feat(compiler): warn invalid children for transition and keep-alive

packages/compiler-core/src/errors.ts

@@ -81,6 +81,7 @@ export const enum ErrorCodes {
   X_V_MODEL_MALFORMED_EXPRESSION,
   X_V_MODEL_ON_SCOPE_VARIABLE,
   X_INVALID_EXPRESSION,
+  X_KEEP_ALIVE_INVALID_CHILDREN,
 
   // generic errors
   X_PREFIX_ID_NOT_SUPPORTED,
@@ -174,6 +175,7 @@ export const errorMessages: { [code: number]: string } = {
   [ErrorCodes.X_V_MODEL_MALFORMED_EXPRESSION]: `v-model value must be a valid JavaScript member expression.`,
   [ErrorCodes.X_V_MODEL_ON_SCOPE_VARIABLE]: `v-model cannot be used on v-for or v-slot scope variables because they are not writable.`,
   [ErrorCodes.X_INVALID_EXPRESSION]: `Invalid JavaScript expression.`,
+  [ErrorCodes.X_KEEP_ALIVE_INVALID_CHILDREN]: `<KeepAlive> expects exactly one child component.`,
 
   // generic errors
   [ErrorCodes.X_PREFIX_ID_NOT_SUPPORTED]: `"prefixIdentifiers" option is not supported in this build of compiler.`,

packages/compiler-core/src/transforms/transformElement.ts

@@ -100,6 +100,17 @@ export const transformElement: NodeTransform = (node, context) => {
       if (!hasProps) {
         args.push(`null`)
       }
+
+      if (__DEV__ && nodeType === KEEP_ALIVE && node.children.length > 1) {
+        context.onError(
+          createCompilerError(ErrorCodes.X_KEEP_ALIVE_INVALID_CHILDREN, {
+            start: node.children[0].loc.start,
+            end: node.children[node.children.length - 1].loc.end,
+            source: ''
+          })
+        )
+      }
+
       // Portal & KeepAlive should have normal children instead of slots
       // Portal is not a real component has dedicated handling in the renderer
       // KeepAlive should not track its own deps so that it can be used inside