checker: add err for unknown generic struct field (#16698)

vlib/v/ast/types.v

@@ -1045,7 +1045,7 @@ pub mut:
 pub struct GenericInst {
 pub mut:
 	parent_idx     int    // idx of the base generic struct
-	concrete_types []Type // concrete types, e.g. <int, string>
+	concrete_types []Type // concrete types, e.g. [int, string]
 }
 
 [minify]
@@ -1148,7 +1148,7 @@ pub fn (mytable &Table) type_to_code(t Type) string {
 	}
 }
 
-// clean type name from generics form. From Type<int> -> Type
+// clean type name from generics form. From Type[int] -> Type
 pub fn (t &Table) clean_generics_type_str(typ Type) string {
 	result := t.type_to_str(typ)
 	return result.all_before('[')
@@ -1436,12 +1436,12 @@ pub fn (t &Table) fn_signature_using_aliases(func &Fn, import_aliases map[string
 }
 
 // symbol_name_except_generic return the name of the complete qualified name of the type,
-// but without the generic parts. For example, `main.Abc<int>` -> `main.Abc`
+// but without the generic parts. For example, `main.Abc[int]` -> `main.Abc`
 pub fn (t &TypeSymbol) symbol_name_except_generic() string {
-	// main.Abc<int>
+	// main.Abc[int]
 	mut embed_name := t.name
 	// remove generic part from name
-	// main.Abc<int> => main.Abc
+	// main.Abc[int] => main.Abc
 	if embed_name.contains('[') {
 		embed_name = embed_name.all_before('[')
 	}
@@ -1449,10 +1449,10 @@ pub fn (t &TypeSymbol) symbol_name_except_generic() string {
 }
 
 pub fn (t &TypeSymbol) embed_name() string {
-	// main.Abc<int> => Abc<int>
+	// main.Abc[int] => Abc[int]
 	mut embed_name := t.name.split('.').last()
 	// remove generic part from name
-	// Abc<int> => Abc
+	// Abc[int] => Abc
 	if embed_name.contains('[') {
 		embed_name = embed_name.split('[')[0]
 	}

vlib/v/checker/struct.v

@@ -78,6 +78,12 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 				if info.is_heap && !field.typ.is_ptr() {
 					struct_sym.info.is_heap = true
 				}
+				for ct in info.concrete_types {
+					ct_sym := c.table.sym(ct)
+					if ct_sym.kind == .placeholder {
+						c.error('unknown type `${ct_sym.name}`', field.type_pos)
+					}
+				}
 			}
 			if sym.kind == .multi_return {
 				c.error('cannot use multi return as field type', field.type_pos)

vlib/v/checker/tests/struct_field_generic_struct_unknown_type_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_field_generic_struct_unknown_type_err.vv:9:24: error: unknown type `UnknownType`
+    7 | }
+    8 | struct MyType {
+    9 |     a Maybe[UnknownType]
+      |                        ^
+   10 | }
+   11 |

vlib/v/checker/tests/struct_field_generic_struct_unknown_type_err.vv

@@ -0,0 +1,14 @@
+module main
+
+
+pub struct Maybe[T] {
+    present bool
+    val     T
+}
+struct MyType {
+    a Maybe[UnknownType]
+}
+
+fn main() {
+    _ = MyType {}
+}