exp/types/staging: index and slice type checks

Also: handle assignments to the blank identifier.

R=rsc
CC=golang-dev
https://golang.org/cl/6658050

src/pkg/exp/types/staging/const.go

@@ -165,6 +165,22 @@ func isZeroConst(x interface{}) bool {
 	return ok && i == 0
 }
 
+// isNegConst reports whether the value of constant x is < 0.
+// x must be a non-complex numeric value.
+//
+func isNegConst(x interface{}) bool {
+	switch x := x.(type) {
+	case int64:
+		return x < 0
+	case *big.Int:
+		return x.Sign() < 0
+	case *big.Rat:
+		return x.Sign() < 0
+	}
+	unreachable()
+	return false
+}
+
 // isRepresentableConst reports whether the value of constant x can
 // be represented as a value of the basic type Typ[as] without loss
 // of precision.

src/pkg/exp/types/staging/expr.go

@@ -397,11 +397,39 @@ func (check *checker) binary(x, y *operand, op token.Token, hint Type) {
 	// x.typ is unchanged
 }
 
-func (check *checker) index(x *operand, e ast.Expr, iota int) {
-	check.expr(x, e, nil, iota)
-	if !isInteger(x.typ) {
-		check.errorf(x.pos(), "array index %s must be integer", x)
+// index checks an index expression for validity. If length >= 0, it is the upper
+// bound for the index. The result is a valid constant index >= 0, or a negative
+// value.
+//
+func (check *checker) index(index ast.Expr, length int64, iota int) int64 {
+	var x operand
+	var i int64 // index value, valid if >= 0
+
+	check.expr(&x, index, nil, iota)
+	if !x.isInteger() {
+		check.errorf(x.pos(), "index %s must be integer", &x)
+		return -1
+	}
+	if x.mode != constant {
+		return -1 // we cannot check more
+	}
+	// x.mode == constant and the index value must be >= 0
+	if isNegConst(x.val) {
+		check.errorf(x.pos(), "index %s must not be negative", &x)
+		return -1
+	}
+	var ok bool
+	if i, ok = x.val.(int64); !ok {
+		// index value doesn't fit into an int64
+		i = length // trigger out of bounds check below if we know length (>= 0)
 	}
+
+	if length >= 0 && i >= length {
+		check.errorf(x.pos(), "index %s is out of bounds (>= %d)", &x, length)
+		return -1
+	}
+
+	return i
 }
 
 func (check *checker) callRecord(x *operand) {
@@ -553,20 +581,34 @@ func (check *checker) exprOrType(x *operand, e ast.Expr, hint Type, iota int, cy
 		goto Error
 
 	case *ast.IndexExpr:
-		var index operand
 		check.expr(x, e.X, hint, iota)
+
+		valid := false
+		length := int64(-1) // valid if >= 0
 		switch typ := underlying(x.typ).(type) {
+		case *Basic:
+			if isString(typ) {
+				valid = true
+				if x.mode == constant {
+					length = int64(len(x.val.(string)))
+				}
+				// an indexed string always yields a byte value
+				// (not a constant) even if the string and the
+				// index are constant
+				x.mode = value
+				x.typ = Typ[Byte]
+			}
+
 		case *Array:
-			check.index(&index, e.Index, iota)
-			if x.mode == constant {
-				// TODO(gri) range check
+			valid = true
+			length = typ.Len
+			if x.mode != variable {
+				x.mode = value
 			}
-			// TODO(gri) only variable if array is variable
-			x.mode = variable
 			x.typ = typ.Elt
 
 		case *Slice:
-			check.index(&index, e.Index, iota)
+			valid = true
 			x.mode = variable
 			x.typ = typ.Elt
 
@@ -574,43 +616,75 @@ func (check *checker) exprOrType(x *operand, e ast.Expr, hint Type, iota int, cy
 			// TODO(gri) check index type
 			x.mode = variable
 			x.typ = typ.Elt
+			return
+		}
 
-		default:
-			check.invalidOp(e.Pos(), "cannot index %s", x.typ)
+		if !valid {
+			check.invalidOp(x.pos(), "cannot index %s", x)
 			goto Error
 		}
 
