warn for possible misname component to lowercase

src/compiler/compile/nodes/Element.ts

@@ -239,6 +239,13 @@ export default class Element extends Node {
 	}
 
 	validate() {
+		if (this.component.var_lookup.has(this.name) || this.scope.names.has(this.name)) {
+			this.component.warn(this, {
+				code: 'lowercase-component-name',
+				message: `A lowercase tag <${this.name}> will be treated as a regular HTML element`,
+			});
+		}
+
 		if (a11y_distracting_elements.has(this.name)) {
 			// no-distracting-elements
 			this.component.warn(this, {

test/validator/samples/component-name-lowercase-2/input.svelte

@@ -0,0 +1,6 @@
+<script>
+  let array;
+</script>
+{#each array as item}
+  <item />
+{/each}

test/validator/samples/component-name-lowercase-2/warnings.json

@@ -0,0 +1,17 @@
+[
+	{
+		"code": "lowercase-component-name",
+		"end": {
+			"character": 64,
+			"column": 10,
+			"line": 5
+		},
+		"message": "A lowercase tag <item> will be treated as a regular HTML element",
+		"pos": 56,
+		"start": {
+			"character": 56,
+			"column": 2,
+			"line": 5
+		}
+	}
+]

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

@@ -0,0 +1,5 @@
+<script>
+  import thisShouldWarnMe from './MyComponent.svelte';
+</script>
+
+<thisShouldWarnMe />

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

@@ -0,0 +1,17 @@
+[
+	{
+		"code": "lowercase-component-name",
+		"message": "A lowercase tag <thisShouldWarnMe> will be treated as a regular HTML element",
+		"pos": 75,
+		"end": {
+			"character": 95,
+			"column": 20,
+			"line": 5
+		},
+		"start": {
+			"character": 75,
+			"column": 0,
+			"line": 5
+		}
+	}
+]