Provide better services for incomplete generic calls

src/compiler/checker.ts

@@ -69,6 +69,9 @@ namespace ts {
         undefinedSymbol.declarations = [];
         const argumentsSymbol = createSymbol(SymbolFlags.Property, "arguments");
 
+        /** This will be set during calls to `getResolvedSignature` where services determines an apparent number of arguments greater than what is actually provided. */
+        let apparentArgumentCount: number | undefined;
+
         // for public members that accept a Node or one of its subtypes, we must guard against
         // synthetic nodes created during transformations by calling `getParseTreeNode`.
         // for most of these, we perform the guard only on `checker` to avoid any possible
@@ -154,9 +157,12 @@ namespace ts {
                 return node ? getContextualType(node) : undefined;
             },
             getFullyQualifiedName,
-            getResolvedSignature: (node, candidatesOutArray?) => {
+            getResolvedSignature: (node, candidatesOutArray, theArgumentCount) => {
                 node = getParseTreeNode(node, isCallLikeExpression);
-                return node ? getResolvedSignature(node, candidatesOutArray) : undefined;
+                apparentArgumentCount = theArgumentCount;
+                const res = node ? getResolvedSignature(node, candidatesOutArray) : undefined;
+                apparentArgumentCount = undefined;
+                return res;
             },
             getConstantValue: node => {
                 node = getParseTreeNode(node, canHaveConstantValue);
@@ -6306,12 +6312,12 @@ namespace ts {
                     // If a type parameter does not have a default type, or if the default type
                     // is a forward reference, the empty object type is used.
                     for (let i = numTypeArguments; i < numTypeParameters; i++) {
-                        typeArguments[i] = isJavaScript ? anyType : emptyObjectType;
+                        typeArguments[i] = getDefaultTypeArgumentType(isJavaScript);
                     }
                     for (let i = numTypeArguments; i < numTypeParameters; i++) {
                         const mapper = createTypeMapper(typeParameters, typeArguments);
                         const defaultType = getDefaultFromTypeParameter(typeParameters[i]);
-                        typeArguments[i] = defaultType ? instantiateType(defaultType, mapper) : isJavaScript ? anyType : emptyObjectType;
+                        typeArguments[i] = defaultType ? instantiateType(defaultType, mapper) : getDefaultTypeArgumentType(isJavaScript);
                     }
                 }
             }
@@ -7961,15 +7967,16 @@ namespace ts {
             };
         }
 
-        function createTypeMapper(sources: Type[], targets: Type[]): TypeMapper {
+        function createTypeMapper(sources: TypeParameter[], targets: Type[]): TypeMapper {
+            Debug.assert(targets === undefined || sources.length === targets.length);
             const mapper: TypeMapper = sources.length === 1 ? makeUnaryTypeMapper(sources[0], targets ? targets[0] : anyType) :
                 sources.length === 2 ? makeBinaryTypeMapper(sources[0], targets ? targets[0] : anyType, sources[1], targets ? targets[1] : anyType) :
                     makeArrayTypeMapper(sources, targets);
             mapper.mappedTypes = sources;
             return mapper;
         }
 
-        function createTypeEraser(sources: Type[]): TypeMapper {
+        function createTypeEraser(sources: TypeParameter[]): TypeMapper {
             return createTypeMapper(sources, /*targets*/ undefined);
         }
 
@@ -10619,7 +10626,7 @@ namespace ts {
                                 context));
                     }
                     else {
-                        inferredType = context.flags & InferenceFlags.AnyDefault ? anyType : emptyObjectType;
+                        inferredType = getDefaultTypeArgumentType(!!(context.flags & InferenceFlags.AnyDefault));
                     }
                 }
                 inference.inferredType = inferredType;
@@ -10635,6 +10642,10 @@ namespace ts {
             return inferredType;
         }
 
