Require references from typealiases to be const (#516)

This adds a language change that requires references from typealiases
to the enclosing module to be `const`.

This is required because typealiases are not late-bound.

Rationale is laid out in SPICE-0007.

Also:
* Update documentation to reflect new rules.
* Fix `Project.pkl`; mark method `const` to not break said rule.

docs/modules/language-reference/pages/index.adoc

@@ -1147,9 +1147,13 @@ Therefore, the same rules that apply to `fixed` also apply to `const`:
 * The const-ness of a property or method must be preserved when it is overridden by a child class.
 
 [[class-and-annotation-const]]
-*Class and Annotation Scoping*
+*Class, Annotation, and Typealias Scoping*
 
-Within a class or annotation body, any reference to a property or method of its enclosing module requires that the referenced member is `const`.
+In these following scenarios, any reference to a property or method of its enclosing module requires that the referenced member is `const`:
+
+* Class body
+* Annotation body
+* Typealiased constrained type
 
 .invalid2.pkl
 [source%parsed,{pkl}]
@@ -1162,15 +1166,18 @@ class Bird {
 
 @Deprecated { message = "Replace with \(pigeonName)" } // <2>
 oldPigeonName: String
+
+typealias IsPigeonName = String(pigeonName) // <3>
 ----
 <1> Error: cannot reference non-const property `pigeonName` from a class.
 <2> Error: cannot reference non-const property `pigeonName` from an annotation.
+<3> Error: cannot reference non-const property `pigeonname` from a typealias.
 
-This rule exists because classes and annotations are not <<late-binding,late bound>>;
+This rule exists because classes, annotations, and typealiases are not <<late-binding,late bound>>;
-it is not possible to change the definition of a class or annotation by amending the module
+it is not possible to change the definition of these members by amending the module
 where it is defined.
 
-Generally, there are two strategies for referencing a property from a class or annotation:
+Generally, there are two strategies for referencing such properties:
 
 *Add the `const` modifier to the referenced property*
 

pkl-core/src/main/java/org/pkl/core/ast/builder/SymbolTable.java

@@ -439,7 +439,7 @@ public final class SymbolTable {
         String qualifiedName,
         FrameDescriptor.Builder frameDescriptorBuilder,
         List<TypeParameter> typeParameters) {
-      super(parent, name, qualifiedName, ConstLevel.NONE, frameDescriptorBuilder, typeParameters);
+      super(parent, name, qualifiedName, ConstLevel.MODULE, frameDescriptorBuilder, typeParameters);
     }
   }
 

pkl-core/src/test/files/LanguageSnippetTests/input/errors/const/constTypeAliasConstraint.pkl

@@ -0,0 +1,5 @@
+typealias MyValue = Any(isValid)
+
+isValid = true
+
+myValue: MyValue = 1

pkl-core/src/test/files/LanguageSnippetTests/output/errors/const/constTypeAliasConstraint.err

@@ -0,0 +1,18 @@
+–– Pkl Error ––
+Cannot reference property `isValid` from here because it is not `const`.
+
+x | typealias MyValue = Any(isValid)
+                            ^^^^^^^
+at constTypeAliasConstraint#myValue (file:///$snippetsDir/input/errors/const/constTypeAliasConstraint.pkl)
+
+To fix, do either of:
+1. Add modifier `const` to property `isValid`
+2. Self-import this module, and reference this property from the import.
+
+x | myValue: MyValue = 1
+                       ^
+at constTypeAliasConstraint#myValue (file:///$snippetsDir/input/errors/const/constTypeAliasConstraint.pkl)
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)

stdlib/Project.pkl

@@ -221,7 +221,7 @@ function newInstance(enclosingModule: Module): Project = new {
   projectFileUri = reflect.Module(enclosingModule).uri
 }
 
-local hasVersion = (it: Uri) ->
+const local hasVersion = (it: Uri) ->
   let (versionSep = it.lastIndexOf("@"))
     if (versionSep == -1) false
     else let (version = it.drop(versionSep + 1))