An attempt to compile the following code leads to runtime panic:
```
package foo
type CustomInt int
func Main() int {
var i CustomInt
i = 5
return i.Do(2)
}
func (CustomInt) Do(arg int) int {
return arg
}
```
The panic:
```
panic: runtime error: index out of range [0] with length 0 [recovered]
panic: runtime error: index out of range [0] with length 0
goroutine 22 [running]:
testing.tRunner.func1.2({0xa341c0, 0xc0001606d8})
/usr/local/go/src/testing/testing.go:1209 +0x24e
testing.tRunner.func1()
/usr/local/go/src/testing/testing.go:1212 +0x218
panic({0xa341c0, 0xc0001606d8})
/usr/local/go/src/runtime/panic.go:1038 +0x215
github.com/nspcc-dev/neo-go/pkg/compiler.(*codegen).convertFuncDecl(0xc00015e3c0, {0xc753b8, 0xc000152c80}, 0xc000266300, 0x30)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/codegen.go:497 +0x10b3
github.com/nspcc-dev/neo-go/pkg/compiler.(*codegen).compile.func2(0xc000152c80, 0xc00023c410)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/codegen.go:2153 +0x3f8
github.com/nspcc-dev/neo-go/pkg/compiler.(*codegen).ForEachFile.func1(0xc000229b80)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/compiler.go:102 +0x82
github.com/nspcc-dev/neo-go/pkg/compiler.(*codegen).ForEachPackage(0xc00015e3c0, 0xc000189bb0)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/compiler.go:93 +0xc6
github.com/nspcc-dev/neo-go/pkg/compiler.(*codegen).ForEachFile(0x999a20, 0xc000130d80)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/compiler.go:99 +0x45
github.com/nspcc-dev/neo-go/pkg/compiler.(*codegen).compile(0xc00015e3c0, 0xc0002669f0, 0x1)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/codegen.go:2140 +0x445
github.com/nspcc-dev/neo-go/pkg/compiler.codeGen(0xc0002669f0)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/codegen.go:2191 +0x353
github.com/nspcc-dev/neo-go/pkg/compiler.CompileWithOptions({0xa6f39a, 0x50b6b3}, {0xc6d1a0, 0xc0002421e0}, 0x0)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/compiler.go:218 +0x65
github.com/nspcc-dev/neo-go/pkg/compiler_test.vmAndCompileInterop(0x5648df, {0xa9bf23, 0x94})
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/vm_test.go:75 +0x113
github.com/nspcc-dev/neo-go/pkg/compiler_test.eval(0xc0002421c0, {0xa9bf23, 0x61be8c7}, {0xa68880, 0xc0002421c0})
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/vm_test.go:36 +0x2d
github.com/nspcc-dev/neo-go/pkg/compiler_test.TestUnnamedMethodReceiver(0x4079f9)
/home/anna/Documents/GitProjects/nspcc-dev/neo-go/pkg/compiler/function_call_test.go:400 +0x4f
testing.tRunner(0xc000204b60, 0xbcebb0)
/usr/local/go/src/testing/testing.go:1259 +0x102
created by testing.(*T).Run
/usr/local/go/src/testing/testing.go:1306 +0x35a
```
The solution is to use the same approach as for unnamed function
parameters handling introduced in #2204. (c *funcScope).newVariable is
able to properly handle "_" receiver.
16dbb35bd8