quick fix for a nullable missing callback function (#51743)

src/services/codefixes/fixAddMissingMember.ts

@@ -330,7 +330,7 @@ function getInfo(sourceFile: SourceFile, tokenPos: number, errorCode: number, ch
     }
 
     if (isIdentifier(token)) {
-        const type = checker.getContextualType(token);
+        const type = checker.getContextualType(token)?.getNonNullableType();
         if (type && getObjectFlags(type) & ObjectFlags.Anonymous) {
             const signature = firstOrUndefined(checker.getSignaturesOfType(type, SignatureKind.Call));
             if (signature === undefined) return undefined;

tests/cases/fourslash/codeFixAddMissingFunctionDeclaration26.ts

@@ -0,0 +1,29 @@
+/// <reference path="fourslash.ts" />
+
+// @strict: true
+
+////interface Foo {
+////    a: ((e: any) => void) | null;
+////}
+////
+////const foo: Foo = {
+////    a: fn
+////}
+
+verify.codeFix({
+    index: 0,
+    description: [ts.Diagnostics.Add_missing_function_declaration_0.message, "fn"],
+    newFileContent:
+`interface Foo {
+    a: ((e: any) => void) | null;
+}
+
+const foo: Foo = {
+    a: fn
+}
+
+function fn(e: any): void {
+    throw new Error("Function not implemented.");
+}
+`
+});

tests/cases/fourslash/codeFixAddMissingFunctionDeclaration27.ts

@@ -0,0 +1,29 @@
+/// <reference path="fourslash.ts" />
+
+// @strict: true
+
+////interface Foo {
+////    a?: ((e: any) => void);
+////}
+////
+////const foo: Foo = {
+////    a: fn
+////}
+
+verify.codeFix({
+    index: 0,
+    description: [ts.Diagnostics.Add_missing_function_declaration_0.message, "fn"],
+    newFileContent:
+`interface Foo {
+    a?: ((e: any) => void);
+}
+
+const foo: Foo = {
+    a: fn
+}
+
+function fn(e: any): void {
+    throw new Error("Function not implemented.");
+}
+`
+});