[Bug]: Using the @opExample decorator crashes the compiler if the operation return type is a union

[dmytrol]

Describe the bug

Consider an operation defined like this:

@get op list(param: string): ListResponse | TypicalErrorResponses

where

model GenericOKResponse<T> {
    ...OkResponse;
    @header contentType: "application/json";

    @bodyRoot
    body: T    
}

model ListResponse is GenericOKResponse<ListItems>

alias TypicalGetErrorResponses = UnauthorizedResponse | NotFoundResponse | BadRequestResponse

When adding an @opExample decorator to such an operation, the compiler crashes:

Emitter "@typespec/openapi3" crashed! This is a bug.
Please file an issue at https://github.com/microsoft/typespec/issues

TypeError: Cannot read properties of undefined (reading 'get')
    at serializeObjectValueAsJson (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/compiler/src/lib/examples.ts:45:40)
    at serializeValueAsJson (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/compiler/src/lib/examples.ts:32:14)
    at getExampleOrExamples (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/openapi3/src/openapi.ts:1206:18)
    at getExamplesForBodyContentEntry (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/openapi3/src/openapi.ts:1183:12)
    at getBodyContentEntry (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/openapi3/src/openapi.ts:1234:14)
    at emitResponseContent (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/openapi3/src/openapi.ts:1038:26)
    at emitResponseObject (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/openapi3/src/openapi.ts:994:5)
    at emitResponses (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/openapi3/src/openapi.ts:946:9)
    at emitOperation (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/openapi3/src/openapi.ts:920:5)
    at getOpenApiFromVersion (file:///Users/dmytrol/externalapi.definitions/node_modules/@typespec/openapi3/src/openapi.ts:771:11)

Reproduction

I've launched compilation under the debugger and here're my observations:

• The "offending" line (in transpiled JavaScript) is const definition = type.properties.get(propValue.name);
• The value of the type parameter in the call to serializeObjectValueAsJson is { kind: 'Union, ... }`
• The value of the type parameter does not have the properties property defined (quite expected IMO because "Union" is not a leaf type in a syntax tree)

I would run the risk to assume that getExamplesForBodyContentEntry should probably perform an additional "walk" over the operation's return type if the return type is a union and substitute the return type passed to serializeObjectValueAsJson with the one matching the data structure provided in returnType in the decorator itself.

Alternatively (this could be easier to implement IMO) the decorator could probably allow specifying the exact return type for a specific operation example.

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[timotheeguerin]

Ouch, sorry about that I meant to implement that and left on leave and forgot I didn't, I'll try to get a hotfix out