fix: don't error on slot prop inside block inside other component (#15148)

`slot` is treated as a regular prop if it is not used directly inside another component, but we were running validations on such regular props that should only be run on real slots.

Fixes #15125

.changeset/good-rocks-talk.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't error on slot prop inside block inside other component

packages/svelte/src/compiler/phases/2-analyze/visitors/shared/attribute.js

@@ -80,18 +80,19 @@ export function validate_slot_attribute(context, attribute, is_component = false
 	}
 
 	if (owner) {
-		if (!is_text_attribute(attribute)) {
-			e.slot_attribute_invalid(attribute);
-		}
-
 		if (
 			owner.type === 'Component' ||
 			owner.type === 'SvelteComponent' ||
 			owner.type === 'SvelteSelf'
 		) {
 			if (owner !== parent) {
+				if (!is_component) {
 					e.slot_attribute_invalid_placement(attribute);
 				}
+			} else {
+				if (!is_text_attribute(attribute)) {
+					e.slot_attribute_invalid(attribute);
+				}
 
 				const name = attribute.value[0].data;
 
@@ -117,6 +118,7 @@ export function validate_slot_attribute(context, attribute, is_component = false
 					}
 				}
 			}
+		}
 	} else if (!is_component) {
 		e.slot_attribute_invalid_placement(attribute);
 	}

packages/svelte/tests/validator/samples/slot-attribute-component/input.svelte

@@ -1,2 +1,8 @@
 <Foo slot="foo">valid</Foo>
 <Foo slot={foo}>valid</Foo>
+<Foo>
+    {#if true}
+        <Foo slot="foo">valid</Foo>
+        <Foo slot={foo}>valid</Foo>
+    {/if}
+</Foo>