warn for possible use of component without uppercase tag name (#5302)

CHANGELOG.md

@@ -5,6 +5,7 @@
 * Support `<slot slot="...">` ([#2079](https://github.com/sveltejs/svelte/issues/2079))
 * Fix unmounting components with a bidirectional transition with a delay ([#4954](https://github.com/sveltejs/svelte/issues/4954))
 * Add types to `get` function in `svelte/store` ([#5269](https://github.com/sveltejs/svelte/pull/5269))
+* Add a warning when a component looks like it's trying to use another component without beginning with a capital letter ([#5302](https://github.com/sveltejs/svelte/pull/5302))
 * Add `EventSource` to known globals ([#5463](https://github.com/sveltejs/svelte/issues/5463))
 * Fix compiler exception with `~`/`+` combinators and `{...spread}` attributes ([#5465](https://github.com/sveltejs/svelte/issues/5465))
 

src/compiler/compile/Component.ts

@@ -634,11 +634,13 @@ export default class Component {
 			}
 
 			const writable = node.type === 'VariableDeclaration' && (node.kind === 'var' || node.kind === 'let');
+			const imported = node.type.startsWith('Import');
 
 			this.add_var({
 				name,
 				initialised: instance_scope.initialised_declarations.has(name),
-				writable
+				writable,
+				imported
 			});
 
 			this.node_for_declaration.set(name, node);

src/compiler/compile/nodes/Element.ts

@@ -186,7 +186,7 @@ export default class Element extends Node {
 
 				case 'Attribute':
 				case 'Spread':
-				// special case
+					// special case
 					if (node.name === 'xmlns') this.namespace = node.value[0].data;
 
 					this.attributes.push(new Attribute(component, this, scope, node));
@@ -241,6 +241,13 @@ export default class Element extends Node {
 	}
 
 	validate() {
+		if (this.component.var_lookup.has(this.name) && this.component.var_lookup.get(this.name).imported) {
+			this.component.warn(this, {
+				code: 'component-name-lowercase',
+				message: `<${this.name}> will be treated as an HTML element unless it begins with a capital letter`
+			});
+		}
+
 		if (a11y_distracting_elements.has(this.name)) {
 			// no-distracting-elements
 			this.component.warn(this, {

src/compiler/interfaces.ts

@@ -161,6 +161,7 @@ export interface Var {
 	hoistable?: boolean;
 	subscribable?: boolean;
 	is_reactive_dependency?: boolean;
+	imported?: boolean;
 }
 
 export interface CssResult { 

test/validator/samples/component-name-lowercase/input.svelte

@@ -0,0 +1,7 @@
+<script>
+	import thisShouldWarnMe from './MyComponent.svelte';
+	let i;
+</script>
+
+<thisShouldWarnMe />
+<i />

test/validator/samples/component-name-lowercase/warnings.json

@@ -0,0 +1,17 @@
+[
+	{
+		"code": "component-name-lowercase",
+		"message": "<thisShouldWarnMe> will be treated as an HTML element unless it begins with a capital letter",
+		"pos": 82,
+		"start": {
+			"character": 82,
+			"column": 0,
+			"line": 6
+		},
+		"end": {
+			"character": 102,
+			"column": 20,
+			"line": 6
+		}
+	}
+]