fix shape accepting composite

ci/release/changelogs/next.md

@@ -6,6 +6,7 @@
 
 - Display version on CLI help invocation [#1400](https://github.com/terrastruct/d2/pull/1400)
 - Improved readability of connection labels when they overlap another connection. [#447](https://github.com/terrastruct/d2/pull/447)
+- Error message when `shape` is given a composite [#1415](https://github.com/terrastruct/d2/pull/1415)
 
 #### Bugfixes ⛑️
 

d2compiler/compile.go

@@ -176,7 +176,11 @@ func (c *compiler) compileMap(obj *d2graph.Object, m *d2ir.Map) {
 	}
 	shape := m.GetField("shape")
 	if shape != nil {
-		c.compileField(obj, shape)
+		if shape.Composite != nil {
+			c.errorf(shape.LastPrimaryKey(), "reserved field shape does not accept composite")
+		} else {
+			c.compileField(obj, shape)
+		}
 	}
 	for _, f := range m.Fields {
 		if f.Name == "shape" {

d2compiler/compile_test.go

@@ -2614,6 +2614,15 @@ object: {
 `,
 			expErr: `d2/testdata/d2compiler/TestCompile/classes-internal-edge.d2:8:3: classes cannot contain an edge`,
 		},
+		{
+			name: "reserved-composite",
+			text: `shape: sequence_diagram {
+  alice -> bob: What does it mean\nto be well-adjusted?
+  bob -> alice: The ability to play bridge or\ngolf as if they were games.
+}
+`,
+			expErr: `d2/testdata/d2compiler/TestCompile/reserved-composite.d2:1:1: reserved field shape does not accept composite`,
+		},
 	}
 
 	for _, tc := range testCases {