add class array help

ci/release/changelogs/next.md

@@ -13,6 +13,7 @@
 - Constraints in `sql_table` sheds their excessive letter-spacing and is padded from the end consistently [#1372](https://github.com/terrastruct/d2/pull/1372)
 - Duplicate image URLs in icons are only fetched once [#1373](https://github.com/terrastruct/d2/pull/1373)
 - In watch mode, images are cached by default across compiles. Can be disabled with flag `--img-cache=0`. [#1373](https://github.com/terrastruct/d2/pull/1373)
+- Common invalid array separator `,` usage in class arrays returns a helpful error message [#1376](https://github.com/terrastruct/d2/pull/1376)
 
 #### Bugfixes ⛑️
 

d2compiler/compile.go

@@ -147,6 +147,21 @@ func (c *compiler) compileMap(obj *d2graph.Object, m *d2ir.Map) {
 			classMap := m.GetClassMap(className)
 			if classMap != nil {
 				c.compileMap(obj, classMap)
+			} else {
+				if strings.Contains(className, ",") {
+					split := strings.Split(className, ",")
+					allFound := true
+					for _, maybeClassName := range split {
+						maybeClassName = strings.TrimSpace(maybeClassName)
+						if m.GetClassMap(maybeClassName) == nil {
+							allFound = false
+							break
+						}
+					}
+					if allFound {
+						c.errorf(class.LastRef().AST(), `class "%s" not found. Did you mean to use ";" to separate array items?`, className)
+					}
+				}
 			}
 		}
 	}

d2compiler/compile_test.go

@@ -2482,6 +2482,23 @@ nostar -> 1star: { class: [path; path2] }
 				tassert.Equal(t, "2", g.Edges[0].Style.StrokeWidth.Value)
 			},
 		},
+		{
+			name: "comma-array-class",
+
+			text: `classes: {
+  dragon_ball: {
+    label: ""
+    shape: circle
+    style.fill: orange
+  }
+  path: {
+    label: "then"
+    style.stroke-width: 4
+  }
+}
+nostar: { class: [dragon_ball, path] }`,
+			expErr: `d2/testdata/d2compiler/TestCompile/comma-array-class.d2:12:11: class "dragon_ball, path" not found. Did you mean to use ";" to separate array items?`,
+		},
 		{
 			name: "reordered-classes",
 			text: `classes: {