go/parser: report error for var/const decls with missing init exprs

Fixes #9639.

Change-Id: I311045d3df26b29b9380c159ef4727e85650d13b
Reviewed-on: https://go-review.googlesource.com/3211Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/parser/parser.go

@@ -2228,6 +2228,7 @@ func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota
 		defer un(trace(p, keyword.String()+"Spec"))
 	}
 
+	pos := p.pos
 	idents := p.parseIdentList()
 	typ := p.tryType()
 	var values []ast.Expr
@@ -2238,6 +2239,17 @@ func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota
 	}
 	p.expectSemi() // call before accessing p.linecomment
 
+	switch keyword {
+	case token.VAR:
+		if typ == nil && values == nil {
+			p.error(pos, "missing variable type or initialization")
+		}
+	case token.CONST:
+		if values == nil && (iota == 0 || typ != nil) {
+			p.error(pos, "missing constant value")
+		}
+	}
+
 	// Go spec: The scope of a constant or variable identifier declared inside
 	// a function begins at the end of the ConstSpec or VarSpec and ends at
 	// the end of the innermost containing block.

src/go/parser/short_test.go

@@ -43,6 +43,7 @@ var valids = []string{
 	`package p; func _() { map[int]int{}[0]++; map[int]int{}[0] += 1 }`,
 	`package p; func _(x interface{f()}) { interface{f()}(x).f() }`,
 	`package p; func _(x chan int) { chan int(x) <- 0 }`,
+	`package p; const (x = 0; y; z)`, // issue 9639
 }
 
 func TestValid(t *testing.T) {
@@ -97,7 +98,11 @@ var invalids = []string{
 	`package p; func f() { go f /* ERROR HERE "function must be invoked" */ }`,
 	`package p; func f() { defer func() {} /* ERROR HERE "function must be invoked" */ }`,
 	`package p; func f() { go func() { func() { f(x func /* ERROR "expected '\)'" */ (){}) } } }`,
-	`package p; func f() (a b string /* ERROR "expected '\)'" */ , ok bool) // issue 8656`,
+	`package p; func f() (a b string /* ERROR "expected '\)'" */ , ok bool)`,         // issue 8656
+	`package p; var x /* ERROR "missing variable type or initialization" */ , y, z;`, // issue 9639
+	`package p; const x /* ERROR "missing constant value" */ ;`,                      // issue 9639
+	`package p; const x /* ERROR "missing constant value" */ int;`,                   // issue 9639
+	`package p; const (x = 0; y; z /* ERROR "missing constant value" */ int);`,       // issue 9639
 }
 
 func TestInvalid(t *testing.T) {