diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index 5c5cd3943d..6c52ca292c 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -1166,6 +1166,7 @@ pub:
 	name          string
 	is_pub        bool
 	pos           token.Pos
+	name_pos      token.Pos
 	comments      []Comment
 	typ           Type
 	generic_types []Type
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 28a24fb855..fb47ebc287 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -493,9 +493,26 @@ pub fn (mut c Checker) sum_type_decl(node ast.SumTypeDecl) {
 		} else if sym.kind == .struct_ && sym.language == .js {
 			c.error('sum type cannot hold an JS struct', variant.pos)
 		} else if mut sym.info is ast.Struct {
-			if sym.info.is_generic && !variant.typ.has_flag(.generic) {
-				c.error('generic struct `$sym.name` must specify generic type names, e.g. Foo<T>',
-					variant.pos)
+			if sym.info.is_generic {
+				if !variant.typ.has_flag(.generic) {
+					c.error('generic struct `$sym.name` must specify generic type names, e.g. Foo<T>',
+						variant.pos)
+				}
+				if node.generic_types.len == 0 {
+					c.error('generic sumtype `$node.name` must specify generic type names, e.g. Foo<T>',
+						node.name_pos)
+				} else {
+					for typ in sym.info.generic_types {
+						if typ !in node.generic_types {
+							sumtype_type_names := node.generic_types.map(c.table.type_to_str(it)).join(', ')
+							generic_sumtype_name := '$node.name<$sumtype_type_names>'
+							variant_type_names := sym.info.generic_types.map(c.table.type_to_str(it)).join(', ')
+							generic_variant_name := '$sym.name<$variant_type_names>'
+							c.error('generic type name `${c.table.sym(typ).name}` of generic struct `$generic_variant_name` is not mentioned in sumtype `$generic_sumtype_name`',
+								variant.pos)
+						}
+					}
+				}
 			}
 		}
 
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err.out b/vlib/v/checker/tests/generic_sumtype_decl_err_a.out
similarity index 50%
rename from vlib/v/checker/tests/generic_sumtype_decl_err.out
rename to vlib/v/checker/tests/generic_sumtype_decl_err_a.out
index 7d3ce8f664..12fac1ed81 100644
--- a/vlib/v/checker/tests/generic_sumtype_decl_err.out
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_a.out
@@ -1,4 +1,4 @@
-vlib/v/checker/tests/generic_sumtype_decl_err.vv:7:24: error: generic struct `Just` must specify generic type names, e.g. Foo<T>
+vlib/v/checker/tests/generic_sumtype_decl_err_a.vv:7:24: error: generic struct `Just` must specify generic type names, e.g. Foo<T>
     5 | struct Nothing {}
     6 |
     7 | type Maybe = Nothing | Just
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err.vv b/vlib/v/checker/tests/generic_sumtype_decl_err_a.vv
similarity index 100%
rename from vlib/v/checker/tests/generic_sumtype_decl_err.vv
rename to vlib/v/checker/tests/generic_sumtype_decl_err_a.vv
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err_b.out b/vlib/v/checker/tests/generic_sumtype_decl_err_b.out
new file mode 100644
index 0000000000..4920a990c7
--- /dev/null
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_b.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generic_sumtype_decl_err_b.vv:7:6: error: generic sumtype `Maybe` must specify generic type names, e.g. Foo<T>
+    5 | struct Nothing {}
+    6 |
+    7 | type Maybe = Nothing | Just<T>
+      |      ~~~~~
+    8 |
+    9 | fn main() {
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err_b.vv b/vlib/v/checker/tests/generic_sumtype_decl_err_b.vv
new file mode 100644
index 0000000000..49873c3b4c
--- /dev/null
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_b.vv
@@ -0,0 +1,10 @@
+struct Just<T> {
+  value T
+}
+
+struct Nothing {}
+
+type Maybe = Nothing | Just<T>
+
+fn main() {
+}
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err_c.out b/vlib/v/checker/tests/generic_sumtype_decl_err_c.out
new file mode 100644
index 0000000000..e0965959b9
--- /dev/null
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_c.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generic_sumtype_decl_err_c.vv:7:27: error: generic type name `T` of generic struct `Just<T>` is not mentioned in sumtype `Maybe<B>`
+    5 | struct Nothing {}
+    6 |
+    7 | type Maybe<B> = Nothing | Just<T>
+      |                           ~~~~~~~
+    8 |
+    9 | fn main() {
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err_c.vv b/vlib/v/checker/tests/generic_sumtype_decl_err_c.vv
new file mode 100644
index 0000000000..99ec38cf7c
--- /dev/null
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_c.vv
@@ -0,0 +1,10 @@
+struct Just<T> {
+  value T
+}
+
+struct Nothing {}
+
+type Maybe<B> = Nothing | Just<T>
+
+fn main() {
+}
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index bde17cebc7..0656b53624 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -3672,6 +3672,7 @@ fn (mut p Parser) type_decl() ast.TypeDecl {
 			generic_types: generic_types
 			attrs: p.attrs
 			pos: decl_pos
+			name_pos: name_pos
 			comments: comments
 		}
 	}