const tag cyclic validation (now runtime, based because of new reactivity system)

packages/svelte/src/compiler/errors.js

@@ -461,10 +461,6 @@ const errors = {
 	// 	message:
 	// 		'Can only bind to an identifier (e.g. `foo`) or a member expression (e.g. `foo.bar` or `foo[baz]`)'
 	// },
-	// cyclical_const_tags: /** @param {string[]} cycle */ (cycle) => ({
-	// 	code: 'cyclical-const-tags',
-	// 	message: `Cyclical dependency detected: ${cycle.join(' → ')}`
-	// }),
 	// invalid_var_declaration: {
 	// 	code: 'invalid_var_declaration',
 	// 	message: '"var" scope should not extend outside the reactive block'

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -1800,6 +1800,12 @@ export const template_visitors = {
 					)
 				)
 			);
+
+			// we need to eagerly evaluate the expression in order to hit any
+			// 'Cannot access x before initialization' errors
+			if (state.options.dev) {
+				state.init.push(b.stmt(b.call('$.get', declaration.id)));
+			}
 		} else {
 			const identifiers = extract_identifiers(declaration.id);
 			const tmp = b.id(state.scope.generate('computed_const'));
@@ -1829,6 +1835,12 @@ export const template_visitors = {
 				b.const(tmp, b.call(state.options.runes ? '$.derived' : '$.derived_safe_equal', fn))
 			);
 
+			// we need to eagerly evaluate the expression in order to hit any
+			// 'Cannot access x before initialization' errors
+			if (state.options.dev) {
+				state.init.push(b.stmt(b.call('$.get', tmp)));
+			}
+
 			for (const node of identifiers) {
 				const binding = /** @type {import('#compiler').Binding} */ (state.scope.get(node.name));
 				binding.expression = b.member(b.call('$.get', tmp), node);

packages/svelte/tests/runtime-legacy/samples/const-tag-used-before-initialised copy/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	error: "Cannot access 'c' before initialization"
+});

packages/svelte/tests/runtime-legacy/samples/const-tag-used-before-initialised copy/main.svelte

@@ -1,8 +1,8 @@
 <script>
-  export let array;
+  export let array = [1];
 </script>
 
 {#each array as a}
   {@const b = a + c}
   {@const c = b + a}
-{/each}
+{/each}

packages/svelte/tests/validator/samples/const-tag-cyclical/_config.js

@@ -1,3 +0,0 @@
-import { test } from '../../test';
-
-export default test({ skip: true });

packages/svelte/tests/validator/samples/const-tag-cyclical/errors.json

@@ -1,8 +0,0 @@
-[
-	{
-		"code": "cyclical-const-tags",
-		"message": "Cyclical dependency detected: b → c → b",
-		"start": { "line": 6, "column": 2 },
-		"end": { "line": 6, "column": 20 }
-	}
-]