3 files changed, 62 insertions, 2 deletions
diff --git a/spec/parser.go b/spec/parser.go
index 9c66bfb..efbd072 100644
--- a/spec/parser.go
+++ b/spec/parser.go
@@ -494,10 +494,14 @@ func (p *parser) parseParameter() *ParameterNode {
 			Group: g,
 			Pos:   pos,
 		}
-	default:
-		return nil
 	}
 	if p.consume(tokenKindExpantion) {
+		switch {
+		case param == nil:
+			raiseSyntaxError(p.pos.Row, synErrStrayExpOp)
+		case param.ID == "":
+			raiseSyntaxError(p.pos.Row, synErrInvalidExpOperand)
+		}
 		param.Expansion = true
 	}
 	return param
diff --git a/spec/parser_test.go b/spec/parser_test.go
index 15f1330..c7d43de 100644
--- a/spec/parser_test.go
+++ b/spec/parser_test.go
@@ -599,6 +599,60 @@ bar: "bar";
 			},
 		},
 		{
+			caption: "an expansion operator must be preceded by an identifier",
+			src: `
+s
+    : foo #ast ...
+    ;
+`,
+			synErr: synErrStrayExpOp,
+		},
+		{
+			caption: "an expansion operator must be preceded by an identifier",
+			src: `
+a
+    : foo #ast ... foo
+    ;
+`,
+			synErr: synErrStrayExpOp,
+		},
+		{
+			caption: "an expansion operator cannot be applied to a pattern",
+			src: `
+a
+    : "foo" #ast "foo"...
+    ;
+`,
+			synErr: synErrInvalidExpOperand,
+		},
+		{
+			caption: "an expansion operator cannot be applied to a string",
+			src: `
+a
+    : 'foo' #ast 'foo'...
+    ;
+`,
+			synErr: synErrInvalidExpOperand,
+		},
+		{
+			caption: "an expansion operator cannot be applied to an ordered symbol",
+			src: `
+a
+    : foo #ast $foo...
+    ;
+`,
+			synErr: synErrInvalidExpOperand,
+		},
+		{
+			caption: "an expansion operator cannot be applied to a directive group",
+			src: `
+a
+    : foo #ast ()...
+    ;
+`,
+			synErr: synErrInvalidExpOperand,
+		},
+		{
 			caption: "an AST has node positions",
 			src: `
 exp
diff --git a/spec/syntax_error.go b/spec/syntax_error.go
index cf64e75..db4c381 100644
--- a/spec/syntax_error.go
+++ b/spec/syntax_error.go
@@ -37,6 +37,8 @@ var (
 	synErrNoDirectiveName        = newSyntaxError("a directive needs a name")
 	synErrNoOrderedSymbolName    = newSyntaxError("an ordered symbol name is missing")
 	synErrUnclosedDirGroup       = newSyntaxError("a directive group must be closed by )")
+	synErrStrayExpOp             = newSyntaxError("an expansion operator ... must be preceded by an identifier")
+	synErrInvalidExpOperand      = newSyntaxError("an expansion operator ... can be applied to only an identifier")
 	synErrSemicolonNoNewline     = newSyntaxError("a semicolon must be followed by a newline")
 	synErrFragmentNoPattern      = newSyntaxError("a fragment needs one pattern element")
 )