dont detach <svelte:head> contents twice - fixes #2086

Author: Rich-Harris

src/compiler/compile/render_dom/wrappers/Element/index.ts

@@ -272,7 +272,7 @@ export default class ElementWrapper extends Wrapper {
 			);
 
 			if (parent_node === '@_document.head') {
-				block.builders.destroy.add_line(`@detach(${node});`);
+				block.builders.destroy.add_conditional('detaching', `@detach(${node});`);
 			}
 		} else {
 			block.builders.mount.add_line(`@insert(#target, ${node}, anchor);`);

test/js/samples/head-no-whitespace/expected.js

@@ -33,8 +33,10 @@ function create_fragment(ctx) {
 		o: noop,
 
 		d(detaching) {
-			detach(meta0);
-			detach(meta1);
+			if (detaching) {
+				detach(meta0);
+				detach(meta1);
+			}
 		}
 	};
 }

test/runtime/samples/head-detached-in-dynamic-component/A.svelte

@@ -0,0 +1,5 @@
+<svelte:head>
+	<meta name="description" content="A"/>
+</svelte:head>
+
+A

test/runtime/samples/head-detached-in-dynamic-component/B.svelte

@@ -0,0 +1,5 @@
+<svelte:head>
+	<meta name="description" content="B"/>
+</svelte:head>
+
+B

test/runtime/samples/head-detached-in-dynamic-component/_config.js

@@ -0,0 +1,15 @@
+export default {
+	html: `
+		A
+	`,
+
+	test({ assert, component, window }) {
+		component.x = false;
+
+		const meta = window.document.querySelectorAll('meta');
+
+		assert.equal(meta.length, 1);
+		assert.equal(meta[0].name, 'description');
+		assert.equal(meta[0].content, 'B');
+	}
+};

test/runtime/samples/head-detached-in-dynamic-component/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	import A from './A.svelte';
+	import B from './B.svelte';
+
+	export let x = true;
+</script>
+
+<svelte:component this="{x ? A : B}"/>