parser: fix parsing interface methods with varargs (#8229)

vlib/v/fmt/tests/interface_variadic_keep.vv

@@ -0,0 +1,4 @@
+interface Element {
+	unnamed_method(...f64)
+	named_method(params ...f64)
+}

vlib/v/parser/struct.v

@@ -476,7 +476,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 			return ast.InterfaceDecl{}
 		}
 		// field_names << name
-		args2, _, _ := p.fn_args() // TODO merge table.Param and ast.Arg to avoid this
+		args2, _, is_variadic := p.fn_args() // TODO merge table.Param and ast.Arg to avoid this
 		mut args := [table.Param{
 			name: 'x'
 			typ: typ
@@ -489,6 +489,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 			params: args
 			file: p.file_name
 			return_type: table.void_type
+			is_variadic: is_variadic
 			is_pub: true
 			pos: method_start_pos.extend(p.prev_tok.position())
 			scope: p.scope
@@ -502,7 +503,13 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 		method.next_comments = mnext_comments
 		methods << method
 		// println('register method $name')
-		ts.register_method(name: name, params: args, return_type: method.return_type, is_pub: true)
+		ts.register_method(
+			name: name
+			params: args
+			return_type: method.return_type
+			is_variadic: is_variadic
+			is_pub: true
+		)
 	}
 	p.top_level_statement_end()
 	p.check(.rcbr)

vlib/v/tests/interface_variadic_test.v

@@ -0,0 +1,25 @@
+interface Element {
+	method(params ...f64) string
+}
+
+struct Foo {}
+
+fn (f &Foo) method(params ...f64) string {
+	return params.str()
+}
+
+fn test_variadic_array_decompose() {
+	mut a := []Element{}
+	a << Foo{}
+
+	input := [0.0, 1.0]
+	assert a[0].method(...input) == '[0, 1]'
+	assert a[0].method(...[0.0, 1.0]) == '[0, 1]'
+}
+
+fn test_variadic_multiple_args() {
+	mut a := []Element{}
+	a << Foo{}
+
+	assert a[0].method(0.0, 1.0) == '[0, 1]'
+}