Syntax error for unnamed argument in Flow function type inside generic type argument of arrow function return type

[noppa]

The title is a bit of a mouthful, but what I mean is this:

// @flow
const foo = (): Foo<number => number> => {}

fails to parse with error

sucrase src -d dist -t flow
$ node_modules/.bin/sucrase src -d dist -t flow
src/index.js -> dist/index.js
SyntaxError: Error transforming src/index.js: Unexpected token, expected ";" (2:15)
    at unexpected (node_modules/sucrase/dist/parser/traverser/util.js:83:15)
    at semicolon (node_modules/sucrase/dist/parser/traverser/util.js:61:5)
    at parseVarStatement (node_modules/sucrase/dist/parser/traverser/statement.js:454:19)
    at parseStatementContent (node_modules/sucrase/dist/parser/traverser/statement.js:171:7)
    at parseStatement (node_modules/sucrase/dist/parser/traverser/statement.js:109:3)
    at parseBlockBody (node_modules/sucrase/dist/parser/traverser/statement.js:504:5)
    at parseTopLevel (node_modules/sucrase/dist/parser/traverser/statement.js:85:3)
    at parseFile (node_modules/sucrase/dist/parser/traverser/index.js:17:35)
    at parse (node_modules/sucrase/dist/parser/index.js:26:35)
    at getSucraseContext (node_modules/sucrase/dist/index.js:88:30) {
  pos: 23,
  loc: Loc { line: 2, column: 15 }

The error only reproduces when

• The argument in function type number => number does not have a name, i.e. not (x: number) => number. This syntax is allowed in Flow
• The function type is inside a generic type, e.g. Foo<number => number> or Promise<number => number>
• The whole generic type is the return type of an arrow function, it does not repro in type declarations or return types of named function declarations

The following similar code examples parse just fine:

// @flow
type X = Foo<number => number>
  
const foo1 = (): X => {}

const foo2 = (): Foo<(x:number) => number> => {}

const foo3 = (): (number => number) => {}

function foo4(): Foo<number => number> {}

Although without parenthesis around the return type, the third example actually produces invalid output, but that's probably a separate issue.