syntax: support add new operators

- Support ES2016 exponentiation (**) and exponentiation assignment (**=)
- Support ES2020 nullish coalescing (??) and logical nullish assignment (??=)
- Support ES2021 logical and assignment (&&=) and logical or assignment (||=)

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

3 files changed, 161 insertions, 0 deletions

diff --git a/tests/custom/00_syntax/24_null_coalesce b/tests/custom/00_syntax/24_null_coalesce
new file mode 100644
index 0000000..26b89f1
--- /dev/null
+++ b/tests/custom/00_syntax/24_null_coalesce
@@ -0,0 +1,50 @@
+Null coalescing operators return the right hand side of an expression of
+the left hand side is null.
+
+
+1. The `??` operator returns the right hand side of the expression if the
+left hand side evaluates to `null`.
+
+-- Expect stdout --
+is null
+false
+0
+-- End --
+
+-- Testcase --
+{%
+	x = null;
+	y = false;
+	z = 0;
+
+	print(x ?? "is null", "\n");
+	print(y ?? "is null", "\n");
+	print(z ?? "is null", "\n");
+%}
+-- End --
+
+
+2. The `??=` nullish assignment operator sets the left hand side variable
+or value to the right hand side expression if the existing value is null.
+
+-- Expect stdout --
+is null
+false
+0
+-- End --
+
+-- Testcase --
+{%
+	x = null;
+	y = false;
+	z = 0;
+
+	x ??= "is null";
+	y ??= "is null";
+	z ??= "is null";
+
+	print(x, "\n");
+	print(y, "\n");
+	print(z, "\n");
+%}
+-- End --
diff --git a/tests/custom/00_syntax/25_and_or_assignment b/tests/custom/00_syntax/25_and_or_assignment
new file mode 100644
index 0000000..4dbc5f3
--- /dev/null
+++ b/tests/custom/00_syntax/25_and_or_assignment
@@ -0,0 +1,55 @@
+The logical AND and logical OR assignment operators set the left hand side
+variable or value to the right hand side expression result depending on
+whether the lhs value is truish.
+
+
+1. The `&&=` operator overwrites the lhs variable or field with the rhs
+expression result if the lhs is truish.
+
+-- Expect stdout --
+[
+	null,
+	false,
+	"is truish"
+]
+-- End --
+
+-- Testcase --
+{%
+	x = null;
+	y = false;
+	z = true;
+
+	x &&= "is truish";
+	y &&= "is truish";
+	z &&= "is truish";
+
+	printf("%.J\n", [ x, y, z ]);
+%}
+-- End --
+
+
+2. The `||=` operator overwrites the lhs variable or field with the rhs
+expression result if the lhs is falsy.
+
+-- Expect stdout --
+[
+	"is falsy",
+	"is falsy",
+	true
+]
+-- End --
+
+-- Testcase --
+{%
+	x = null;
+	y = false;
+	z = true;
+
+	x ||= "is falsy";
+	y ||= "is falsy";
+	z ||= "is falsy";
+
+	printf("%.J\n", [ x, y, z ]);
+%}
+-- End --
diff --git a/tests/custom/00_syntax/26_exponentiation b/tests/custom/00_syntax/26_exponentiation
new file mode 100644
index 0000000..1aab257
--- /dev/null
+++ b/tests/custom/00_syntax/26_exponentiation
@@ -0,0 +1,56 @@
+The exponentiation and exponentiation assignment operands allow raising
+the base operand value to the given power.
+
+
+1. The `**` operator returns the result of raising the first operand to
+the power of the second operand.
+
+-- Expect stdout --
+[
+	1,
+	4,
+	9223372036854775808,
+	-9223372036854775808,
+	-0.25,
+	2.75568
+]
+-- End --
+
+-- Testcase --
+{%
+	printf("%.J\n", [
+		2 ** 0,
+		2 ** 2,
+		2 ** 63,
+		-2 ** 63,
+		-2 ** -2,
+		1.5 ** 2.5
+	]);
+%}
+-- End --
+
+
+2. The `**=` operator raises the lhs variable or field value to the
+power value in the rhs expression.
+
+-- Expect stdout --
+[
+	4,
+	-0.25,
+	2.75568
+]
+-- End --
+
+-- Testcase --
+{%
+	x = 2;
+	y = -2;
+	z = 1.5;
+
+	x **= 2;
+	y **= -2;
+	z **= 2.5;
+
+	printf("%.J\n", [ x, y, z ]);
+%}
+-- End --