fix parse arrays with const length correctly

mockgen/internal/tests/const_array_length/input.go

@@ -0,0 +1,8 @@
+//go:generate mockgen -package const_length -destination mock.go -source input.go
+package const_length
+
+const C = 2
+
+type I interface {
+	F() [C]int
+}

mockgen/parse.go

@@ -410,7 +410,16 @@ func (p *fileParser) parseType(pkg string, typ ast.Expr) (model.Type, error) {
 	case *ast.ArrayType:
 		ln := -1
 		if v.Len != nil {
-			x, err := strconv.Atoi(v.Len.(*ast.BasicLit).Value)
+			var value string
+			switch val := v.Len.(type) {
+			case (*ast.BasicLit):
+				value = val.Value
+			case (*ast.Ident):
+				// when the length is a const
+				value = val.Obj.Decl.(*ast.ValueSpec).Values[0].(*ast.BasicLit).Value
+			}
+
+			x, err := strconv.Atoi(value)
 			if err != nil {
 				return nil, p.errorf(v.Len.Pos(), "bad array size: %v", err)
 			}

mockgen/parse_test.go

@@ -263,3 +263,27 @@ func TestParsePackageImport_FallbackMultiGoPath(t *testing.T) {
 		t.Errorf("expect %s, got %s", expected, pkgPath)
 	}
 }
+
+func TestParseArrayWithConstLength(t *testing.T) {
+	fs := token.NewFileSet()
+
+	file, err := parser.ParseFile(fs, "internal/tests/const_array_length/input.go", nil, 0)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	p := fileParser{
+		fileSet: fs,
+	}
+
+	pkg, err := p.parseFile("", file)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	expect := "[2]int"
+	got := pkg.Interfaces[0].Methods[0].Out[0].Type.String(nil, "")
+	if got != expect {
+		t.Fatalf("Expected return type %v but got %v", expect, got)
+	}
+}