Add :test:function, :test:select, and :test:format function definitions

test/README.md

@@ -35,3 +35,104 @@ Some examples of test harnesses using these tests, from the source repository:
 - [CST parse/stringify tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/cst/cst.test.ts)
 - [Data model stringify tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/data-model/stringify.test.ts)
 - [Formatting tests](https://github.com/messageformat/messageformat/blob/11c95dab2b25db8454e49ff4daadb817e1d5b770/packages/mf2-messageformat/src/messageformat.test.ts)
+
+## Test Functions
+
+As the behaviour of some of the default registry functions
+such as `:number` and `:datetime`
+is dependent on locale-specific data and may vary between implementations,
+the following functions are defined for **test use only**:
+
+### `:test:function`
+
+This function is valid both as a selector and as a formatter.
+
+#### Operands
+
+The function `:test:function` requires a [Number Operand](/spec/registry.md#number-operands) as its _operand_.
+
+#### Options
+
+The only option `:test:function` recognizes is `fd`,
+a [digit size option](/spec/registry.md) for which only `0` and `1` are valid values.
+
+All other options and their values are ignored.
+
+#### Behavior
+
+When resolving a `:test:function` expression,
+its _Input_ and _FD_ values are determined as follows:
+
+1. Let _FD_ be `0`.
+1. Let _arg_ be the resolved value of the expression operand.
+1. If _arg_ is the resolved value of an expression
+   with a `:test:function`, `:test:select`, or `:test:format` annotation
+   for which resolution has succeeded, then
+   1. Let _Input_ be the _Input_ value of _arg_.
+   1. Set _FD_ to be _FD_ value of _arg_.
+1. Else if _arg_ is a numerical value
+   or a string matching the `number-literal` production, then
+   1. Let _Input_ be the numerical value of _arg_.
+1. Else,
+   1. Emit "bad-input" Resolution Error.
+   1. Use a fallback representation as the resolved value of the expression.
+      Further steps of this algorithm are not followed.
+1. If the `fd` option is set, then
+   1. If its value resolves to a numerical integer value `0` or `1`
+      or their corresponding string representations `'0'` or `'1'`, then
+      1. Set _FD_ to be the numerical value of the option.
+   1. Else if its value is not an unresolved value set by [Option Resolution](/spec/formatting.md#option-resolution),
+      1. Emit "bad-option" Resolution Error.
+      1. Use a fallback representation as the resolved value of the expression.
+
+When `:test:function` is used as a selector,
+the behaviour of calling it as the `rv` value of MatchSelectorKeys(`rv`, `keys`)
+(see [Resolve Preferences](/spec/formatting.md#resolve-preferences) for more information)
+depends on its _Input_ and _FD_ values.
+
+- If the _Input_ is `1` and _FD_ is `1`,
+  the method will return some slice of the list « `'1.0'`, `'1'` »,
+  depending on whether those values are included in `keys`.
+- If the _Input_ is `1` and _FD_ is `0`,
+  the method will return the list « `'1'` » if `keys` includes `'1'`, or an empty list otherwise.
+- If the _Input_ is any other value, the method will return an empty list.
+
+When an expression with a `:test:function` annotation is assigned to a variable by a declaration
+and that variable is used as an option value,
+its resolved value is the _Input_ value.
+
+When `:test:function` is used as a formatter,
+a placeholder resolving to a value with a `:test:function` expression
+is formatted as a concatenation of the following parts:
+
+1. If _Input_ is less than `0`, the character `-` U+002D Hyphen-Minus.
+1. The truncated absolute integer value of _Input_, i.e. floor(abs(_Input_)),
+   formatted as a sequence of decimal digit caharacters.
+1. If _FD_ is `1`, then
+   1. The character `.` U+002E Full Stop.
+   1. The single decimal digit character representing the value floor((abs(_Input_) - floor(abs(_Input_))) \* 10)
+
+If the formatting target is a sequence of parts,
+each of the above parts will be emitted separately
+rather than being concatenated into a single string.
+
+Note that for purposes of clarity, the formatting of `:test:function` does not perform any rounding.
+
+### `:test:select`
+
+This function accepts the same operands and options,
+and behaves exactly the same as `:test:function`,
+except that it cannot be used for formatting.
+
+When `:test:select` is used as a formatter,
+a "not-formattable" error is emitted and the placeholder is formatted with
+a fallback representation.
+
+### `:test:format`
+
+This function accepts the same operands and options,
+and behaves exactly the same as `:test:function`,
+except that it cannot be used for selection.
+
+When `:test:format` is used as a selector,
+the steps under 2.iii. of [Resolve Selectors](/spec/formatting.md#resolve-selectors) are followed.