checker: check assigning generic function to a variable (#16507)

vlib/v/checker/assign.v

@@ -65,6 +65,11 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				c.error('cannot use `none` in `unsafe` blocks', right.expr.pos)
 			}
 		}
+		if mut right is ast.AnonFn {
+			if right.decl.generic_names.len > 0 {
+				c.error('cannot assign generic function to a variable', right.decl.pos)
+			}
+		}
 	}
 	if node.left.len != right_len {
 		if right_first is ast.CallExpr {

vlib/v/checker/tests/assign_generic_fn_err.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/assign_generic_fn_err.vv:2:9: error: cannot assign generic function to a variable
+    1 | fn main() {
+    2 |     fun := fn <T> (value T) T {
+      |            ~~~~~~~~~~~~~~~~~~~~
+    3 |         return value
+    4 |     }
+vlib/v/checker/tests/assign_generic_fn_err.vv:6:13: error: a non generic function called like a generic one
+    4 |     }
+    5 |
+    6 |     println(fun<int>(100))
+      |                ~~~~~
+    7 | }

vlib/v/checker/tests/assign_generic_fn_err.vv

@@ -0,0 +1,7 @@
+fn main() {
+	fun := fn <T> (value T) T {
+		return value
+	}
+
+	println(fun<int>(100))
+}