reflect: implement CanInterface and fix string Index()

This commit fixes two related issues:

 1. CanInterface was unimplemented. It now uses the same check as is
    used in Interface() itself.
    This issue led to https://github.com/tinygo-org/tinygo/issues/3033
 2. Allow making an interface out of a string char element.

Doing this in one commit (instead of two) because they are shown to be
correct with the same tests.

src/reflect/value.go

@@ -168,8 +168,7 @@ func (v Value) IsValid() bool {
 }
 
 func (v Value) CanInterface() bool {
-	// No Value types of private data can be constructed at the moment.
-	return true
+	return v.isExported()
 }
 
 func (v Value) CanAddr() bool {
@@ -502,6 +501,9 @@ func (v Value) Index(i int) Value {
 		// Extract a character from a string.
 		// A string is never stored directly in the interface, but always as a
 		// pointer to the string value.
+		// Keeping valueFlagExported if set, but don't set valueFlagIndirect
+		// otherwise CanSet will return true for string elements (which is bad,
+		// strings are read-only).
 		s := *(*stringHeader)(v.value)
 		if uint(i) >= uint(s.len) {
 			panic("reflect: string index out of range")
@@ -509,6 +511,7 @@ func (v Value) Index(i int) Value {
 		return Value{
 			typecode: Uint8.basicType(),
 			value:    unsafe.Pointer(uintptr(*(*uint8)(unsafe.Pointer(uintptr(s.data) + uintptr(i))))),
+			flags:    v.flags & valueFlagExported,
 		}
 	case Array:
 		// Extract an element from the array.

testdata/reflect.go

@@ -415,6 +415,9 @@ func showValue(rv reflect.Value, indent string) {
 	if rv.CanAddr() {
 		print(" addrable=true")
 	}
+	if !rv.CanInterface() {
+		print(" caninterface=false")
+	}
 	if !rt.Comparable() {
 		print(" comparable=false")
 	}

testdata/reflect.txt

@@ -236,7 +236,7 @@ reflect type: struct
   tag: 
   embedded: true
   exported: false
-  reflect type: interface
+  reflect type: interface caninterface=false
     interface
     nil: true
 reflect type: struct
@@ -245,19 +245,19 @@ reflect type: struct
   tag: 
   embedded: false
   exported: false
-  reflect type: uint8
+  reflect type: uint8 caninterface=false
     uint: 42
   field: 1 b
   tag: 
   embedded: false
   exported: false
-  reflect type: int16
+  reflect type: int16 caninterface=false
     int: 321
   field: 2 c
   tag: 
   embedded: false
   exported: false
-  reflect type: int8
+  reflect type: int8 caninterface=false
     int: 123
 reflect type: struct comparable=false
   struct: 5
@@ -265,45 +265,45 @@ reflect type: struct comparable=false
   tag: foo:"bar"
   embedded: false
   exported: false
-  reflect type: int
+  reflect type: int caninterface=false
     int: 5
   field: 1 some
   tag: 
   embedded: false
   exported: false
-  reflect type: struct
+  reflect type: struct caninterface=false
     struct: 2
     field: 0 X
     tag: 
     embedded: false
     exported: true
-    reflect type: int16
+    reflect type: int16 caninterface=false
       int: -5
     field: 1 Y
     tag: 
     embedded: false
     exported: true
-    reflect type: int16
+    reflect type: int16 caninterface=false
       int: 3
   field: 2 zero
   tag: 
   embedded: false
   exported: false
-  reflect type: struct
+  reflect type: struct caninterface=false
     struct: 0
   field: 3 buf
   tag: 
   embedded: false
   exported: false
-  reflect type: slice comparable=false
+  reflect type: slice caninterface=false comparable=false
     slice: uint8 2 2
     pointer: true
     nil: false
     indexing: 0
-    reflect type: uint8 addrable=true
+    reflect type: uint8 addrable=true caninterface=false
       uint: 71
     indexing: 1
-    reflect type: uint8 addrable=true
+    reflect type: uint8 addrable=true caninterface=false
       uint: 111
   field: 4 Buf
   tag: 
@@ -325,14 +325,14 @@ reflect type: ptr
     tag: description:"chain"
     embedded: false
     exported: false
-    reflect type: ptr addrable=true
+    reflect type: ptr addrable=true caninterface=false
       pointer: false struct
       nil: true
     field: 1 foo
     tag: 
     embedded: false
     exported: false
-    reflect type: int addrable=true
+    reflect type: int addrable=true caninterface=false
       int: 42
 reflect type: slice comparable=false
   slice: interface 3 3