proposal: ignore interfaces that contains type constraints (#93)

This is a proposal PR to generate mocks without error for following
cases:
```go
type Water[R any, C UnsignedInteger] interface {
	Fish(R) []C
}

type UnsignedInteger interface {
	~uint | ~uint32 | ~uint64
}
```

Go `types` package seems to wrap interfaces that contain type
constraints by checking a type set literal of the form `~T` and `A|B`.
https://github.com/golang/go/blob/master/src/go/types/decl.go#L668

So gomock can just ignore that pattern to generate mocks safely without
`don't know how to mock method of type` errors. I think this can be a
better solution for custom type constraints than [`-exclude`
flag](#72).

---------

Co-authored-by: Jacob Oaks <jacoboaks.8@gmail.com>

mockgen/generic_go118.go

@@ -11,8 +11,10 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"go/ast"
+	"go/token"
 	"strings"
 
 	"go.uber.org/mock/mockgen/model"
@@ -98,6 +100,16 @@ func (p *fileParser) parseGenericMethod(field *ast.Field, it *namedInterface, if
 	case *ast.IndexListExpr:
 		indices = v.Indices
 		typ = v.X
+	case *ast.UnaryExpr:
+		if v.Op == token.TILDE {
+			return nil, errConstraintInterface
+		}
+		return nil, fmt.Errorf("~T may only appear as constraint for %T", field.Type)
+	case *ast.BinaryExpr:
+		if v.Op == token.OR {
+			return nil, errConstraintInterface
+		}
+		return nil, fmt.Errorf("A|B may only appear as constraint for %T", field.Type)
 	default:
 		return nil, fmt.Errorf("don't know how to mock method of type %T", field.Type)
 	}
@@ -114,3 +126,5 @@ func (p *fileParser) parseGenericMethod(field *ast.Field, it *namedInterface, if
 
 	return p.parseMethod(nf, it, iface, pkg, tps)
 }
+
+var errConstraintInterface = errors.New("interface contains constraints")

mockgen/internal/tests/generics/generics.go

@@ -57,3 +57,11 @@ type SolarSystem[T constraints.Ordered] interface {
 type Earth[R any] interface {
 	Water(R) []R
 }
+
+type Water[R any, C UnsignedInteger] interface {
+	Fish(R) []C
+}
+
+type UnsignedInteger interface {
+	~uint | ~uint32 | ~uint64
+}

mockgen/parse.go

@@ -232,6 +232,9 @@ func (p *fileParser) parseFile(importPath string, file *ast.File) (*model.Packag
 			continue
 		}
 		i, err := p.parseInterface(ni.name.String(), importPath, ni)
+		if errors.Is(err, errConstraintInterface) {
+			continue
+		}
 		if err != nil {
 			return nil, err
 		}