Fix signature cloning to ensure target and mapper are propagated

Author: ahejlsberg

src/compiler/checker.ts

@@ -6778,8 +6778,19 @@ namespace ts {
         }
 
         function cloneSignature(sig: Signature): Signature {
-            return createSignature(sig.declaration, sig.typeParameters, sig.thisParameter, sig.parameters, /*resolvedReturnType*/ undefined,
+            const result = createSignature(sig.declaration, sig.typeParameters, sig.thisParameter, sig.parameters, /*resolvedReturnType*/ undefined,
                 /*resolvedTypePredicate*/ undefined, sig.minArgumentCount, sig.hasRestParameter, sig.hasLiteralTypes);
+            result.target = sig.target;
+            result.mapper = sig.mapper;
+            return result;
+        }
+
+        function createUnionSignature(signature: Signature, unionSignatures: Signature[]) {
+            const result = cloneSignature(signature);
+            result.unionSignatures = unionSignatures;
+            result.target = undefined;
+            result.mapper = undefined;
+            return result;
         }
 
         function getExpandedParameters(sig: Signature): ReadonlyArray<Symbol> {
@@ -6889,9 +6900,8 @@ namespace ts {
                                     const thisType = getUnionType(map(unionSignatures, sig => sig.thisParameter ? getTypeOfSymbol(sig.thisParameter) : anyType), UnionReduction.Subtype);
                                     thisParameter = createSymbolWithType(signature.thisParameter!, thisType);
                                 }
-                                s = cloneSignature(signature);
+                                s = createUnionSignature(signature, unionSignatures);
                                 s.thisParameter = thisParameter;
-                                s.unionSignatures = unionSignatures;
                             }
                             (result || (result = [])).push(s);
                         }
@@ -8399,8 +8409,6 @@ namespace ts {
                 if (returnSignature) {
                     const newReturnSignature = cloneSignature(returnSignature);
                     newReturnSignature.typeParameters = inferredTypeParameters;
-                    newReturnSignature.target = returnSignature.target;
-                    newReturnSignature.mapper = returnSignature.mapper;
                     const newInstantiatedSignature = cloneSignature(instantiatedSignature);
                     newInstantiatedSignature.resolvedReturnType = getOrCreateTypeFromSignature(newReturnSignature);
                     return newInstantiatedSignature;
@@ -18345,14 +18353,8 @@ namespace ts {
                     }
                 }
             }
-
             // Result is union of signatures collected (return type is union of return types of this signature set)
-            let result: Signature | undefined;
-            if (signatureList) {
-                result = cloneSignature(signatureList[0]);
-                result.unionSignatures = signatureList;
-            }
-            return result;
+            return signatureList && createUnionSignature(signatureList[0], signatureList);
         }
 
         function checkSpreadExpression(node: SpreadElement, checkMode?: CheckMode): Type {