Correctly handle type aliases with a type variable as the root (#1714)

pkl-core/src/main/java/org/pkl/core/runtime/VmTypeAlias.java

@@ -182,10 +182,18 @@ public final class VmTypeAlias extends VmValue {
     // which should be good for interpreted and compiled performance alike:
     // * Fewer root nodes to call
     // * ControlFlowException used to implement union types doesn't escape root node
+
+    if (typeParameters.isEmpty()) return (TypeNode) typeNode.deepCopy();
+
+    // handle if alias root is itself a type variable (https://github.com/apple/pkl/issues/1711)
+    if (typeNode instanceof TypeVariableNode typeVarNode) {
+      // no need to run validation since the arg itself has already been checked
+      return typeArgumentNodes.length == 0
+          ? new UnknownTypeNode(sourceSection)
+          : typeArgumentNodes[typeVarNode.getTypeParameterIndex()];
+    }
+
     var clone = (TypeNode) typeNode.deepCopy();
-
-    if (typeParameters.isEmpty()) return clone;
-
     clone.accept(
         node -> {
           if (node instanceof TypeVariableNode typeVarNode) {

pkl-core/src/test/files/LanguageSnippetTests/input/types/typeAlias3.pkl

@@ -0,0 +1,2 @@
+typealias Id<T> = T
+foo: Id<Int> = 1

pkl-core/src/test/files/LanguageSnippetTests/input/types/typeAlias3a.pkl

@@ -0,0 +1,2 @@
+typealias Id<T> = T
+foo: Id<Int> = "oops"

pkl-core/src/test/files/LanguageSnippetTests/output/types/typeAlias3.pcf

@@ -0,0 +1 @@
+foo = 1

pkl-core/src/test/files/LanguageSnippetTests/output/types/typeAlias3a.err

@@ -0,0 +1,19 @@
+–– Pkl Error ––
+Expected value of type `Int`, but got type `String`.
+Value: "oops"
+
+x | foo: Id<Int> = "oops"
+            ^^^
+at typeAlias3a#foo (file:///$snippetsDir/input/types/typeAlias3a.pkl)
+
+x | foo: Id<Int> = "oops"
+                   ^^^^^^
+at typeAlias3a#foo (file:///$snippetsDir/input/types/typeAlias3a.pkl)
+
+xxx | renderer.renderDocument(value)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)
+
+xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
+                                                                         ^^^^
+at pkl.base#Module.output.bytes (pkl:base)