checker: add missing check for global var on assignment to shared var (#18125)

vlib/v/checker/assign.v

@@ -221,6 +221,13 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 			// 	c.error('cannot assign a `none` value to a non-option variable', right.pos())
 			// }
 		}
+		if mut left is ast.Ident
+			&& (left as ast.Ident).info is ast.IdentVar && right is ast.Ident && (right as ast.Ident).name in c.global_names {
+			ident_var_info := left.info as ast.IdentVar
+			if ident_var_info.share == .shared_t {
+				c.error('cannot assign global variable to shared variable', right.pos())
+			}
+		}
 		if right_type.is_ptr() && left_type.is_ptr() {
 			if mut right is ast.Ident {
 				if mut right.obj is ast.Var {

vlib/v/checker/tests/globals/assign_global_to_shared_err.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/globals/assign_global_to_shared_err.vv:5:9: warning: unused variable: `b`
+    3 | 
+    4 | fn main() {
+    5 |     shared b := a
+      |            ^
+    6 | }
+vlib/v/checker/tests/globals/assign_global_to_shared_err.vv:5:14: error: cannot assign global variable to shared variable
+    3 | 
+    4 | fn main() {
+    5 |     shared b := a
+      |                 ^
+    6 | }

vlib/v/checker/tests/globals/assign_global_to_shared_err.vv

@@ -0,0 +1,6 @@
+[has_globals]
+__global a = 0
+
+fn main() {
+	shared b := a
+}