seen: check for explicit tables on dotted keys (#639)

The TOML spec is being clarified to say that dotted keys "define" their
intermediate tables. Therefore the seen tracker needs to verify that none of
them reference an explicit table.

Also added a missing seen expression check for key-values parsed as part of a
table section.

See https://github.com/toml-lang/toml/issues/846

internal/tracker/seen.go

@@ -106,7 +106,6 @@ func (s *SeenTracker) create(parentIdx int, name []byte, kind keyKind, explicit
 // that have been seen in previous calls, and validates that types are consistent.
 func (s *SeenTracker) CheckExpression(node *ast.Node) error {
 	if s.entries == nil {
-		// s.entries = make([]entry, 0, 8)
 		// Skip ID = 0 to remove the confusion between nodes whose parent has
 		// id 0 and root nodes (parent id is 0 because it's the zero value).
 		s.nextID = 1
@@ -221,6 +220,9 @@ func (s *SeenTracker) checkKeyValue(node *ast.Node) error {
 			if s.entries[idx].kind != tableKind {
 				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), s.entries[idx].kind)
 			}
+			if s.entries[idx].explicit {
+				return fmt.Errorf("toml: cannot redefine table %s that has already been explicitly defined", string(k))
+			}
 		} else {
 			idx = s.create(parentIdx, k, tableKind, false)
 		}