+        function getDefaultTypeArgumentType(isInJavaScriptFile: boolean): Type {
+            return isInJavaScriptFile ? anyType : emptyObjectType;
+        }
+
         function getInferredTypes(context: InferenceContext): Type[] {
             const result: Type[] = [];
             for (let i = 0; i < context.inferences.length; i++) {
@@ -12784,7 +12795,9 @@ namespace ts {
             const args = getEffectiveCallArguments(callTarget);
             const argIndex = indexOf(args, arg);
             if (argIndex >= 0) {
-                const signature = getResolvedOrAnySignature(callTarget);
+                // If we're already in the process of resolving the given signature, don't resolve again as
+                // that could cause infinite recursion. Instead, return anySignature.
+                const signature = getNodeLinks(callTarget).resolvedSignature === resolvingSignature ? resolvingSignature : getResolvedSignature(callTarget);
                 return getTypeAtPosition(signature, argIndex);
             }
             return undefined;
@@ -14948,16 +14961,14 @@ namespace ts {
         }
 
         function inferTypeArguments(node: CallLikeExpression, signature: Signature, args: Expression[], excludeArgument: boolean[], context: InferenceContext): Type[] {
-            const inferences = context.inferences;
-
             // Clear out all the inference results from the last time inferTypeArguments was called on this context
-            for (let i = 0; i < inferences.length; i++) {
+            for (const inference of context.inferences) {
                 // As an optimization, we don't have to clear (and later recompute) inferred types
                 // for type parameters that have already been fixed on the previous call to inferTypeArguments.
                 // It would be just as correct to reset all of them. But then we'd be repeating the same work
                 // for the type parameters that were fixed, namely the work done by getInferredType.
-                if (!inferences[i].isFixed) {
-                    inferences[i].inferredType = undefined;
+                if (!inference.isFixed) {
+                    inference.inferredType = undefined;
                 }
             }
 
@@ -15626,27 +15637,43 @@ namespace ts {
             }
 
             // No signature was applicable. We have already reported the errors for the invalid signature.
-            // If this is a type resolution session, e.g. Language Service, try to get better information that anySignature.
-            // Pick the first candidate that matches the arity. This way we can get a contextual type for cases like:
-            //  declare function f(a: { xa: number; xb: number; });
-            //  f({ |
+            // If this is a type resolution session, e.g. Language Service, try to get better information than anySignature.
+            // Pick the longest signature. This way we can get a contextual type for cases like:
+            //     declare function f(a: { xa: number; xb: number; }, b: number);
+            //     f({ |
+            // Also, use explicitly-supplied type arguments if they are provided, so we can get a contextual signature in cases like:
+            //     declare function f<T>(k: keyof T);
+            //     f<Foo>("
             if (!produceDiagnostics) {
-                for (let candidate of candidates) {
-                    if (hasCorrectArity(node, args, candidate)) {
-                        if (candidate.typeParameters && typeArguments) {
-                            candidate = getSignatureInstantiation(candidate, map(typeArguments, getTypeFromTypeNode));
-                        }
-                        return candidate;
+                Debug.assert(candidates.length > 0); // Else would have exited above.
+                const bestIndex = getLongestCandidateIndex(candidates, apparentArgumentCount === undefined ? args.length : apparentArgumentCount);
+                const candidate = candidates[bestIndex];
+
+                const { typeParameters } = candidate;
+                if (typeParameters && callLikeExpressionMayHaveTypeArguments(node) && node.typeArguments) {
+                    const typeArguments = node.typeArguments.map(getTypeOfNode);
+                    while (typeArguments.length > typeParameters.length) {
+                        typeArguments.pop();
+                    }
+                    while (typeArguments.length < typeParameters.length) {
+                        typeArguments.push(getDefaultTypeArgumentType(isInJavaScriptFile(node)));
                     }
+
+                    const instantiated = createSignatureInstantiation(candidate, typeArguments);
+                    candidates[bestIndex] = instantiated;
+                    return instantiated;
                 }
+
+                return candidate;
             }
 
             return resolveErrorCall(node);
 
             function chooseOverload(candidates: Signature[], relation: Map<RelationComparisonResult>, signatureHelpTrailingComma = false) {
                 candidateForArgumentError = undefined;
                 candidateForTypeArgumentError = undefined;
-                for (const originalCandidate of candidates) {
+                for (let candidateIndex = 0; candidateIndex < candidates.length; candidateIndex++) {
+                    const originalCandidate = candidates[candidateIndex];
                     if (!hasCorrectArity(node, args, originalCandidate, signatureHelpTrailingComma)) {
                         continue;
                     }
@@ -15678,6 +15705,7 @@ namespace ts {
                         }
                         const index = excludeArgument ? indexOf(excludeArgument, /*value*/ true) : -1;
                         if (index < 0) {
+                            candidates[candidateIndex] = candidate;
                             return candidate;
                         }
                         excludeArgument[index] = false;
@@ -15689,6 +15717,24 @@ namespace ts {
 
