mockgen generates duplicate implementation for overlapping interfaces in source mode

[marten-seemann]

Actual behavior
Since Go 1.14 embedded interfaces can have overlapping methods, as the io.Closer in the following example:

type readCloser interface {
	io.Reader
	io.Closer
}

type writeCloser interface {
	io.Writer
	io.Closer
}

type ReadWriteCloser interface {
	readCloser
	writeCloser
}

When generating a mock for ReadWriteCloser, the mock implementation will declare Close() error twice. The mock therefore fails to compile: (*MockReadWriteCloser).Close redeclared in this block

Expected behavior
The overlapping method should only be implemented once.

To Reproduce Steps to reproduce the behavior

mockgen -source=xxx.go

Additional Information

• gomock mode (reflect or source): reflect
• gomock version or git ref: v1.4.4
• golang version: 1.15.3

[codyoss]

Thanks for the report, I will take a look at your PR today!