fix: nil ptr when discarded key and value not present on range

[Villaquiranm]

fixes #5664
The range of the form

for _ = range a.e

Should cause an early return on (x *RangeStmt) AssertCompatible, as both values key and value of the range are discarded. The problem is when the range is on the form for _ := range . the key is considered blankIdentifier but the value is not present (nil).

The fix is to add this case to the early return

[Gno2D2]

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[davd-gzl]

Try:

# from a local clone of gnolang/gno:
gh pr checkout 5751 -R gnolang/gno
cat > gnovm/tests/files/range_blank_key_5751.gno <<'EOF'
package main

func main() {
	a := []int{1, 2, 3}
	var v int
	for _, v = range a {
	}
	_ = v
	println("ok")
}

// Output:
// ok
EOF
go test -run 'TestFiles/range_blank_key_5751.gno$' ./gnovm/pkg/gnolang/
rm gnovm/tests/files/range_blank_key_5751.gno

Output:

--- FAIL: TestFiles/range_blank_key_5751.gno (0.00s)
    files_test.go:111: unexpected panic: runtime error: invalid memory address or nil pointer dereference
    ...
    gnolang.assertIndexTypeIsInt({0x0, 0x0})  type_check.go:823
    gnolang.(*RangeStmt).AssertCompatible(...) type_check.go:853

[Villaquiranm]

Try:

# from a local clone of gnolang/gno:
gh pr checkout 5751 -R gnolang/gno
cat > gnovm/tests/files/range_blank_key_5751.gno <<'EOF'
package main

func main() {
	a := []int{1, 2, 3}
	var v int
	for _, v = range a {
	}
	_ = v
	println("ok")
}

// Output:
// ok
EOF
go test -run 'TestFiles/range_blank_key_5751.gno$' ./gnovm/pkg/gnolang/
rm gnovm/tests/files/range_blank_key_5751.gno

Output:

--- FAIL: TestFiles/range_blank_key_5751.gno (0.00s)
    files_test.go:111: unexpected panic: runtime error: invalid memory address or nil pointer dereference
    ...
    gnolang.assertIndexTypeIsInt({0x0, 0x0})  type_check.go:823
    gnolang.(*RangeStmt).AssertCompatible(...) type_check.go:853

Thanks for the review and the test, this is indeed a problem :)

But in my opinion is not inside the scope of the issue this PR was trying to address (even if it is closely related). I was going to check and implement a fix for this but I saw this PR #5764 from @ltzmaxwell that already fix it I verified with your test 👍

[ltzmaxwell]

i will lean to a filetest for this kind a single file test for VM:

package main

type matrix struct {
	e []int
}

func (a matrix) equal() bool {
	for _ = range a.e {
	}
	for _, _ = range a.e {
	}
	for range a.e {
	}
	return true
}

func main() {
	var a matrix
	var i interface{}
	i = true && a.equal()
	println(i)
}

// Output:
// true