@stevehollaar any chance you could share code that reproduces the issue? That would help us out a ton

Unfortunately the codebase is proprietary and I wasn’t able to isolate a small repro.

I ended up debugging this by adding a console.log line:

console.log({source, sourceType });

in tsc.js, just prior to this line:

reportIncompatibleError(ts.Diagnostics.Type_at_position_0_in_source_is_not_compatible_with_type_at_position_1_in_target, sourceIndex, i);

That gave me:

{
  source: Type {
    flags: 524288,
    id: 117489,
    symbol: undefined,
    objectFlags: 1114116,
    target: <ref *1> Type {
      flags: 524288,
      id: 1048,
      objectFlags: 12,
      symbol: undefined,
      members: undefined,
      properties: undefined,
      callSignatures: undefined,
      constructSignatures: undefined,
      stringIndexInfo: undefined,
      numberIndexInfo: undefined,
      typeParameters: [Array],
      outerTypeParameters: undefined,
      localTypeParameters: [Array],
      instantiations: [Map],
      target: [Circular *1],
      resolvedTypeArguments: [Array],
      thisType: [Type],
      declaredProperties: [Array],
      declaredCallSignatures: [],
      declaredConstructSignatures: [],
      declaredStringIndexInfo: undefined,
      declaredNumberIndexInfo: undefined,
      elementFlags: [Array],
      minLength: 2,
      fixedLength: 2,
      hasRestElement: false,
      combinedFlags: 1,
      readonly: false,
      labeledElementDeclarations: undefined,
      resolvedBaseTypes: [Array],
      baseTypesResolved: true
    },
    resolvedTypeArguments: [ [Type], [Type] ],
    members: Map(36) {
      '0' => [Symbol],
      '1' => [Symbol],
      'length' => [Symbol],
      'toString' => [Symbol],
      'toLocaleString' => [Symbol],
      'pop' => [Symbol],
      'push' => [Symbol],
      'concat' => [Symbol],
      'join' => [Symbol],
      'reverse' => [Symbol],
      'shift' => [Symbol],
      'slice' => [Symbol],
      'sort' => [Symbol],
      'splice' => [Symbol],
      'unshift' => [Symbol],
      'indexOf' => [Symbol],
      'lastIndexOf' => [Symbol],
      'every' => [Symbol],
      'some' => [Symbol],
      'forEach' => [Symbol],
      'map' => [Symbol],
      'filter' => [Symbol],
      'reduce' => [Symbol],
      'reduceRight' => [Symbol],
      'find' => [Symbol],
      'findIndex' => [Symbol],
      'fill' => [Symbol],
      'copyWithin' => [Symbol],
      '__@iterator' => [Symbol],
      'entries' => [Symbol],
      'keys' => [Symbol],
      'values' => [Symbol],
      '__@unscopables' => [Symbol],
      'includes' => [Symbol],
      'flatMap' => [Symbol],
      'flat' => [Symbol]
    },
    properties: [
      [Symbol], [Symbol], [Symbol], [Symbol],
      [Symbol], [Symbol], [Symbol], [Symbol],
      [Symbol], [Symbol], [Symbol], [Symbol],
      [Symbol], [Symbol], [Symbol], [Symbol],
      [Symbol], [Symbol], [Symbol], [Symbol],
      [Symbol], [Symbol], [Symbol], [Symbol],
      [Symbol], [Symbol], [Symbol], [Symbol],
      [Symbol], [Symbol], [Symbol], [Symbol],
      [Symbol], [Symbol], [Symbol], [Symbol]
    ],
    callSignatures: [],
    constructSignatures: [],
    stringIndexInfo: undefined,
    numberIndexInfo: { type: [Type], isReadonly: false, declaration: [Node] }
  },
  sourceType: Type {
    flags: 524288,
    id: 117487,
    objectFlags: 16,
    symbol: Symbol {
      flags: 4096,
      escapedName: '__object',
      declarations: [Array],
      valueDeclaration: [Node],
      id: undefined,
      mergeId: undefined,
      parent: undefined,
      members: [Map]
    },
    members: Map(3) {
      'notificationId' => [Symbol],
      'getTemplateData' => [Symbol],
      'generateStartEndDate' => [Symbol]
    },
    properties: [ [Symbol], [Symbol], [Symbol] ],
    callSignatures: [],
    constructSignatures: [],
    stringIndexInfo: undefined,
    numberIndexInfo: undefined
  }
}

just prior to the tsc error, and from there I was able to find the problematic code via the sourceType.members.

I think there must have been something funky about the way types were shared across module boundaries (possibly related to a circular reference?), as when I tried to repro in the playground, I could not. My “fix” was to explicitly type some exported objects with a shared interface, instead of leaving the type as implicit. I hope this helps!