+		if e.Index == nil {
+			check.invalidAST(e.Pos(), "missing index expression for %s", x)
+			return
+		}
+
+		check.index(e.Index, length, iota)
+		// ok to continue
+
 	case *ast.SliceExpr:
-		var lo, hi operand
 		check.expr(x, e.X, hint, iota)
+
+		valid := false
+		length := int64(-1) // valid if >= 0
+		switch typ := underlying(x.typ).(type) {
+		case *Basic:
+			if isString(typ) {
+				valid = true
+				if x.mode == constant {
+					length = int64(len(x.val.(string))) + 1 // +1 for slice
+				}
+				// a sliced string always yields a string value
+				// of the same type as the original string (not
+				// a constant) even if the string and the indexes
+				// are constant
+				x.mode = value
+				// x.typ doesn't change
+			}
+
+		case *Array:
+			valid = true
+			length = typ.Len + 1 // +1 for slice
+			if x.mode != variable {
+				check.invalidOp(x.pos(), "cannot slice %s (value not addressable)", x)
+				goto Error
+			}
+			x.typ = &Slice{Elt: typ.Elt}
+
+		case *Slice:
+			valid = true
+			x.mode = variable
+			// x.typ doesn't change
+		}
+
+		if !valid {
+			check.invalidOp(x.pos(), "cannot slice %s", x)
+			goto Error
+		}
+
+		var lo int64
 		if e.Low != nil {
-			check.index(&lo, e.Low, iota)
-		} else {
-			lo.mode = constant
-			lo.expr = nil // TODO(gri) should not use nil here
-			lo.typ = Typ[UntypedInt]
-			lo.val = zeroConst
+			lo = check.index(e.Low, length, iota)
 		}
+
+		var hi int64 = length
 		if e.High != nil {
-			check.index(&hi, e.High, iota)
-		} else {
-			unimplemented()
+			hi = check.index(e.High, length, iota)
 		}
-		switch typ := x.typ.(type) {
-		case *Array:
-			unimplemented()
-		case *Slice:
-			assert(x.mode == variable)
-			// x.typ does not change
-		case *Pointer:
-			if typ, ok := underlying(typ.Base).(*Array); ok {
-				// TODO(gri) array slice
-				_ = typ
-			}
-			unimplemented()
-		default:
-			check.invalidOp(e.Pos(), "cannot slice %s", x.typ)
-			goto Error
+
+		if hi >= 0 && lo > hi {
+			check.errorf(e.Low.Pos(), "inverted slice range: %d > %d", lo, hi)
+			// ok to continue
 		}
 
 	case *ast.TypeAssertExpr:

src/pkg/exp/types/staging/stmt.go

@@ -108,6 +108,15 @@ func (check *checker) assignment(lhs ast.Expr, x *operand, decl bool) {
 }
 
 func (check *checker) assign1to1(lhs, rhs ast.Expr, decl bool, iota int) {
+	ident, _ := lhs.(*ast.Ident)
+
+	if ident != nil && ident.Name == "_" {
+		// anything can be assigned to a blank identifier - check rhs only
+		var x operand
+		check.expr(&x, rhs, nil, iota)
+		return
+	}
+
 	if !decl {
 		// regular assignment - start with lhs[0] to obtain a type hint
 		var z operand
@@ -127,8 +136,7 @@ func (check *checker) assign1to1(lhs, rhs ast.Expr, decl bool, iota int) {
 	}
 
 	// declaration - rhs may or may not be typed yet
-	ident, ok := lhs.(*ast.Ident)
-	if !ok {
+	if ident == nil {
 		check.errorf(lhs.Pos(), "cannot declare %s", lhs)
 		return
 	}

