table: fix generics struct with anon fn fields (#10024)

vlib/v/ast/table.v

@@ -63,7 +63,6 @@ pub fn (t &Table) panic(message string) {
 
 pub struct Fn {
 pub:
-	return_type    Type
 	is_variadic     bool
 	language        Language
 	generic_names   []string
@@ -79,10 +78,10 @@ pub:
 	mod             string
 	ctdefine        string // compile time define. "myflag", when [if myflag] tag
 	attrs           []Attr
-	//
 	pos             token.Position
 	return_type_pos token.Position
 pub mut:
+	return_type Type
 	name        string
 	params      []Param
 	source_fn   voidptr // set in the checker, while processing fn declarations
@@ -1108,6 +1107,26 @@ pub fn (mut t Table) resolve_generic_to_concrete(generic_type Type, generic_name
 			}
 			return new_type(idx).derive(generic_type).clear_flag(.generic)
 		}
+	} else if mut sym.info is FnType {
+		mut func := sym.info.func
+		if func.return_type.has_flag(.generic) {
+			if typ := t.resolve_generic_to_concrete(func.return_type, generic_names, concrete_types,
+				is_inst)
+			{
+				func.return_type = typ
+			}
+		}
+		for mut param in func.params {
+			if param.typ.has_flag(.generic) {
+				if typ := t.resolve_generic_to_concrete(param.typ, generic_names, concrete_types,
+					is_inst)
+				{
+					param.typ = typ
+				}
+			}
+		}
+		idx := t.find_or_register_fn_type('', func, true, false)
+		return new_type(idx).derive(generic_type).clear_flag(.generic)
 	}
 	return none
 }

vlib/v/tests/generics_struct_anon_fn_fields_test.v

@@ -0,0 +1,14 @@
+struct Scope<T> {
+	before fn () T
+	specs  []fn (T) T
+	after  fn (T)
+}
+
+fn test_generics_struct_anon_fn_fields() {
+	s := Scope<u32>{}
+	println(s)
+	ts := '$s'
+	assert ts.contains('before: fn () u32')
+	assert ts.contains('specs: []')
+	assert ts.contains('after: fn (u32)')
+}