reflect: support Len and Cap on pointer-to-array Value

Fixes #52411

Change-Id: I2fd13a453622992c52d49aade7cd058cfc8a77ca
GitHub-Last-Rev: d5987c2ec817ebd01d9e1adb3bd2e75274dbbabd
GitHub-Pull-Request: golang/go#52423
Reviewed-on: https://go-review.googlesource.com/c/go/+/400954
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/reflect/all_test.go

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 72d01c7..9eb01bd 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -7913,3 +7913,69 @@
 		sinkAll.RawInt = sourceAll.SliceAny.Cap()
 	}
 }
+
+func TestValue_Cap(t *testing.T) {
+	a := &[3]int{1, 2, 3}
+	v := ValueOf(a)
+	if v.Cap() != cap(a) {
+		t.Errorf("Cap = %d want %d", v.Cap(), cap(a))
+	}
+
+	a = nil
+	v = ValueOf(a)
+	if v.Cap() != cap(a) {
+		t.Errorf("Cap = %d want %d", v.Cap(), cap(a))
+	}
+
+	getError := func(f func()) (errorStr string) {
+		defer func() {
+			e := recover()
+			if str, ok := e.(string); ok {
+				errorStr = str
+			}
+		}()
+		f()
+		return
+	}
+	e := getError(func() {
+		var ptr *int
+		ValueOf(ptr).Cap()
+	})
+	wantStr := "reflect: call of reflect.Value.Cap on ptr to non-array Value"
+	if e != wantStr {
+		t.Errorf("error is %q, want %q", e, wantStr)
+	}
+}
+
+func TestValue_Len(t *testing.T) {
+	a := &[3]int{1, 2, 3}
+	v := ValueOf(a)
+	if v.Len() != len(a) {
+		t.Errorf("Len = %d want %d", v.Len(), len(a))
+	}
+
+	a = nil
+	v = ValueOf(a)
+	if v.Len() != len(a) {
+		t.Errorf("Len = %d want %d", v.Len(), len(a))
+	}
+
+	getError := func(f func()) (errorStr string) {
+		defer func() {
+			e := recover()
+			if str, ok := e.(string); ok {
+				errorStr = str
+			}
+		}()
+		f()
+		return
+	}
+	e := getError(func() {
+		var ptr *int
+		ValueOf(ptr).Len()
+	})
+	wantStr := "reflect: call of reflect.Value.Len on ptr to non-array Value"
+	if e != wantStr {
+		t.Errorf("error is %q, want %q", e, wantStr)
+	}
+}

src/reflect/value.go

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 6b5ebfa..f92fa16 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -1144,7 +1144,7 @@
 }
 
 // Cap returns v's capacity.
-// It panics if v's Kind is not Array, Chan, or Slice.
+// It panics if v's Kind is not Array, Chan, Slice or pointer to Array.
 func (v Value) Cap() int {
 	// capNonSlice is split out to keep Cap inlineable for slice kinds.
 	if v.kind() == Slice {
@@ -1160,6 +1160,11 @@
 		return v.typ.Len()
 	case Chan:
 		return chancap(v.pointer())
+	case Ptr:
+		if v.typ.Elem().Kind() == Array {
+			return v.typ.Elem().Len()
+		}
+		panic("reflect: call of reflect.Value.Cap on ptr to non-array Value")
 	}
 	panic(&ValueError{"reflect.Value.Cap", v.kind()})
 }
@@ -1600,7 +1605,7 @@
 }
 
 // Len returns v's length.
-// It panics if v's Kind is not Array, Chan, Map, Slice, or String.
+// It panics if v's Kind is not Array, Chan, Map, Slice, String, or pointer to Array.
 func (v Value) Len() int {
 	// lenNonSlice is split out to keep Len inlineable for slice kinds.
 	if v.kind() == Slice {
@@ -1621,6 +1626,11 @@
 	case String:
 		// String is bigger than a word; assume flagIndir.
 		return (*unsafeheader.String)(v.ptr).Len
+	case Ptr:
+		if v.typ.Elem().Kind() == Array {
+			return v.typ.Elem().Len()
+		}
+		panic("reflect: call of reflect.Value.Len on ptr to non-array Value")
 	}
 	panic(&ValueError{"reflect.Value.Len", v.kind()})
 }