go.tools/go/types: set array type even if length is invalid

This avoids some follow-up errors due to invalid
array declarations.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/57640043

go/types/testdata/const0.src

@@ -234,7 +234,7 @@ const (
 	_b1 = assert(iota + iota2 == 5)
 	_b2 = len([iota]int{}) // iota may appear in a type!
 	_b3 = assert(_b2 == 2)
-	_b4 = len(A /* ERROR "invalid composite literal" */ {})
+	_b4 = len(A{})
 )
 
 type A [iota /* ERROR "cannot use iota" */ ]int

go/types/typexpr.go

@@ -245,30 +245,12 @@ func (check *checker) typInternal(e ast.Expr, def *Named, cycleOk bool) Type {
 
 	case *ast.ArrayType:
 		if e.Len != nil {
-			var x operand
-			check.expr(&x, e.Len)
-			if x.mode != constant {
-				if x.mode != invalid {
-					check.errorf(x.pos(), "array length %s must be constant", &x)
-				}
-				break
-			}
-			if !x.isInteger() {
-				check.errorf(x.pos(), "array length %s must be integer", &x)
-				break
-			}
-			n, ok := exact.Int64Val(x.val)
-			if !ok || n < 0 {
-				check.errorf(x.pos(), "invalid array length %s", &x)
-				break
-			}
-
 			typ := new(Array)
 			if def != nil {
 				def.underlying = typ
 			}
 
-			typ.len = n
+			typ.len = check.arrayLength(e.Len)
 			typ.elem = check.typ(e.Elt, nil, cycleOk)
 			return typ
 
@@ -383,6 +365,27 @@ func (check *checker) typOrNil(e ast.Expr) Type {
 	return Typ[Invalid]
 }
 
+func (check *checker) arrayLength(e ast.Expr) int64 {
+	var x operand
+	check.expr(&x, e)
+	if x.mode != constant {
+		if x.mode != invalid {
+			check.errorf(x.pos(), "array length %s must be constant", &x)
+		}
+		return 0
+	}
+	if !x.isInteger() {
+		check.errorf(x.pos(), "array length %s must be integer", &x)
+		return 0
+	}
+	n, ok := exact.Int64Val(x.val)
+	if !ok || n < 0 {
+		check.errorf(x.pos(), "invalid array length %s", &x)
+		return 0
+	}
+	return n
+}
+
 func (check *checker) collectParams(scope *Scope, list *ast.FieldList, variadicOk bool) (params []*Var, isVariadic bool) {
 	if list == nil {
 		return