Summary

Revisits #1665.

In issue #1665, the feature was closed with "There are no plans to support this feature atm." However, after investigating the implementation, it appears feasible with a relatively small change. I'd like to propose this as a candidate for support — happy to iterate if the approach needs adjustment.

What this does

The CLI plugin now automatically infers enum: [...] metadata for properties whose type is a string or number literal union — without requiring a manual @ApiProperty({ enum: [...] }).

Before

type Color = 'red' | 'green' | 'blue';

export class CatDto {
  // Previously required manual annotation:
  @ApiProperty({ enum: ['red', 'green', 'blue'] })
  color: Color;
}

After

type Color = 'red' | 'green' | 'blue';

export class CatDto {
  color: Color;
}

Generated metadata: { color: { required: true, enum: ["red", "green", "blue"] } } ✅

Cases covered

Implementation

• lib/plugin/utils/plugin-utils.ts: Added getStringLiteralUnionValues() — detects whether a ts.Type is a pure string or number literal union (TypeScript enum members are explicitly excluded via SymbolFlags.EnumMember), and returns the values along with a isNullable flag.
• lib/plugin/visitors/model-class.visitor.ts:
  • createTypePropertyAssignments: treats string/number literal unions like enums (skips emitting type: () => Object; emits nullable: true when needed)
  • createEnumPropertyAssignment: when a literal union is detected, emits enum: ["a", "b", ...] or enum: [1, 2, ...] as an ArrayLiteralExpression instead of a type reference identifier

Non-goals / out of scope

• Mixed unions like 'a' | 1 | true — To keep the diff small, and since this is still a proposal, this currently only supports simple unions where all members are string literals or all members are number literals.