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Tree grammars match subtrees constructed by a parser. In order to create an output template using a tree grammar, the tree grammar must know about the token stream from which its trees were created. If you rewrite the tree, all of the token indexes will be incorrect. If a node for ID was originally created from a token at index 32, but you move it around in the tree, this pretty much preventing ANTLR from creating a valid string derived from the input. So, Automatic construction of templates only works if you have not manipulated the tree.

ANTLR tree grammar rules compute the automatic template by asking for the default template as with a parser grammar. The elements inserted into the output templates are a sequence of token objects including the whitespace object. Each subtree root has a start and stop index into the token stream, which naturally includes all of the off channel tokens in between the real tokens. The automatic templates do not include whitespace before or after the tokens associated with the nodes matched by a treat member rule.

prog : ^(PROGRAM (d+=decl)+) -> file(decls={$d}) ;
decl : ^(DECL type ID) ; // auto creates template from input tokens for decl

A warning

Each tree grammar rule knows the text from which the associated subtree was created but only if the subtree has a single root. The following rule, because it has a single root, gives ANTLR a problem.

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decl : ^(DECL type ID) ;