In services, when overload resolution fails, create a union signature

src/compiler/checker.ts

@@ -3200,7 +3200,7 @@ namespace ts {
             if ((symbol as TransientSymbol).syntheticLiteralTypeOrigin) {
                 const stringValue = (symbol as TransientSymbol).syntheticLiteralTypeOrigin.value;
                 if (!isIdentifierText(stringValue, compilerOptions.target)) {
-                    return `"${escapeString(stringValue, CharacterCodes.doubleQuote)}"`;
+                    return '"' + escapeString(stringValue, CharacterCodes.doubleQuote) + '"';
                 }
             }
             return symbolName(symbol);
@@ -5568,9 +5568,9 @@ namespace ts {
         /**
          * Gets the symbolic name for a late-bound member from its type.
          */
-        function getLateBoundNameFromType(type: LiteralType | UniqueESSymbolType) {
+        function getLateBoundNameFromType(type: LiteralType | UniqueESSymbolType): __String | undefined {
             if (type.flags & TypeFlags.UniqueESSymbol) {
-                return `__@${type.symbol.escapedName}@${getSymbolId(type.symbol)}` as __String;
+                return "__@" + type.symbol.escapedName + "@" + getSymbolId(type.symbol) as __String;
             }
             if (type.flags & TypeFlags.StringOrNumberLiteral) {
                 return escapeLeadingUnderscores("" + (<LiteralType>type).value);
@@ -7037,13 +7037,17 @@ namespace ts {
         }
 
         function getRestTypeOfSignature(signature: Signature): Type {
+            return tryGetRestTypeOfSignature(signature) || anyType;
+        }
+
+        function tryGetRestTypeOfSignature(signature: Signature): Type | undefined {
             if (signature.hasRestParameter) {
                 const type = getTypeOfSymbol(lastOrUndefined(signature.parameters));
                 if (getObjectFlags(type) & ObjectFlags.Reference && (<TypeReference>type).target === globalArrayType) {
                     return (<TypeReference>type).typeArguments[0];
                 }
             }
-            return anyType;
+            return undefined;
         }
 
         function getSignatureInstantiation(signature: Signature, typeArguments: Type[], isJavascript: boolean): Signature {
@@ -7772,7 +7776,7 @@ namespace ts {
 
         // Add the given types to the given type set. Order is preserved, duplicates are removed,
         // and nested types of the given kind are flattened into the set.
-        function addTypesToUnion(typeSet: TypeSet, types: Type[]) {
+        function addTypesToUnion(typeSet: TypeSet, types: ReadonlyArray<Type>) {
             for (const type of types) {
                 addTypeToUnion(typeSet, type);
             }
@@ -7851,7 +7855,7 @@ namespace ts {
         // expression constructs such as array literals and the || and ?: operators). Named types can
         // circularly reference themselves and therefore cannot be subtype reduced during their declaration.
         // For example, "type Item = string | (() => Item" is a named type that circularly references itself.
-        function getUnionType(types: Type[], unionReduction: UnionReduction = UnionReduction.Literal, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
+        function getUnionType(types: ReadonlyArray<Type>, unionReduction: UnionReduction = UnionReduction.Literal, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
             if (types.length === 0) {
                 return neverType;
             }
@@ -17060,35 +17064,8 @@ namespace ts {
                 diagnostics.add(createDiagnosticForNode(node, fallbackError));
             }
 
-            // 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 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) {
-                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 getCandidateForOverloadFailure(node, candidates, args, !!candidatesOutArray);
             }
 
             return resolveErrorCall(node);
@@ -17162,7 +17139,103 @@ namespace ts {
             }
         }
 
