optional from eca test

lang/README.md

@@ -614,6 +614,18 @@ main := fn(): uint {
 }
 ```
 
+#### optional_from_eca
+```hb
+main := fn(): uint {
+	a := @as(?uint, @eca(0, 0, 0, 0))
+
+	if a == null {
+		die
+	}
+	return a
+}
+```
+
 #### inlining_issues
 ```hb
 main := fn(): void {

lang/src/son.rs

@@ -4594,5 +4594,6 @@ mod tests {
         global_aliasing_overptimization;
         overwrite_aliasing_overoptimization;
         more_if_opts;
+        optional_from_eca;
     }
 }

lang/tests/son_tests_optional_from_eca.txt

@@ -0,0 +1,3 @@
+test.hb:8:2: can't prove the value is not 'null', use '@unwrap(<opt>)' if you believe compiler is stupid, or explicitly check for null and handle it ('if <opt> == null { /* handle */ } else { /* use opt */ }')
+    return a
+    ^