Introduce `TypelessStructLiteralArg` and `NewConstructorArg` interfaces

[makenowjust]

Close #119 Close #177

~TypelessStructValue (#177) and NewConstructorValue (#119) share their struct shapes. Then, this PR introduces a new struct named AsExpr for expression AS name-kind values and replaces them with it.~

~I believe this is not a hasty generalization because these struct shapes are exactly the same, and they are used for common semantics. However, I'm not sure AsExpr is the best name. If you have a better name, please tell me it!~

~Thank you.~

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See https://github.com/cloudspannerecosystem/memefish/pull/183#issuecomment-2441417390.

[apstndb]

TypelessStructValue (#177) and NewConstructorValue (#119) share their struct shapes. Then, this PR introduces a new struct named AsExpr for expression AS name-kind values and replaces them with it.

I believe this is not a hasty generalization because these struct shapes are exactly the same, and they are used for common semantics.

I have concerned about there are undocumented differences in NEW constructors and typeless STRUCT literals.

https://github.com/google/zetasql/blob/194cd32b5d766d60e3ca442651d792c7fe54ea74/zetasql/parser/bison_parser.y#L8927-L8942

    | expression "AS" "(" path_expression ")"
      {
        // Do not parenthesize $4 because it is not really a parenthesized
        // path expression. The parentheses are just part of the syntax here.
        $$ = MAKE_NODE(ASTNewConstructorArg, @$, {$1, $4});
      }

I believe it is a syntax for proto extensions and it is not yet available in Cloud Spanner, but I think these semantics are not same. It may be a reason to distinguish between TypelessStructValue and NewConstructorValue.

spanner> SELECT NEW examples.shipping.`Order`("Japan" AS (shipping_address.country));
ERROR: spanner: code = "InvalidArgument", desc = "NEW constructor does not support proto extensions [at 1:50]\\nSELECT NEW examples.shipping.`Order`(\\\"Japan\\\" AS (shipping_address.country))\\n      

[apstndb]

This difference is natural because:

• NewConstructor is a syntax for PROTO type.
• TypelessStructLiteral is a syntax for STRUCT type.

[makenowjust]

IMHO, when expression AS (path) is needed, we can introduce a new interface NewConstructorArg, and a new struct ExprAsPath, so we can go this way even if STRUCT and NEW syntax are different.

[apstndb]

Personally I think it is acceptable to generalize them temporarily, even if their semantics are not same, But I am wondering if this generalization really have benefit.

we can introduce a new interface NewConstructorArg, and a new struct ExprAsPath

I think it can't be a general name because it is expression AS (path_expression), not expression AS path_expression. It seems that (path_expression) is restricted form of generalized_extension_path. In ZetaSQL, new_constructor_arg is really special structure with no other similar syntax, because its syntax is specialized to Protocol Buffers.

[apstndb]

I understood. We can add another interface when it is needed. NewConstructorArg won't be concrete struct so it is not needed to define.

[apstndb]

However, I'm not sure AsExpr is the best name. If you have a better name, please tell me it! Rename AsExpr to ExprAsName, and use AsAlias

IMHO, ExprAsName doesn't imply the name is optional, and there are other expr AS name structure(e.g. in * REPLACE(expr AS name, ...)). I think it can be confusing so more explicit name, like ExprOptName, may be prefered.

syntax	AS	ident	Rule in ZetaSQL	ast node in ZetaSQL	Parser func in memefish	Name in memefish
AS ident	required		opt_as_alias_with_required_as	ASTAlias	tryParseAsAlias()	AsAlias
[AS] ident	optional		opt_as_alias	ASTAlias	tryParseAsAlias()	AsAlias
expr	none	none		ASTExpression	ExprArg or Expr
expr AS ident	required	required	(star_replace_item, select_column_expr_with_as_alias, ...)	ASTExpressionWithAlias (not used externally?)
expr [AS ident]	required	optional	expression_with_opt_alias, (expression_using_argument, ...)	ASTExpressionWithOptAlias		ExprAsName?
expr [AS] ident	optional	required				Alias
expr [[AS] ident]	optional	optional	(select_column_expr, ...)	concrete nodes

(There are many variants.)

[makenowjust]

Finally, I conclude there is no better name for the struct expression [AS name]. That is, we should not introduce such a struct.

Therefore, we introduce new interfaces TypelessStructLiteralArg and NewConstructorArg. Currently, two interfaces have the same implementations and parsers, but they are separated for future extensibility.