-        function getLongestCandidateIndex(candidates: Signature[], argsCount: number): number {
+        // 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 than anySignature.
+        function getCandidateForOverloadFailure(
+            node: CallLikeExpression,
+            candidates: Signature[],
+            args: ReadonlyArray<Expression>,
+            hasCandidatesOutArray: boolean,
+        ): Signature {
+            Debug.assert(candidates.length > 0); // Else should not have called this.
+            // Normally we will combine overloads. Skip this if they have type parameters since that's hard to combine.
+            // Don't do this if there is a `candidatesOutArray`,
+            // because then we want the chosen best candidate to be one of the overloads, not a combination.
+            return hasCandidatesOutArray || candidates.length === 1 || candidates.some(c => !!c.typeParameters)
+                ? pickLongestCandidateSignature(node, candidates, args)
+                : createUnionOfSignaturesForOverloadFailure(candidates);
+        }
+
+        function createUnionOfSignaturesForOverloadFailure(candidates: ReadonlyArray<Signature>): Signature {
+            const thisParameters = mapDefined(candidates, c => c.thisParameter);
+            let thisParameter: Symbol | undefined;
+            if (thisParameters.length) {
+                thisParameter = createCombinedSymbolFromTypes(thisParameters, thisParameters.map(getTypeOfParameter));
+            }
+
+            const { min: minArgumentCount, max: maxNonRestParam } = minAndMax(candidates, getNumNonRestParameters);
+            const parameters: ts.Symbol[] = [];
+            for (let i = 0; i < maxNonRestParam; i++) {
+                const symbols = mapDefined(candidates, ({ parameters, hasRestParameter }) => hasRestParameter ?
+                    i < parameters.length - 1 ? parameters[i] : last(parameters) :
+                    i < parameters.length ? parameters[i] : undefined);
+                Debug.assert(symbols.length !== 0);
+                parameters.push(createCombinedSymbolFromTypes(symbols, mapDefined(candidates, candidate => tryGetTypeAtPosition(candidate, i))));
+            }
+
+            const restParameterSymbols = mapDefined(candidates, c => c.hasRestParameter ? last(c.parameters) : undefined);
+            const hasRestParameter = restParameterSymbols.length !== 0;
+            if (hasRestParameter) {
+                const type = createArrayType(getUnionType(mapDefined(candidates, tryGetRestTypeOfSignature), UnionReduction.Subtype));
+                parameters.push(createCombinedSymbolForOverloadFailure(restParameterSymbols, type));
+            }
+
+            return createSignature(
+                candidates[0].declaration,
+                /*typeParameters*/ undefined, // Before calling this we tested for `!candidates.some(c => !!c.typeParameters)`.
+                thisParameter,
+                parameters,
+                /*resolvedReturnType*/ getIntersectionType(candidates.map(getReturnTypeOfSignature)),
+                /*typePredicate*/ undefined,
+                minArgumentCount,
+                hasRestParameter,
+                /*hasLiteralTypes*/ candidates.some(c => c.hasLiteralTypes));
+        }
+
+        function createCombinedSymbolFromTypes(sources: ReadonlyArray<Symbol>, types: ReadonlyArray<Type>): Symbol {
+            return createCombinedSymbolForOverloadFailure(sources, getUnionType(types, UnionReduction.Subtype));
+        }
+
+        function createCombinedSymbolForOverloadFailure(sources: ReadonlyArray<Symbol>, type: Type): Symbol {
+            // This function is currently only used for erroneous overloads, so it's good enough to just use the first source.
+            return createSymbolWithType(first(sources), type);
+        }
+
+        function pickLongestCandidateSignature(node: CallLikeExpression, candidates: Signature[], args: ReadonlyArray<Expression>): Signature {
+            // 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>("
+            const bestIndex = getLongestCandidateIndex(candidates, apparentArgumentCount === undefined ? args.length : apparentArgumentCount);
+            const candidate = candidates[bestIndex];
+
+            const { typeParameters } = candidate;
+            if (!typeParameters) {
+                return candidate;
+            }
+
+            if (!callLikeExpressionMayHaveTypeArguments(node) || !node.typeArguments) {
+                // TODO: This leaks a type parameter! See GH#19854
+                // Could use `callLikeExpressionMayHaveTypeArguments(node) ? node.typeArguments || emptyArray : emptyArray;` instead of `node.typeArguments`.
+                return candidate;
+            }
+
+            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;
+        }
+
+        function getLongestCandidateIndex(candidates: ReadonlyArray<Signature>, argsCount: number): number {
             let maxParamsIndex = -1;
             let maxParams = -1;
 
@@ -17815,9 +17888,13 @@ namespace ts {
         }
 
         function getTypeAtPosition(signature: Signature, pos: number): Type {
+            return tryGetTypeAtPosition(signature, pos) || anyType;
+        }
+
+        function tryGetTypeAtPosition(signature: Signature, pos: number): Type | undefined {
             return signature.hasRestParameter ?
                 pos < signature.parameters.length - 1 ? getTypeOfParameter(signature.parameters[pos]) : getRestTypeOfSignature(signature) :
-                pos < signature.parameters.length ? getTypeOfParameter(signature.parameters[pos]) : anyType;
+                pos < signature.parameters.length ? getTypeOfParameter(signature.parameters[pos]) : undefined;
         }
 
         function getTypeOfFirstParameterOfSignature(signature: Signature) {

src/compiler/core.ts

@@ -912,6 +912,22 @@ namespace ts {
         return to;
     }
 
+    export function minAndMax<T>(arr: ReadonlyArray<T>, getValue: (value: T) => number): { min: number, max: number } {
+        Debug.assert(arr.length !== 0);
+        let min = getValue(arr[0]);
+        let max = min;
+        for (let i = 1; i < arr.length; i++) {
+            const b = getValue(arr[i]);
+            if (b < min) {
+                min = b;
+            }
+            else if (b > max) {
+                max = b;
+            }
+        }
+        return { min, max };
+    }
+
     /**
      * Gets the actual offset into an array for a relative offset. Negative offsets indicate a
      * position offset from the end of the array.

tests/cases/fourslash/completionsCombineOverloads.ts

@@ -0,0 +1,9 @@
+/// <reference path="fourslash.ts" />
+
+////interface A { a: number }
+////interface B { b: number }
+////declare function f(a: A): void;
+////declare function f(b: B): void;
+////f({ /**/ });
+
+verify.completionsAt("", ["a", "b"]);

tests/cases/fourslash/completionsCombineOverloads_restParameter.ts

@@ -0,0 +1,13 @@
+/// <reference path="fourslash.ts" />
+
+////interface A { a: number }
+////interface B { b: number }
+////interface C { c: number }
+////declare function f(a: A): void;
+////declare function f(...bs: B[]): void;
+////declare function f(...cs: C[]): void;
+////f({ /*1*/ });
+////f({ a: 1 }, { /*2*/ });
+
+verify.completionsAt("1", ["a", "b", "c"]);
+verify.completionsAt("2", ["b", "c"]);

tests/cases/fourslash/completionsCombineOverloads_returnType.ts

@@ -0,0 +1,9 @@
+/// <reference path="fourslash.ts" />
+
+////interface A { a: number }
+////interface B { b: number }
+////declare function f(n: number): A;
+////declare function f(s: string): B;
+////f()./**/
+
+verify.completionsAt("", ["a", "b"]);

tests/cases/fourslash/quickInfo_errorSignatureFillsInTypeParameter.ts

@@ -0,0 +1,6 @@
+/// <reference path='fourslash.ts'/>
+
+////declare function f<T>(x: number): T;
+////const x/**/ = f();
+
+verify.quickInfoAt("", "const x: T"); // TODO: GH#19854