checker: disallow foo[T] as a value

vlib/v/checker/checker.v

@@ -3834,6 +3834,17 @@ fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 					c.error('`${node.name}` is a generic fn, you should pass its concrete types, e.g. ${node.name}[int]',
 						node.pos)
 				}
+				mut has_generic := false // foo[T] instead of foo[int]
+				for concrete_type in node.concrete_types {
+					if concrete_type.has_flag(.generic) {
+						has_generic = true
+					}
+				}
+				if c.table.cur_fn != unsafe { nil } && c.table.cur_concrete_types.len == 0
+					&& has_generic {
+					c.error('generic fn using generic types cannot be called outside of generic fn',
+						node.pos)
+				}
 				concrete_types := node.concrete_types.map(c.unwrap_generic(it))
 				if concrete_types.all(!it.has_flag(.generic)) {
 					c.table.register_fn_concrete_types(func.fkey(), concrete_types)
@@ -4012,6 +4023,17 @@ fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 					c.error('`${node.name}` is a generic fn, you should pass its concrete types, e.g. ${node.name}[int]',
 						node.pos)
 				}
+				mut has_generic := false // foo[T] instead of foo[int]
+				for concrete_type in node.concrete_types {
+					if concrete_type.has_flag(.generic) {
+						has_generic = true
+					}
+				}
+				if c.table.cur_fn != unsafe { nil } && c.table.cur_concrete_types.len == 0
+					&& has_generic {
+					c.error('generic fn using generic types cannot be called outside of generic fn',
+						node.pos)
+				}
 			}
 			return c.resolve_var_fn(func, mut node, name)
 		}

vlib/v/checker/tests/generic_fn_generic_name_unresolved_err.out

@@ -0,0 +1,21 @@
+vlib/v/checker/tests/generic_fn_generic_name_unresolved_err.vv:8:7: error: generic fn using generic types cannot be called outside of generic fn
+    6 | 
+    7 | fn main() {
+    8 |     _ := func[T]
+      |          ~~~~
+    9 |     _ := {
+   10 |         'test': func[T]
+vlib/v/checker/tests/generic_fn_generic_name_unresolved_err.vv:10:11: error: generic fn using generic types cannot be called outside of generic fn
+    8 |     _ := func[T]
+    9 |     _ := {
+   10 |         'test': func[T]
+      |                 ~~~~
+   11 |     }
+   12 |     _ := {
+vlib/v/checker/tests/generic_fn_generic_name_unresolved_err.vv:13:3: error: generic fn using generic types cannot be called outside of generic fn
+   11 |     }
+   12 |     _ := {
+   13 |         func[T]: 'test'
+      |         ~~~~
+   14 |     }
+   15 | }

vlib/v/checker/tests/generic_fn_generic_name_unresolved_err.vv

@@ -0,0 +1,15 @@
+module main
+
+fn func[T](arg string, val T) int {
+	return 2
+}
+
+fn main() {
+	_ := func[T]
+	_ := {
+		'test': func[T]
+	}
+	_ := {
+		func[T]: 'test'
+	}
+}