Add getTypeArguments method to TypeChecker interface

Author: ahejlsberg

src/compiler/checker.ts

@@ -382,6 +382,7 @@ namespace ts {
             getReturnTypeOfSignature,
             getNullableType,
             getNonNullableType,
+            getTypeArguments,
             typeToTypeNode: nodeBuilder.typeToTypeNode,
             indexInfoToIndexSignatureDeclaration: nodeBuilder.indexInfoToIndexSignatureDeclaration,
             signatureToSignatureDeclaration: nodeBuilder.signatureToSignatureDeclaration,

src/compiler/types.ts

@@ -3191,6 +3191,7 @@ namespace ts {
         /* @internal */ getParameterType(signature: Signature, parameterIndex: number): Type;
         getNullableType(type: Type, flags: TypeFlags): Type;
         getNonNullableType(type: Type): Type;
+        getTypeArguments(type: TypeReference): readonly Type[];
 
         // TODO: GH#18217 `xToDeclaration` calls are frequently asserted as defined.
         /** Note that the resulting nodes cannot be checked. */

src/services/codefixes/inferFromUsage.ts

@@ -993,8 +993,8 @@ namespace ts.codefix {
             }
             else if (getObjectFlags(genericType) & ObjectFlags.Reference && getObjectFlags(usageType) & ObjectFlags.Reference) {
                 // this is wrong because we need a reference to the targetType to, so we can check that it's also a reference
-                const genericArgs = (genericType as TypeReference).typeArguments;
-                const usageArgs = (usageType as TypeReference).typeArguments;
+                const genericArgs = checker.getTypeArguments(genericType as TypeReference);
+                const usageArgs = checker.getTypeArguments(usageType as TypeReference);
                 const types = [];
                 if (genericArgs && usageArgs) {
                     for (let i = 0; i < genericArgs.length; i++) {