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

Start with the linked TypeScript Playground and reproduce the example using the function return type on the left side of the equals sign. Compare behavior with the return type on the right side and with the first switch case removed. Done means the missing requiredNumber property is reported in the original form.