Fix 38219: signature helper works for optional function condition. (microsoft#39672)

* fix optional fuction condition.

* Adds a test

Co-authored-by: Orta <git@orta.io>

Author: ShuiRuTian

src/services/signatureHelp.ts

@@ -304,14 +304,17 @@ namespace ts.SignatureHelp {
         if (!info) return undefined;
         const { contextualType, argumentIndex, argumentCount, argumentsSpan } = info;
 
-        const signatures = contextualType.getCallSignatures();
+        // for optional function condition.
+        const nonNullableContextualType = contextualType.getNonNullableType();
+
+        const signatures = nonNullableContextualType.getCallSignatures();
         if (signatures.length !== 1) return undefined;
 
-        const invocation: ContextualInvocation = { kind: InvocationKind.Contextual, signature: first(signatures), node: startingToken, symbol: chooseBetterSymbol(contextualType.symbol) };
+        const invocation: ContextualInvocation = { kind: InvocationKind.Contextual, signature: first(signatures), node: startingToken, symbol: chooseBetterSymbol(nonNullableContextualType.symbol) };
         return { isTypeParameterList: false, invocation, argumentsSpan, argumentIndex, argumentCount };
     }
 
-    interface ContextualSignatureLocationInfo {readonly contextualType: Type; readonly argumentIndex: number; readonly argumentCount: number; readonly argumentsSpan: TextSpan; }
+    interface ContextualSignatureLocationInfo { readonly contextualType: Type; readonly argumentIndex: number; readonly argumentCount: number; readonly argumentsSpan: TextSpan; }
     function getContextualSignatureLocationInfo(startingToken: Node, sourceFile: SourceFile, checker: TypeChecker): ContextualSignatureLocationInfo | undefined {
         if (startingToken.kind !== SyntaxKind.OpenParenToken && startingToken.kind !== SyntaxKind.CommaToken) return undefined;
         const { parent } = startingToken;

tests/cases/fourslash/signatureHelpForOptionalMethods.ts

@@ -0,0 +1,22 @@
+// #39672
+// <reference path='fourslash.ts'/>
+// @strict: true
+
+//// interface Obj {
+////     optionalMethod?: (current: any) => any;
+//// };
+////
+//// const o: Obj = {
+////   optionalMethod(/*1*/) {
+////     return {};
+////   }
+//// };
+
+verify.signatureHelp(
+  {
+      marker: "1",
+      text: 'optionalMethod(current: any): any',
+      parameterName: "current",
+      parameterSpan: "current: any",
+  },
+);