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

Author: Andy Hanson

src/compiler/checker.ts

@@ -6682,13 +6682,17 @@ 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 {
@@ -10430,6 +10434,11 @@ namespace ts {
             return symbol;
         }
 
+        function createCombinedSymbolWithType(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 transformTypeOfMembers(type: Type, f: (propertyType: Type) => Type) {
             const members = createSymbolTable();
             for (const property of getPropertiesOfObjectType(type)) {
@@ -16433,35 +16442,8 @@ 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);
@@ -16535,6 +16517,89 @@ 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 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.
+            if (!hasCandidatesOutArray && candidates.length > 1 && !candidates.some(c => !!c.typeParameters)) {
+                return createUnionOfSignaturesForOverloadFailure(candidates);
+            }
+
+            // 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 && 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;
+        }
+
+        function createUnionOfSignaturesForOverloadFailure(candidates: ReadonlyArray<Signature>): Signature {
+            const thisParameters = mapDefined(candidates, c => c.thisParameter);
+            let thisParameter: Symbol | undefined;
+            if (thisParameters.length) {
+                thisParameter = createCombinedSymbolWithType(thisParameters, getUnionType(thisParameters.map(getTypeOfParameter), /*subtypeReduction*/ true));
+            }
+
+            const { min: minArgumentCount, max: maxNonRestParam }  = minAndMax(candidates, getNumNonRestParameters);
+            const hasRestParameter = candidates.some(c => c.hasRestParameter);
+            const hasLiteralTypes = candidates.some(c => c.hasLiteralTypes);
+            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);
+                const types = mapDefined(candidates, candidate => tryGetTypeAtPosition(candidate, i));
+                parameters.push(createCombinedSymbolWithType(symbols, getUnionType(types, /*subtypeReduction*/ true)));
+            }
+
+            if (hasRestParameter) {
+                const symbols = mapDefined(candidates, c => c.hasRestParameter ? last(c.parameters) : undefined);
+                Debug.assert(symbols.length !== 0);
+                const type = createArrayType(getUnionType(mapDefined(candidates, tryGetRestTypeOfSignature), /*subtypeReduction*/ true));
+                parameters.push(createCombinedSymbolWithType(symbols, type));
+            }
+
+            return createSignature(
+                candidates[0].declaration,
+                /*typeParameters*/ undefined,
+                thisParameter,
+                parameters,
+                /*resolvedReturnType*/ unknownType,
+                /*typePredicate*/ undefined,
+                minArgumentCount,
+                hasRestParameter,
+                hasLiteralTypes);
+        }
+
         function getLongestCandidateIndex(candidates: Signature[], argsCount: number): number {
             let maxParamsIndex = -1;
             let maxParams = -1;
@@ -17175,9 +17240,13 @@ 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

@@ -778,6 +778,22 @@ 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"]);