compiler: allow to use multiple underscores in func arguments

It should still be present in the argument array in VM so just don't
save them in the map.

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/compiler/function_call_test.go

@@ -313,3 +313,14 @@ func TestJumpOptimize(t *testing.T) {
 		require.Equal(t, b[mi.Range.End], byte(opcode.RET))
 	}
 }
+
+func TestFunctionUnusedParameters(t *testing.T) {
+	src := `package foo
+	func add13(a int, _ int, _1 int, _ int) int {
+		return a + _1
+	}
+	func Main() int {
+		return add13(1, 10, 100, 1000)
+	}`
+	eval(t, src, big.NewInt(101))
+}

pkg/compiler/vars.go

@@ -2,6 +2,7 @@ package compiler
 
 import (
 	"go/ast"
+	"strconv"
 )
 
 type varScope struct {
@@ -70,6 +71,12 @@ func (c *varScope) newVariable(t varType, name string) int {
 	case varLocal:
 		return c.newLocal(name)
 	case varArgument:
+		if name == "_" {
+			// See #2204. This name won't actually be referenced.
+			// This approach simplifies argument allocation and
+			// makes debugging easier.
+			name = "%_" + strconv.FormatUint(uint64(len(c.arguments)), 10)
+		}
 		_, ok := c.arguments[name]
 		if ok {
 			panic("argument is already allocated")