Support completions for string literal in rest parameter

src/compiler/checker.ts

@@ -84,6 +84,7 @@ namespace ts {
             getIndexTypeOfType,
             getBaseTypes,
             getTypeFromTypeNode,
+            getParameterType: getTypeAtPosition,
             getReturnTypeOfSignature,
             getNonNullableType,
             getSymbolsInScope,

src/compiler/types.ts

@@ -2341,6 +2341,11 @@ namespace ts {
         getIndexTypeOfType(type: Type, kind: IndexKind): Type;
         getBaseTypes(type: InterfaceType): ObjectType[];
         getReturnTypeOfSignature(signature: Signature): Type;
+        /**
+         * Gets the type of a parameter at a given position in a signature.
+         * Returns `any` if the index is not valid.
+         */
+        /* @internal */ getParameterType(signature: Signature, parameterIndex: number): Type;
         getNonNullableType(type: Type): Type;
 
         getSymbolsInScope(location: Node, meaning: SymbolFlags): Symbol[];

src/services/completions.ts

@@ -204,10 +204,7 @@ namespace ts.Completions {
             typeChecker.getResolvedSignature(argumentInfo.invocation, candidates);
 
             for (const candidate of candidates) {
-                if (candidate.parameters.length > argumentInfo.argumentIndex) {
-                    const parameter = candidate.parameters[argumentInfo.argumentIndex];
-                    addStringLiteralCompletionsFromType(typeChecker.getTypeAtLocation(parameter.valueDeclaration), entries);
-                }
+                addStringLiteralCompletionsFromType(typeChecker.getParameterType(candidate, argumentInfo.argumentIndex), entries);
             }
 
             if (entries.length) {

tests/cases/fourslash/completionForStringLiteral7.ts

@@ -0,0 +1,10 @@
+/// <reference path='fourslash.ts'/>
+
+////type T = "foo" | "bar";
+////type U = "oof" | "rab";
+////function f(x: T, ...args: U[]) { };
+////f("/*1*/", "/*2*/", "/*3*/");
+
+verify.completionsAt("1", ["foo", "bar"]);
+verify.completionsAt("2", ["oof", "rab"]);
+verify.completionsAt("3", ["oof", "rab"]);