From 0454f43fa394d2883ca06a3ba1d7d4d396732d3d Mon Sep 17 00:00:00 2001
From: Turiiya <34311583+tobealive@users.noreply.github.com>
Date: Sun, 18 Jun 2023 21:22:22 +0200
Subject: [PATCH] checker: fix error assigning generic function variable with
 generic struct return (#18472)

---
 vlib/v/checker/assign.v                                | 10 ++++++++++
 .../tests/generic_fn_infinite_loop_limit_err.out       |  7 +++++++
 .../tests/unknown_array_fn_type_in_struct_field.out    |  7 +++++++
 3 files changed, 24 insertions(+)

diff --git a/vlib/v/checker/assign.v b/vlib/v/checker/assign.v
index 802907711988a3..7d6481ff996168 100644
--- a/vlib/v/checker/assign.v
+++ b/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=
diff --git a/vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.out b/vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.out
index ae3fea784074a8..baff173944387d 100644
--- a/vlib/v/checker/tests/generic_fn_infinite_loop_limit_err.out
+++ b/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 {
       | ~~~~~~~~~~~~~~~~~~~~~~
diff --git a/vlib/v/checker/tests/unknown_array_fn_type_in_struct_field.out b/vlib/v/checker/tests/unknown_array_fn_type_in_struct_field.out
index 6ed81f7f9ae67a..84c2a0ffe16fda 100644
--- a/vlib/v/checker/tests/unknown_array_fn_type_in_struct_field.out
+++ b/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 | }