checker: add check for implicit fixed array decomp in function varargs (#16806)

vlib/v/checker/checker.v

@@ -2273,7 +2273,11 @@ pub fn (mut c Checker) expr(node_ ast.Expr) ast.Type {
 		ast.ArrayDecompose {
 			typ := c.expr(node.expr)
 			type_sym := c.table.sym(typ)
-			if type_sym.kind != .array {
+			if type_sym.kind == .array_fixed {
+				c.error('direct decomposition of fixed array is not allowed, convert the fixed array to normal array via ${node.expr}[..]',
+					node.expr.pos())
+				return ast.void_type
+			} else if type_sym.kind != .array {
 				c.error('decomposition can only be used on arrays', node.expr.pos())
 				return ast.void_type
 			}

vlib/v/checker/tests/fixed_array_decompose_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/fixed_array_decompose_err.vv:11:9: error: direct decomposition of fixed array is not allowed, convert the fixed array to normal array via arr[..]
+    9 | fn main() {
+   10 |     arr := [1, 2, 3, 4]!
+   11 |     sum(...arr)
+      |            ~~~
+   12 | }

vlib/v/checker/tests/fixed_array_decompose_err.vv

@@ -0,0 +1,12 @@
+fn sum(a ...int) int {
+	mut total := 0
+	for x in a {
+		total += x
+	}
+	return total
+}
+
+fn main() {
+	arr := [1, 2, 3, 4]!
+	sum(...arr)
+}

vlib/v/tests/fixed_array_explicit_decompose_test.v

@@ -0,0 +1,12 @@
+fn sum(a ...int) int {
+	mut total := 0
+	for x in a {
+		total += x
+	}
+	return total
+}
+
+fn test_fixed_array_explicit_decompose() {
+	arr := [1, 2, 3, 4]!
+	assert sum(...arr[..]) == 10
+}