syntax: support array elements without values

For example:

	declare -A x=([index]=)

Fixes mvdan#361.

syntax/filetests_test.go

@@ -4008,6 +4008,19 @@ var fileTests = []testCase{
 			}}},
 		}}},
 	},
+	{
+		Strs: []string{
+			"a=([x]= [y]=)",
+			"a=(\n[x]=\n[y]=\n)",
+		},
+		bash: &CallExpr{Assigns: []*Assign{{
+			Name: lit("a"),
+			Array: &ArrayExpr{Elems: []*ArrayElem{
+				{Index: litWord("x")},
+				{Index: litWord("y")},
+			}},
+		}}},
+	},
 	{
 		Strs:   []string{"a]b"},
 		common: litStmt("a]b"),
@@ -4579,7 +4592,9 @@ func clearPosRecurse(tb testing.TB, src string, v interface{}) {
 		if x.Index != nil {
 			recurse(x.Index)
 		}
-		recurse(x.Value)
+		if x.Value != nil {
+			recurse(x.Value)
+		}
 	case *ExtGlob:
 		setPos(&x.OpPos, x.Op.String())
 		checkSrc(posAddCol(x.End(), -1), ")")

syntax/nodes.go

@@ -756,8 +756,12 @@ func (a *ArrayExpr) Pos() Pos { return a.Lparen }
 func (a *ArrayExpr) End() Pos { return posAddCol(a.Rparen, 1) }
 
 // ArrayElem represents a Bash array element.
+//
+// Index can be nil; for example, declare -a x=(value).
+// Value can be nil; for example, declare -A x=([index]=).
+// Finally, neither can be nil; for example, declare -A x=([index]=value)
 type ArrayElem struct {
-	Index    ArithmExpr // [i]=, ["k"]=
+	Index    ArithmExpr
 	Value    *Word
 	Comments []Comment
 }
@@ -768,7 +772,12 @@ func (a *ArrayElem) Pos() Pos {
 	}
 	return a.Value.Pos()
 }
-func (a *ArrayElem) End() Pos { return a.Value.End() }
+func (a *ArrayElem) End() Pos {
+	if a.Value != nil {
+		return a.Value.End()
+	}
+	return posAddCol(a.Index.Pos(), 1)
+}
 
 // ExtGlob represents a Bash extended globbing expression. Note that these are
 // parsed independently of whether shopt has been called or not.

syntax/parser.go

@@ -1609,11 +1609,16 @@ func (p *Parser) getAssign(needEqual bool) *Assign {
 				p.follow(left, `"[x]"`, assgn)
 			}
 			if ae.Value = p.getWord(); ae.Value == nil {
-				if p.tok == leftParen {
+				switch p.tok {
+				case leftParen:
 					p.curErr("arrays cannot be nested")
+					return nil
+				case _Newl, rightParen, leftBrack:
+					// TODO: support [index]=[
+				default:
+					p.curErr("array element values must be words")
+					break
 				}
-				p.curErr("array element values must be words")
-				break
 			}
 			if len(p.accComs) > 0 {
 				c := p.accComs[0]

syntax/printer.go

@@ -788,7 +788,9 @@ func (p *Printer) elemJoin(elems []*ArrayElem, last []Comment) {
 		if p.wroteIndex(el.Index) {
 			p.WriteByte('=')
 		}
-		p.word(el.Value)
+		if el.Value != nil {
+			p.word(el.Value)
+		}
 		p.comments(left...)
 	}
 	if len(last) > 0 {

syntax/walk.go

@@ -201,7 +201,9 @@ func Walk(node Node, f func(Node) bool) {
 		if x.Index != nil {
 			Walk(x.Index, f)
 		}
-		Walk(x.Value, f)
+		if x.Value != nil {
+			Walk(x.Value, f)
+		}
 	case *ExtGlob:
 		Walk(x.Pattern, f)
 	case *ProcSubst: