @@ export class Resolver extends DiagnosticEmitter {
           // Inherit from 'Function' if not overridden, i.e. fn.call
           let ownMember = target.getMember(propertyName);
           if (!ownMember) {
-            let functionInstance = this.resolveFunction(<FunctionPrototype>target, null, uniqueMap<string,Type>(), ReportMode.SWALLOW);
+            let functionInstance = this.resolveFunction(<FunctionPrototype>target, null, new Map(), ReportMode.SWALLOW);
             if (functionInstance) {
               let wrapper = functionInstance.type.getClassOrWrapper(this.program);
               if (wrapper) target = wrapper;
@@ -2538,7 +2538,7 @@ export class Resolver extends DiagnosticEmitter {
         <ClassPrototype>element,
         node.typeArguments,
         ctxFlow.actualFunction,
-        uniqueMap<string,Type>(ctxFlow.contextualTypeArguments),
+        cloneMap(ctxFlow.contextualTypeArguments),
         node,
         reportMode
       );
@@ -2625,7 +2625,7 @@ export class Resolver extends DiagnosticEmitter {
     /** Type arguments provided. */
     typeArguments: Type[] | null,
     /** Contextual types, i.e. `T`. */
-    ctxTypes: Map<string,Type> = uniqueMap<string,Type>(),
+    ctxTypes: Map<string,Type> = new Map(),
     /** How to proceed with eventual diagnostics. */
     reportMode: ReportMode = ReportMode.REPORT
   ): Function | null {
@@ -2927,7 +2927,7 @@ export class Resolver extends DiagnosticEmitter {
     /** Type arguments provided. */
     typeArguments: Type[] | null,
     /** Contextual types, i.e. `T`. */
-    ctxTypes: Map<string,Type> = uniqueMap<string,Type>(),
+    ctxTypes: Map<string,Type> = new Map(),
     /** How to proceed with eventual diagnostics. */
     reportMode: ReportMode = ReportMode.REPORT
   ): Class | null {
@@ -2988,7 +2988,7 @@ export class Resolver extends DiagnosticEmitter {
         basePrototype,
         extendsNode.typeArguments,
         prototype.parent, // relative to derived class
-        uniqueMap(ctxTypes), // don't inherit
+        cloneMap(ctxTypes), // don't inherit
         extendsNode,
         reportMode
       );
@@ -3024,7 +3024,7 @@ export class Resolver extends DiagnosticEmitter {
           interfacePrototype,
           implementsNode.typeArguments,
           prototype.parent,
-          uniqueMap(ctxTypes),
+          cloneMap(ctxTypes),
           implementsNode,
           reportMode
         );
@@ -3346,14 +3346,14 @@ export class Resolver extends DiagnosticEmitter {
         operatorInstance = this.resolveFunction(
           boundPrototype,
           null,
-          uniqueMap<string,Type>(),
+          new Map(),
           reportMode
         );
       } else {
         operatorInstance = this.resolveFunction(
           overloadPrototype,
           null,
-          uniqueMap<string,Type>(),
+          new Map(),
           reportMode
         );
       }
@@ -3491,7 +3491,7 @@ export class Resolver extends DiagnosticEmitter {
       let getterInstance = this.resolveFunction(
         getterPrototype,
         null,
-        uniqueMap<string,Type>(),
+        new Map(),
         reportMode
       );
       if (getterInstance) {
@@ -3504,7 +3504,7 @@ export class Resolver extends DiagnosticEmitter {
       let setterInstance = this.resolveFunction(
         setterPrototype,
         null,
-        uniqueMap<string,Type>(),
+        new Map(),
         reportMode
       );
       if (setterInstance) {
diff --git a/src/util/collections.ts b/src/util/collections.ts
index 134ea073b0..1a296b1292 100644
--- a/src/util/collections.ts
+++ b/src/util/collections.ts
@@ -3,26 +3,47 @@
  * @license Apache-2.0
  */
 
-/** Makes a unique map. Typically used to track contextual type arguemnts. */
-export function uniqueMap<K,V>(original: Map<K,V> | null = null, overrides: Map<K,V> | null = null): Map<K,V> {
-  var cloned = new Map<K,V>();
-  if (original) {
-    // TODO: for (let [k, v] of original) {
-    for (let _keys = Map_keys(original), i = 0, k = _keys.length; i < k; ++i) {
-      let k = unchecked(_keys[i]);
-      let v = assert(original.get(k));
-      cloned.set(k, v);
+/** Clone map. Typically used to track contextual type arguments. */
+export function cloneMap<K,V>(map: Map<K,V> | null): Map<K,V> {
+  if (!ASC_TARGET) { // JS
+    // fast path for js target
+    return new Map<K,V>(map);
+  } else {
+    let out = new Map<K,V>();
+    if (map) {
+      // TODO: for (let [k, v] of map) {
+      for (let _keys = Map_keys(map), i = 0, k = _keys.length; i < k; ++i) {
+        let k = unchecked(_keys[i]);
+        let v = assert(map.get(k));
+        out.set(k, v);
+      }
     }
+    return out;
   }
-  if (overrides) {
-    // TODO: for (let [k, v] of overrides) {
-    for (let _keys = Map_keys(overrides), i = 0, k = _keys.length; i < k; ++i) {
+}
+
+/** Merge two maps in into new one. */
+export function mergeMaps<K,V>(map1: Map<K,V>, map2: Map<K,V>): Map<K,V> {
+  if (!ASC_TARGET) { // JS
+    let out = new Map<K,V>(map1);
+    map2.forEach((v, k) => out.set(k, v));
+    return out;
+  } else {
+    let out = new Map<K,V>();
+    // TODO: for (let [k, v] of map1) {
+    for (let _keys = Map_keys(map1), i = 0, k = _keys.length; i < k; ++i) {
+      let k = unchecked(_keys[i]);
+      let v = assert(map1.get(k));
+      out.set(k, v);
+    }
+    // TODO: for (let [k, v] of map2) {
+    for (let _keys = Map_keys(map2), i = 0, k = _keys.length; i < k; ++i) {
       let k = unchecked(_keys[i]);
-      let v = assert(overrides.get(k));
-      cloned.set(k, v);
+      let v = assert(map2.get(k));
+      out.set(k, v);
     }
+    return out;
   }
-  return cloned;
 }
 
 /** BitSet represent growable sequence of N bits. It's faster alternative of Set<i32> when elements