drop legacy Go support

Go 1.9 was released in August 2017 so it's safe to make a claim that projects
are already using new enough Go version.

Tested with:

$ go version
go version go1.13.8 linux/amd64
$ GO111MODULE=off ./test.sh
ok      github.com/modern-go/reflect2-tests     0.005s  coverage: 12.3% of statements in github.com/modern-go/reflect2
ok      github.com/modern-go/reflect2-tests/test15      0.004s  coverage: 6.4% of statements in github.com/modern-go/reflect2
ok      github.com/modern-go/reflect2-tests/tests       0.010s  coverage: 65.4% of statements in github.com/modern-go/reflect2

Signed-off-by: Mikko Ylinen <[email protected]>

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 
 go:
-  - 1.8.x
+  - 1.9.x
   - 1.x
 
 before_install:

Gopkg.toml

@@ -26,10 +26,6 @@
 
 ignored = []
 
-[[constraint]]
-  name = "github.com/modern-go/concurrent"
-  version = "1.0.0"
-
 [prune]
   go-tests = true
   unused-packages = true

go_above_19.go

@@ -6,6 +6,9 @@ import (
 	"unsafe"
 )
 
+//go:linkname resolveTypeOff reflect.resolveTypeOff
+func resolveTypeOff(rtype unsafe.Pointer, off int32) unsafe.Pointer
+
 //go:linkname makemap reflect.makemap
 func makemap(rtype unsafe.Pointer, cap int) (m unsafe.Pointer)
 

reflect2.go

@@ -1,9 +1,9 @@
 package reflect2
 
 import (
-	"github.com/modern-go/concurrent"
 	"reflect"
 	"runtime"
+	"sync"
 	"unsafe"
 )
 
@@ -131,13 +131,13 @@ var ConfigSafe = Config{UseSafeImplementation: true}.Froze()
 
 type frozenConfig struct {
 	useSafeImplementation bool
-	cache                 *concurrent.Map
+	cache                 *sync.Map
 }
 
 func (cfg Config) Froze() *frozenConfig {
 	return &frozenConfig{
 		useSafeImplementation: cfg.UseSafeImplementation,
-		cache: concurrent.NewMap(),
+		cache:                 new(sync.Map),
 	}
 }
 

type_map.go

@@ -4,16 +4,10 @@ package reflect2
 
 import (
 	"reflect"
-	"runtime"
-	"strings"
 	"sync"
 	"unsafe"
 )
 
-// typelinks1 for 1.5 ~ 1.6
-//go:linkname typelinks1 reflect.typelinks
-func typelinks1() [][]unsafe.Pointer
-
 // typelinks2 for 1.7 ~
 //go:linkname typelinks2 reflect.typelinks
 func typelinks2() (sections []unsafe.Pointer, offset [][]int32)
@@ -29,49 +23,10 @@ func discoverTypes() {
 	types = make(map[string]reflect.Type)
 	packages = make(map[string]map[string]reflect.Type)
 
-	ver := runtime.Version()
-	if ver == "go1.5" || strings.HasPrefix(ver, "go1.5.") {
-		loadGo15Types()
-	} else if ver == "go1.6" || strings.HasPrefix(ver, "go1.6.") {
-		loadGo15Types()
-	} else {
-		loadGo17Types()
-	}
-}
-
-func loadGo15Types() {
-	var obj interface{} = reflect.TypeOf(0)
-	typePtrss := typelinks1()
-	for _, typePtrs := range typePtrss {
-		for _, typePtr := range typePtrs {
-			(*emptyInterface)(unsafe.Pointer(&obj)).word = typePtr
-			typ := obj.(reflect.Type)
-			if typ.Kind() == reflect.Ptr && typ.Elem().Kind() == reflect.Struct {
-				loadedType := typ.Elem()
-				pkgTypes := packages[loadedType.PkgPath()]
-				if pkgTypes == nil {
-					pkgTypes = map[string]reflect.Type{}
-					packages[loadedType.PkgPath()] = pkgTypes
-				}
-				types[loadedType.String()] = loadedType
-				pkgTypes[loadedType.Name()] = loadedType
-			}
-			if typ.Kind() == reflect.Slice && typ.Elem().Kind() == reflect.Ptr &&
-				typ.Elem().Elem().Kind() == reflect.Struct {
-				loadedType := typ.Elem().Elem()
-				pkgTypes := packages[loadedType.PkgPath()]
-				if pkgTypes == nil {
-					pkgTypes = map[string]reflect.Type{}
-					packages[loadedType.PkgPath()] = pkgTypes
-				}
-				types[loadedType.String()] = loadedType
-				pkgTypes[loadedType.Name()] = loadedType
-			}
-		}
-	}
+	loadGoTypes()
 }
 
-func loadGo17Types() {
+func loadGoTypes() {
 	var obj interface{} = reflect.TypeOf(0)
 	sections, offset := typelinks2()
 	for i, offs := range offset {