Type annotation that worked in v11 now gives error `Did you mean to annotate the parameter type or the return type?`

[mediremi]

In v11, the following compiles fine:

let f = (): array<nullable<int>> => {
  []
}

Playground link

but from v12.0.0-alpha.13 onwards it gives this error:

Did you mean to annotate the parameter type or the return type?
  1) (pattern): array<nullable<int>> => {
    []
  }
  2) (pattern: array<nullable<int>>) => {
    []
  }

Alpha 13 playground link
Beta 10 playground link

[mediremi]

Using git bisect, I've managed to track down the commit that introduced this issue to #7183.

---

test.sh

#!/bin/bash

make build

if [ -f "./cli/bsc.js" ]; then
    output=$(./cli/bsc.js test.res 2>&1)
        if echo "$output" | grep -q "Syntax error"; then
                exit 1
        else
                exit 0
        fi
else
        output=$(./cli/bsc test.res 2>&1)
        if echo "$output" | grep -q "Syntax error"; then
                exit 1
        else
                exit 0
        fi
fi

test.res

let f = (): array<nullable<int>> => {
  []
}

git bisect steps

1. git bisect start
2. git checkout 20d644e13235e85bcca87c5ed195d27973782f85 && git bisect good
3. git checkout 6ddfd80b3bdb06e7f7092d8c1d4175b1b26a2d8a && git bisect bad
4. git bisect run ./test.sh

[zth]

Wonder if #7894 solves this.