Prohibit ambiguous symbol in an #ast directive

grammar/grammar.go

@@ -701,6 +701,7 @@ func (b *GrammarBuilder) genProductionsAndActions(root *spec.RootNode, symTabAnd
 		for _, alt := range prod.RHS {
 			altSyms := make([]symbol, len(alt.Elements))
 			offsets := map[string]int{}
+			ambiguousIDOffsets := map[string]struct{}{}
 			for i, elem := range alt.Elements {
 				var sym symbol
 				if elem.Pattern != "" {
@@ -756,7 +757,15 @@ func (b *GrammarBuilder) genProductionsAndActions(root *spec.RootNode, symTabAnd
 				// A symbol having a label can be specified by both the label and the symbol name.
 				// So record the symbol's position, whether or not it has a label.
 				if elem.ID != "" {
-					offsets[elem.ID] = i
+					if _, exist := offsets[elem.ID]; exist {
+						// When the same symbol appears multiple times in an alternative, the symbol is ambiguous. When we need
+						// to specify the symbol in a directive, we cannot use the name of the ambiguous symbol. Instead, specify
+						// a label to resolve the ambiguity.
+						delete(offsets, elem.ID)
+						ambiguousIDOffsets[elem.ID] = struct{}{}
+					} else {
+						offsets[elem.ID] = i
+					}
 				}
 			}
 
@@ -842,6 +851,16 @@ func (b *GrammarBuilder) genProductionsAndActions(root *spec.RootNode, symTabAnd
 							continue LOOP_RHS
 						}
 
+						if _, ambiguous := ambiguousIDOffsets[param.ID]; ambiguous {
+							b.errs = append(b.errs, &verr.SpecError{
+								Cause:  semErrAmbiguousElem,
+								Detail: fmt.Sprintf("'%v' is ambiguous", param.ID),
+								Row:    param.Pos.Row,
+								Col:    param.Pos.Col,
+							})
+							continue LOOP_RHS
+						}
+
 						offset, ok := offsets[param.ID]
 						if !ok {
 							b.errs = append(b.errs, &verr.SpecError{

grammar/grammar_test.go

@@ -1023,6 +1023,34 @@ foo
 `,
 			errs: []*SemanticError{semErrDuplicateElem},
 		},
+		{
+			caption: "symbol `foo` is ambiguous because it appears in an alternative twice",
+			specSrc: `
+%name test
+
+s
+    : foo foo #ast foo
+    ;
+
+foo
+    : 'foo';
+`,
+			errs: []*SemanticError{semErrAmbiguousElem},
+		},
+		{
+			caption: "symbol `foo` is ambiguous because it appears in an alternative twice, even if one of them has a label",
+			specSrc: `
+%name test
+
+s
+    : foo@x foo #ast foo
+    ;
+
+foo
+    : 'foo';
+`,
+			errs: []*SemanticError{semErrAmbiguousElem},
+		},
 		{
 			caption: "the expansion operator cannot be applied to a terminal symbol",
 			specSrc: `

grammar/semantic_error.go

@@ -35,6 +35,7 @@ var (
 	semErrDirInvalidParam     = newSemanticError("invalid parameter")
 	semErrDuplicateDir        = newSemanticError("a directive must not be duplicated")
 	semErrDuplicateElem       = newSemanticError("duplicate element")
+	semErrAmbiguousElem       = newSemanticError("ambiguous element")
 	semErrInvalidProdDir      = newSemanticError("invalid production directive")
 	semErrInvalidAltDir       = newSemanticError("invalid alternative directive")
 )