Actual behavior
example.go

package example

type Example interface {
	ParenthesizedParameterType(param (int))
}

Result:
Loading input failed: parenthesized.go:4:28: failed parsing arguments: don't know how to parse type *ast.ParenExpr

Expected behavior
The above codes are accepted.
https://golang.org/ref/spec#Types

Type = TypeName | TypeLit | "(" Type ")" .

Additional Information

• gomock mode (reflect or source):
• gomock version or git ref:

$ mockgen -version
-> v1.4.3

• golang version:

$ go version
-> go version go1.14.1 darwin/amd64

Triage Notes for the Maintainers
I know the linter removes these parentheses. But in case of *(<typename>), it doesn't.
And if this wasn't so hard, I'd be up for the challenge.