Contextual typing fails to throw error with function return type on left side of equals sign

TypeScript Version: I got the same behavior on 3.7.4 and 4.0.0-dev.20200601

Search Terms:
left side
right side
contextual typing
type inference

Code

interface ReportState {
  reportDefinition: {
    [key: number]: ReportDetails;
  };
}

interface ReportDetails {
  requiredNumber: number;
  columnWidths?: number[];
}

export const Report: (state: ReportState, action: any) => ReportState = (
  state,
  action
) => { // See note below about this line
  switch (action.type) {
    case "TYPE0": {
      const { reportDef, sequenceId } = action.payload;
      return {
        ...state,
        reportDefinition: {
          ...state.reportDefinition,
          [sequenceId]: {
            ...reportDef,
            selectedElements: ["0x0"],
          },
        },
      };
    }

    case "TYPE1": {
      return {
        ...state,
        reportDefinition: {
          ...state.reportDefinition,
          [action.payload.sequenceId]: {
            // requiredNumber is missing here. This should be an error.
            columnWidths: action.payload.columnWidths,
          },
        },
      };
    }

    default:
      return state;
  }
};

Expected behavior:
There should be an error, because requiredNumber is missing from the second return statement.

Actual behavior:
Compilation succeeds without error.

Curiosity 1:
If you add the return type on the right side of the =, i.e. by changing
) => {
to
): ReportState => {
, it correctly finds the error. This should not be necessary, because the return type is already specified on the left side of the =.

Curiosity 2:
If you remove the first case block, it correctly finds the error (which is in the second case block).

Playground Link: Click here

Related Issues:
No

Beginne mit dem verlinkten TypeScript Playground und reproduziere das Beispiel, indem du den Rückgabetyp der Funktion auf der linken Seite des Gleichheitszeichens verwendest. Vergleiche das Verhalten mit dem Rückgabetyp auf der rechten Seite und mit entferntem ersten switch case. Als abgeschlossen gilt die Aufgabe, wenn die fehlende Eigenschaft requiredNumber in der ursprünglichen Form gemeldet wird.