Drop hacks to work around Go reflection bugs in Go1.9 (#282)

Now that Go 1.11 is the minimally supported version,
we can drop some local hacks to work around bugs in
reflect that were present in Go1.9.

cmp/compare.go

@@ -36,7 +36,6 @@ import (
 	"strings"
 
 	"github.com/google/go-cmp/cmp/internal/diff"
-	"github.com/google/go-cmp/cmp/internal/flags"
 	"github.com/google/go-cmp/cmp/internal/function"
 	"github.com/google/go-cmp/cmp/internal/value"
 )
@@ -319,7 +318,6 @@ func (s *state) tryMethod(t reflect.Type, vx, vy reflect.Value) bool {
 }
 
 func (s *state) callTRFunc(f, v reflect.Value, step Transform) reflect.Value {
-	v = sanitizeValue(v, f.Type().In(0))
 	if !s.dynChecker.Next() {
 		return f.Call([]reflect.Value{v})[0]
 	}
@@ -343,8 +341,6 @@ func (s *state) callTRFunc(f, v reflect.Value, step Transform) reflect.Value {
 }
 
 func (s *state) callTTBFunc(f, x, y reflect.Value) bool {
-	x = sanitizeValue(x, f.Type().In(0))
-	y = sanitizeValue(y, f.Type().In(1))
 	if !s.dynChecker.Next() {
 		return f.Call([]reflect.Value{x, y})[0].Bool()
 	}
@@ -372,19 +368,6 @@ func detectRaces(c chan<- reflect.Value, f reflect.Value, vs ...reflect.Value) {
 	ret = f.Call(vs)[0]
 }
 
-// sanitizeValue converts nil interfaces of type T to those of type R,
-// assuming that T is assignable to R.
-// Otherwise, it returns the input value as is.
-func sanitizeValue(v reflect.Value, t reflect.Type) reflect.Value {
-	// TODO(≥go1.10): Workaround for reflect bug (https://golang.org/issue/22143).
-	if !flags.AtLeastGo110 {
-		if v.Kind() == reflect.Interface && v.IsNil() && v.Type() != t {
-			return reflect.New(t).Elem()
-		}
-	}
-	return v
-}
-
 func (s *state) compareStruct(t reflect.Type, vx, vy reflect.Value) {
 	var addr bool
 	var vax, vay reflect.Value // Addressable versions of vx and vy

cmp/compare_test.go

@@ -1447,14 +1447,6 @@ func embeddedTests() []test {
 		return s
 	}
 
-	// TODO(≥go1.10): Workaround for reflect bug (https://golang.org/issue/21122).
-	wantPanicNotGo110 := func(s string) string {
-		if !flags.AtLeastGo110 {
-			return ""
-		}
-		return s
-	}
-
 	return []test{{
 		label:     label + "/ParentStructA/PanicUnexported1",
 		x:         ts.ParentStructA{},
@@ -1745,8 +1737,7 @@ func embeddedTests() []test {
 		label:     label + "/ParentStructG/PanicUnexported1",
 		x:         ts.ParentStructG{},
 		y:         ts.ParentStructG{},
-		wantPanic: wantPanicNotGo110("cannot handle unexported field"),
-		wantEqual: !flags.AtLeastGo110,
+		wantPanic: "cannot handle unexported field",
 		reason:    "ParentStructG has unexported fields",
 	}, {
 		label: label + "/ParentStructG/Ignored",
@@ -1836,8 +1827,7 @@ func embeddedTests() []test {
 		label:     label + "/ParentStructI/PanicUnexported1",
 		x:         ts.ParentStructI{},
 		y:         ts.ParentStructI{},
-		wantPanic: wantPanicNotGo110("cannot handle unexported field"),
-		wantEqual: !flags.AtLeastGo110,
+		wantPanic: "cannot handle unexported field",
 		reason:    "ParentStructI has unexported fields",
 	}, {
 		label: label + "/ParentStructI/Ignored1",