parser: change array decompose syntax (#7965)

vlib/v/checker/tests/decompose_type_err.out

@@ -1,6 +1,6 @@
-vlib/v/checker/tests/decompose_type_err.vv:4:10: error: decomposition can only be used on arrays
+vlib/v/checker/tests/decompose_type_err.vv:4:13: error: decomposition can only be used on arrays
     2 |
     3 | fn main() {
-    4 |     varargs(123...)
-      |             ~~~
-    5 | }
+    4 |     varargs(...123)
+      |                ~~~
+    5 | }

vlib/v/checker/tests/decompose_type_err.vv

@@ -1,5 +1,5 @@
 fn varargs(a ...int) { println(a) }
 
 fn main() {
-	varargs(123...)
+	varargs(...123)
 }

vlib/v/fmt/fmt.v

@@ -1237,8 +1237,8 @@ pub fn (mut f Fmt) expr(node ast.Expr) {
 			f.write('}')
 		}
 		ast.ArrayDecompose {
-			f.expr(node.expr)
 			f.write('...')
+			f.expr(node.expr)
 		}
 	}
 }

vlib/v/fmt/tests/array_decomposition_keep.vv

@@ -4,5 +4,5 @@ fn varargs(a ...int) {
 
 fn main() {
 	a := [1, 2, 3]
-	varargs(a...)
+	varargs(...a)
 }

vlib/v/parser/fn.v

@@ -127,9 +127,13 @@ pub fn (mut p Parser) call_args() []ast.CallArg {
 		}
 		mut comments := p.eat_comments()
 		arg_start_pos := p.tok.position()
-		mut e := p.expr(0)
+		mut array_decompose := false
 		if p.tok.kind == .ellipsis {
 			p.next()
+			array_decompose = true
+		}
+		mut e := p.expr(0)
+		if array_decompose {
 			e = ast.ArrayDecompose{
 				expr: e
 				pos: p.tok.position()

vlib/v/tests/fn_variadic_test.v

@@ -34,7 +34,7 @@ fn test_fn_variadic_generic() {
 */
 // forwarding
 fn variadic_forward_a(a ...string) string {
-	return variadic_fn_a(a...)
+	return variadic_fn_a(...a)
 }
 
 fn variadic_fn_a(a ...string) string {
@@ -66,7 +66,7 @@ fn test_variadic_only_with_no_vargs() {
 
 fn test_array_decomposition_to_vargs() {
 	a := ['a', 'b', 'c']
-	assert variadic_fn_a(a...) == 'abc'
+	assert variadic_fn_a(...a) == 'abc'
 }
 
 struct VaTestStruct {