Parse error of function definition with here doc in command substitution

[D630]

Hi,

In version `5.2.0(1)-rc2 (x86_64-pc-linux-gnu)` from Debian, this code

```
_=$(
        f1() {
                c <<-!
                !
        }
        f2() {
                :
        }
)
```

results in

```
bash: command substitution: line 15: syntax error near unexpected token 
`('
bash: command substitution: line 15: `}function f2 () '
```

This one looks good at first:

```
_=$(
        f2() {
                :
        }
        f1() {
                c <<-!
                !
        }
)
```

But this not:

```
_=$(
        f2() {
                :
        }
        f1() {
                c <<-!
                !
        }
        f1
)
```

This works:

```
_=$(
        f2() {
                :
        }
        f1() {
                c
        } <<-!
        !
        f1
)
```