Instantiate contextual this parameters if needed

Author: sandersn

src/compiler/checker.ts

@@ -4632,6 +4632,9 @@ namespace ts {
 
         function getThisTypeOfSignature(signature: Signature): Type | undefined {
             if (signature.thisParameter) {
+                if (signature.mapper) {
+                    signature = instantiateSignature(signature, signature.mapper);
+                }
                 return getTypeOfSymbol(signature.thisParameter);
             }
         }
@@ -11770,6 +11773,10 @@ namespace ts {
 
         function assignContextualParameterTypes(signature: Signature, context: Signature, mapper: TypeMapper) {
             const len = signature.parameters.length - (signature.hasRestParameter ? 1 : 0);
+            if (context.thisParameter) {
+                // save the mapper in case we need to type `this` later
+                context.mapper = mapper;
+            }
             for (let i = 0; i < len; i++) {
                 const parameter = signature.parameters[i];
                 const contextualParameterType = getTypeAtPosition(context, i);