checker: add a notice for global variable redeclarations (#23162)

vlib/v/checker/assign.v

@@ -139,6 +139,10 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				c.error('cannot reassign using range expression on the left side of an assignment',
 					left.pos)
 			}
+		} else if mut left is ast.Ident && node.op == .decl_assign {
+			if left.name in c.global_names {
+				c.note('the global variable named `${left.name}` already exists', left.pos)
+			}
 		}
 		is_blank_ident := left.is_blank_ident()
 		mut left_type := ast.void_type

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

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/globals/global_var_redeclare.vv:4:2: notice: the global variable named `foobar` already exists
+    2 | 
+    3 | fn main() {
+    4 |     foobar := 2
+      |     ~~~~~~
+    5 |     println(foobar)
+    6 | }

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

@@ -0,0 +1,6 @@
+__global foobar = 1
+
+fn main() {
+	foobar := 2
+	println(foobar)
+}