add test

vlib/v/checker/tests/nil_to_option_err.out

@@ -0,0 +1,34 @@
+vlib/v/checker/tests/nil_to_option_err.vv:8:7: warning: cannot assign a reference to a value (this will be an error soon) left=int false right=nil true ptr=false
+    6 | fn main() {
+    7 |     mut a := ?int(none)
+    8 |     a = unsafe { nil }
+      |       ^
+    9 | 
+   10 |     mut b := Test{}
+vlib/v/checker/tests/nil_to_option_err.vv:7:9: warning: unused variable: `a`
+    5 | 
+    6 | fn main() {
+    7 |     mut a := ?int(none)
+      |         ^
+    8 |     a = unsafe { nil }
+    9 |
+vlib/v/checker/tests/nil_to_option_err.vv:3:7: error: cannot assign `nil` to a non-pointer field
+    1 | struct Test {
+    2 | mut:
+    3 |     a ?int = unsafe { nil }
+      |       ~~~~
+    4 | }
+    5 |
+vlib/v/checker/tests/nil_to_option_err.vv:8:9: error: cannot assign `nil` to option value
+    6 | fn main() {
+    7 |     mut a := ?int(none)
+    8 |     a = unsafe { nil }
+      |         ~~~~~~
+    9 | 
+   10 |     mut b := Test{}
+vlib/v/checker/tests/nil_to_option_err.vv:11:11: error: cannot assign `nil` to option value
+    9 | 
+   10 |     mut b := Test{}
+   11 |     b.a = unsafe { nil }
+      |           ~~~~~~
+   12 | }

vlib/v/checker/tests/nil_to_option_err.vv

@@ -0,0 +1,12 @@
+struct Test {
+mut:
+    a ?int = unsafe { nil }
+}
+
+fn main() {
+    mut a := ?int(none)
+    a = unsafe { nil }
+
+    mut b := Test{}
+    b.a = unsafe { nil }
+}