Improved argument description for parameters originating from tuples that were extracted from functions. (ie mapped functions)

src/compiler/checker.ts

@@ -112,6 +112,7 @@ namespace ts {
             getMergedSymbol,
             getDiagnostics,
             getGlobalDiagnostics,
+            getExpandedParameters,
             getTypeOfSymbolAtLocation: (symbol, location) => {
                 location = getParseTreeNode(location);
                 return location ? getTypeOfSymbolAtLocation(symbol, location) : errorType;

src/compiler/types.ts

@@ -3032,6 +3032,7 @@ namespace ts {
         /* @internal */
         getPromisedTypeOfPromise(promise: Type, errorNode?: Node): Type | undefined;
         getReturnTypeOfSignature(signature: Signature): Type;
+        /* @internal */ getExpandedParameters(sig: Signature): ReadonlyArray<Symbol>;
         /**
          * Gets the type of a parameter at a given position in a signature.
          * Returns `any` if the index is not valid.

src/services/signatureHelp.ts

@@ -566,7 +566,7 @@ namespace ts.SignatureHelp {
     }
 
     function itemInfoForParameters(candidateSignature: Signature, checker: TypeChecker, enclosingDeclaration: Node, sourceFile: SourceFile): SignatureHelpItemInfo {
-        const isVariadic = candidateSignature.hasRestParameter;
+        let isVariadic = candidateSignature.hasRestParameter;
         const printer = createPrinter({ removeComments: true });
         const typeParameterParts = mapToDisplayParts(writer => {
             if (candidateSignature.typeParameters && candidateSignature.typeParameters.length) {
@@ -574,7 +574,16 @@ namespace ts.SignatureHelp {
                 printer.writeList(ListFormat.TypeParameters, args, sourceFile, writer);
             }
         });
-        const parameters = candidateSignature.parameters.map(p => createSignatureHelpParameterForParameter(p, checker, enclosingDeclaration, sourceFile, printer));
+
+        const expandedParameters = checker.getExpandedParameters(candidateSignature);
+        const parameters = expandedParameters.map(p => createSignatureHelpParameterForParameter(p, checker, enclosingDeclaration, sourceFile, printer));
+
+        // If parameters are not the same as the expanded parameters, we need to reevaluate whether this a variadic signature based on the last parameter
+        if (expandedParameters !== candidateSignature.parameters) {
+            const restCandidate = lastOrUndefined(expandedParameters);
+            isVariadic = !!restCandidate && !!(getCheckFlags(restCandidate) & CheckFlags.RestParameter);
+        }
+
         return { isVariadic, parameters, prefix: [...typeParameterParts, punctuationPart(SyntaxKind.OpenParenToken)], suffix: [punctuationPart(SyntaxKind.CloseParenToken)] };
     }
 

tests/cases/fourslash/signatureHelpCallExpressionTuples.ts

@@ -0,0 +1,56 @@
+/// <reference path='fourslash.ts'/>
+
+
+//// function fnTest(str: string, num: number) { }
+//// declare function wrap<A extends any[], R>(fn: (...a: A) => R) : (...a: A) => R;
+//// var fnWrapped = wrap(fnTest);
+//// fnWrapped/*3*/(/*1*/'', /*2*/5);
+//// function fnTestVariadic (str: string, ...num: number[]) { }
+//// var fnVariadicWrapped = wrap(fnTestVariadic);
+//// fnVariadicWrapped/*4*/(/*5*/'', /*6*/5);
+//// function fnNoParams () { }
+//// var fnNoParamsWrapped = wrap(fnNoParams);
+//// fnNoParamsWrapped/*7*/(/*8*/);
+
+verify.quickInfoAt("3", "var fnWrapped: (str: string, num: number) => void");
+verify.signatureHelp(
+    {
+        marker: "1",
+        text: "fnWrapped(str: string, num: number): void",
+        parameterCount: 2,
+        parameterName: "str",
+        parameterSpan: "str: string",
+    },
+    {
+        marker: "2",
+        parameterName: "num",
+        parameterSpan: "num: number",
+    },
+);
+
+verify.quickInfoAt("4", "var fnVariadicWrapped: (str: string, ...num: number[]) => void");
+verify.signatureHelp(
+    {
+        marker: "5",
+        text: "fnVariadicWrapped(str: string, ...num: number[]): void",
+        parameterCount: 2,
+        parameterName: "str",
+        parameterSpan: "str: string",
+        isVariadic: true,
+    },
+    {
+        marker: "6",
+        parameterName: "num",
+        parameterSpan: "...num: number[]",
+        isVariadic: true,
+    },
+);
+
+verify.quickInfoAt("7", "var fnNoParamsWrapped: () => void");
+verify.signatureHelp(
+    {
+        marker: "8",
+        text: "fnNoParamsWrapped(): void",
+        parameterCount: 0,
+    }
+);