Add related diagnostic to "used before defined" if type is a function that returns a union with undefined (#33171)

* Add "use before defined" diagnostic

* Make "use before defined" diagnostic as related information to TS2454

* Add baseline tests for "use before defined"

* Add test for type alias union with undefined for "use before defined" diagnostic

* Update baselines

src/compiler/checker.ts

@@ -20190,7 +20190,26 @@ namespace ts {
                 }
             }
             else if (!assumeInitialized && !(getFalsyFlags(type) & TypeFlags.Undefined) && getFalsyFlags(flowType) & TypeFlags.Undefined) {
-                error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
+                const diag = error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
+
+                // See GH:32846 - if the user is using a variable whose type is () => T1 | ... | undefined
+                // they may have meant to specify the type as (() => T1 | ...) | undefined
+                // This is assumed if: the type is a FunctionType, the return type is a Union, the last constituent of
+                // the union is `undefined`
+                if (type.symbol && type.symbol.declarations.length === 1 && isFunctionTypeNode(type.symbol.declarations[0])) {
+                    const funcTypeNode = <FunctionTypeNode>type.symbol.declarations[0];
+                    const returnType = getReturnTypeFromAnnotation(funcTypeNode);
+                    if (returnType && returnType.flags & TypeFlags.Union) {
+                        const unionTypes = (<UnionTypeNode>funcTypeNode.type).types;
+                        if (unionTypes && unionTypes[unionTypes.length - 1].kind === SyntaxKind.UndefinedKeyword) {
+                            const parenedFuncType = getMutableClone(funcTypeNode);
+                            // Highlight to the end of the second to last constituent of the union
+                            parenedFuncType.end = unionTypes[unionTypes.length - 2].end;
+                            addRelatedInfo(diag, createDiagnosticForNode(parenedFuncType, Diagnostics.Did_you_mean_to_parenthesize_this_function_type));
+                        }
+                    }
+                }
+
                 // Return the declared type to reduce follow-on errors
                 return type;
             }

src/compiler/diagnosticMessages.json

@@ -1047,6 +1047,10 @@
         "category": "Error",
         "code": 1359
     },
+    "Did you mean to parenthesize this function type?": {
+        "category": "Error",
+        "code": 1360
+    },
 
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",