src/pkg/exp/types/staging/testdata/decls1.src

@@ -37,7 +37,7 @@ var (
 	s4 = s + t
 	s5 = s /* ERROR "invalid operation" */ / t
 	s6 = array[t1]
-	s7 = array[x /* ERROR "array index" */]
+	s7 = array[x /* ERROR "index" */]
 	s8 = &a
 	s10 = &42 /* ERROR "cannot take address" */
 	s11 = &v
@@ -56,7 +56,7 @@ var (
 	t4 string = s + t
 	t5 string = s /* ERROR "invalid operation" */ / t
 	t6 byte = array[t1]
-	t7 byte = array[x /* ERROR "array index" */]
+	t7 byte = array[x /* ERROR "index" */]
 	t8 *int = & /* ERROR "cannot assign" */ a
 	t10 *int = &42 /* ERROR "cannot take address" */
 	t11 *complex64 = &v

src/pkg/exp/types/staging/testdata/expr3.src

@@ -2,10 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// shifts
+// various expressions
 
 package expr3
 
+// TODO(gri) Move the code below into function "shifts" once we check
+//           declarations with initilizations inside functions.
 var (
 	i0 int
 	u0 uint
@@ -41,3 +43,73 @@ var (
 	v float32 = 1 /* ERROR "must be integer" */ <<s   // illegal: 1 has type float32, cannot shift
 	w int64 = 1.0<<33  // 1.0<<33 is a constant shift expression
 )
+
+// TODO(gri) The error messages below depond on adjusting the spec
+//           to reflect what gc is doing at the moment (the spec
+//           asks for run-time errors at the moment - see issue 4231).
+//
+func indexes() {
+	_ = 1 /* ERROR "cannot index" */ [0]
+	_ = indexes /* ERROR "cannot index" */ [0]
+	_ = ( /* ERROR "cannot slice" */ 12 + 3)[1:2]
+
+	var a [10]int
+	_ = a[true /* ERROR "must be integer" */ ]
+	_ = a["foo" /* ERROR "must be integer" */ ]
+	_ = a[1.1 /* ERROR "must be integer" */ ]
+	_ = a[1.0]
+	_ = a[- /* ERROR "index .* negative" */ 1]
+	_ = a[- /* ERROR "index .* negative" */ 1 :]
+	_ = a[: - /* ERROR "index .* negative" */ 1]
+	var a0 int
+	a0 = a[0]
+	var a1 int32
+	a1 = a /* ERROR "cannot assign" */ [1] 
+	_ = a[9]
+	_ = a[10 /* ERROR "index .* out of bounds" */ ]
+	_ = a[10:]
+	_ = a[:10]
+	_ = a[10:10]
+	_ = a[11 /* ERROR "index .* out of bounds" */ :]
+	_ = a[: 11 /* ERROR "index .* out of bounds" */ ]
+
+	var b [0]int
+	_ = b[0 /* ERROR "index .* out of bounds" */ ]
+	_ = b[:]
+	_ = b[0:]
+	_ = b[:0]
+	_ = b[0:0]
+
+	var s []int
+	_ = s[- /* ERROR "index .* negative" */ 1]
+	_ = s[- /* ERROR "index .* negative" */ 1 :]
+	_ = s[: - /* ERROR "index .* negative" */ 1]
+	_ = s[0]
+	_ = s[1 : 2]
+	_ = s[2 /* ERROR "inverted slice range" */ : 1]
+	_ = s[2 :]
+
+	var t string
+	_ = t[- /* ERROR "index .* negative" */ 1]
+	_ = t[- /* ERROR "index .* negative" */ 1 :]
+	_ = t[: - /* ERROR "index .* negative" */ 1]
+	var t0 byte
+	t0 = t[0]
+	var t1 rune
+	t1 = t /* ERROR "cannot assign" */ [2]
+	_ = ("foo" + "bar")[5]
+	_ = ("foo" + "bar")[6 /* ERROR "index .* out of bounds" */ ]
+
+	const c = "foo"
+	_ = c[- /* ERROR "index .* negative" */ 1]
+	_ = c[- /* ERROR "index .* negative" */ 1 :]
+	_ = c[: - /* ERROR "index .* negative" */ 1]
+	var c0 byte
+	c0 = c[0]
+	var c2 float32
+	c2 = c /* ERROR "cannot assign" */ [2]
+	_ = c[3 /* ERROR "index .* out of bounds" */ ]
+	_ = ""[0 /* ERROR "index .* out of bounds" */ ]
+
+	_ = s[1<<30] // no compile-time error here
+}