checker: fix error assigning generic function variable with generic struct return (#18472)

vlib/v/checker/assign.v

@@ -507,6 +507,16 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 			c.error('cannot copy map: call `move` or `clone` method (or use a reference)',
 				right.pos())
 		}
+		if left_sym.kind == .function && right_sym.info is ast.FnType {
+			return_sym := c.table.sym(right_sym.info.func.return_type)
+			if return_sym.kind == .placeholder {
+				c.error('unkown return type: cannot assign `${right}` as a function variable',
+					right.pos())
+			} else if (!right_sym.info.is_anon && return_sym.kind == .any)
+				|| (return_sym.info is ast.Struct && (return_sym.info as ast.Struct).is_generic) {
+				c.error('cannot assign `${right}` as a generic function variable', right.pos())
+			}
+		}
 		if left_type.is_any_kind_of_pointer() && !left.is_auto_deref_var() {
 			if !c.inside_unsafe && node.op !in [.assign, .decl_assign] {
 				// ptr op=

vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.out

@@ -5,6 +5,13 @@ vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.vv:6:2: warning: unused
       |     ~~~
     7 |     // ff1 := f1[int] <-- is a valid usage with generic return types that are not generic structs
     8 | }
+vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.vv:6:9: error: cannot assign `f1` as a generic function variable
+    4 | 
+    5 | fn main() {
+    6 |     ff1 := f1 // <-- missing e.g. `[int]`
+      |            ~~
+    7 |     // ff1 := f1[int] <-- is a valid usage with generic return types that are not generic structs
+    8 | }
 vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.vv:1:1: error: generic function visited more than 10000 times
     1 | fn f1[T](x T, i int) T {
       | ~~~~~~~~~~~~~~~~~~~~~~

vlib/v/checker/tests/unknown_array_fn_type_in_struct_field.out

@@ -5,3 +5,10 @@ Did you mean `[2]fn (u32) UnknownThing`?
       |                         ~~~~~~~~~~~~
     3 | }
     4 |
+vlib/v/checker/tests/unknown_array_fn_type_in_struct_field.vv:6:18: error: unkown return type: cannot assign `virt.fns[0]` as a function variable
+    4 | 
+    5 | fn (virt Virt) caller() {
+    6 |     func := virt.fns[0]
+      |                     ~~~
+    7 |     func(5)
+    8 | }