checker: check generic struct no_keys init (fix #17061) (#17067)

vlib/v/checker/struct.v

@@ -387,6 +387,15 @@ fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 				field.typ = c.expr(field.expr)
 				field.expected_type = field.typ
 			}
+			sym := c.table.sym(c.unwrap_generic(node.typ))
+			if sym.kind == .struct_ {
+				info := sym.info as ast.Struct
+				if node.no_keys && node.fields.len != info.fields.len {
+					fname := if info.fields.len != 1 { 'fields' } else { 'field' }
+					c.error('initializing struct `${sym.name}` needs `${info.fields.len}` ${fname}, but got `${node.fields.len}`',
+						node.pos)
+				}
+			}
 		}
 		// string & array are also structs but .kind of string/array
 		.struct_, .string, .array, .alias {

vlib/v/checker/tests/generics_struct_nokeys_init_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generics_struct_nokeys_init_err.vv:6:9: error: initializing struct `Am` needs `1` field, but got `2`
+    4 | 
+    5 | fn convert[T](num int) T {
+    6 |     return T{num, 1}
+      |            ~~~~~~~~~
+    7 | }
+    8 |

vlib/v/checker/tests/generics_struct_nokeys_init_err.vv

@@ -0,0 +1,12 @@
+struct Am {
+	inner int
+}
+
+fn convert[T](num int) T {
+	return T{num, 1}
+}
+
+fn main() {
+	println(convert[Am](3).inner)
+	assert convert[Am](3).inner == 3
+}