fix(49392): show optional class methods with enabled strict option (#49768)

src/services/codefixes/helpers.ts

@@ -138,7 +138,7 @@ namespace ts.codefix {
                 // If there is more than one overload but no implementation signature
                 // (eg: an abstract method or interface declaration), there is a 1-1
                 // correspondence of declarations and signatures.
-                const signatures = checker.getSignaturesOfType(type, SignatureKind.Call);
+                const signatures = type.isUnion() ? flatMap(type.types, t => t.getCallSignatures()) : type.getCallSignatures();
                 if (!some(signatures)) {
                     break;
                 }

tests/cases/fourslash/completionsOverridingMethod14.ts

@@ -0,0 +1,29 @@
+/// <reference path="fourslash.ts" />
+
+// @Filename: a.ts
+// @strictNullChecks: true
+// @newline: LF
+
+////interface IFoo {
+////    foo?(arg: string): number;
+////}
+////class Foo implements IFoo {
+////    /**/
+////}
+
+verify.completions({
+    marker: "",
+    isNewIdentifierLocation: true,
+    preferences: {
+        includeCompletionsWithInsertText: true,
+        includeCompletionsWithSnippetText: false,
+        includeCompletionsWithClassMemberSnippets: true,
+    },
+    includes: [
+        {
+            name: "foo",
+            sortText: completion.SortText.ClassMemberSnippets,
+            insertText: "foo(arg: string): number {\n}"
+        },
+    ],
+});