         }
 
+        function getLongestCandidateIndex(candidates: Signature[], argsCount: number): number {
+            let maxParamsIndex = -1;
+            let maxParams = -1;
+
+            for (let i = 0; i < candidates.length; i++) {
+                const candidate = candidates[i];
+                if (candidate.hasRestParameter || candidate.parameters.length >= argsCount) {
+                    return i;
+                }
+                if (candidate.parameters.length > maxParams) {
+                    maxParams = candidate.parameters.length;
+                    maxParamsIndex = i;
+                }
+            }
+
+            return maxParamsIndex;
+        }
+
         function resolveCallExpression(node: CallExpression, candidatesOutArray: Signature[]): Signature {
             if (node.expression.kind === SyntaxKind.SuperKeyword) {
                 const superType = checkSuperExpression(node.expression);
@@ -16003,9 +16049,7 @@ namespace ts {
 
             const callSignatures = elementType && getSignaturesOfType(elementType, SignatureKind.Call);
             if (callSignatures && callSignatures.length > 0) {
-                let callSignature: Signature;
-                callSignature = resolveCall(openingLikeElement, callSignatures, candidatesOutArray);
-                return callSignature;
+                return resolveCall(openingLikeElement, callSignatures, candidatesOutArray);
             }
 
             return undefined;
@@ -16055,12 +16099,6 @@ namespace ts {
             return result;
         }
 
-        function getResolvedOrAnySignature(node: CallLikeExpression) {
-            // If we're already in the process of resolving the given signature, don't resolve again as
-            // that could cause infinite recursion. Instead, return anySignature.
-            return getNodeLinks(node).resolvedSignature === resolvingSignature ? resolvingSignature : getResolvedSignature(node);
-        }
-
         /**
          * Indicates whether a declaration can be treated as a constructor in a JavaScript
          * file.

src/compiler/types.ts

@@ -2582,7 +2582,11 @@ namespace ts {
         getAugmentedPropertiesOfType(type: Type): Symbol[];
         getRootSymbols(symbol: Symbol): Symbol[];
         getContextualType(node: Expression): Type | undefined;
-        getResolvedSignature(node: CallLikeExpression, candidatesOutArray?: Signature[]): Signature | undefined;
+        /**
+         * returns unknownSignature in the case of an error. Don't know when it returns undefined.
+         * @param argumentCount Apparent number of arguments, passed in case of a possibly incomplete call. This should come from an ArgumentListInfo. See `signatureHelp.ts`.
+         */
+        getResolvedSignature(node: CallLikeExpression, candidatesOutArray?: Signature[], argumentCount?: number): Signature | undefined;
         getSignatureFromDeclaration(declaration: SignatureDeclaration): Signature | undefined;
         isImplementationOfOverload(node: FunctionLikeDeclaration): boolean | undefined;
         isUndefinedSymbol(symbol: Symbol): boolean;
@@ -3377,8 +3381,8 @@ namespace ts {
     /* @internal */
     export interface TypeMapper {
         (t: TypeParameter): Type;
-        mappedTypes?: Type[];       // Types mapped by this mapper
-        instantiations?: Type[];    // Cache of instantiations created using this type mapper.
+        mappedTypes?: TypeParameter[]; // Types mapped by this mapper
+        instantiations?: Type[];       // Cache of instantiations created using this type mapper.
     }
 
     export const enum InferencePriority {

src/compiler/utilities.ts

@@ -4735,6 +4735,11 @@ namespace ts {
 // All node tests in the following list should *not* reference parent pointers so that
 // they may be used with transformations.
 namespace ts {
+    /* @internal */
+    export function isSyntaxList(n: Node): n is SyntaxList {
+        return n.kind === SyntaxKind.SyntaxList;
+    }
+
     /* @internal */
     export function isNode(node: Node) {
         return isNodeKind(node.kind);

src/services/completions.ts

@@ -229,7 +229,7 @@ namespace ts.Completions {
         const entries: CompletionEntry[] = [];
         const uniques = createMap<true>();
 
-        typeChecker.getResolvedSignature(argumentInfo.invocation, candidates);
+        typeChecker.getResolvedSignature(argumentInfo.invocation, candidates, argumentInfo.argumentCount);
 
         for (const candidate of candidates) {
             addStringLiteralCompletionsFromType(typeChecker.getParameterType(candidate, argumentInfo.argumentIndex), entries, typeChecker, uniques);