むかーしの記述 https://github.com/apple/swift/blob/9b42d1b273cb6e5554bb717ab31341f1db65689b/include/swift/AST/Expr.h#L121-L125
/// TupleExpr - Parenthesized expressions like '(a=x+x)' and '(x, y, 4)'. Tuple /// types automatically decay if they have a single element, this means that /// single element tuple literals, such as "(4)", will exist in the AST, but /// have a result type that is the same as the input operand type. /// /// When a tuple element is formed with a default value for the type, the /// corresponding SubExpr element will be null. class TupleExpr : public Expr {
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