fix(developer): check vars string usage before definition

Validation of vars was not properly checking for forward references to
string variables. This, coupled with a null vs undefined bug in
subsequent use, meant that forward reference variables were ending up
with a literal string value of 'undefined'.

This also fixes the test for visual-keyboard-compiler, where the fixture
was actually buggy and was the trigger for investigating this problem.

Fixes: #12403
Relates-to: #12395

common/web/types/src/kmx/kmx-plus/kmx-plus.ts

@@ -291,7 +291,7 @@ export class Vars extends Section {
     });
   }
   findStringVariableValue(id: string): string {
-    return Vars.findVariable(this.strings, id)?.value?.value; // Unwrap: Variable, StrsItem
+    return Vars.findVariable(this.strings, id)?.value?.value ?? null; // Unwrap: Variable, StrsItem
   }
   substituteSetRegex(str: string, sections: DependencySections): string {
     return str.replaceAll(VariableParser.SET_REFERENCE, (_entire, id) => {

developer/src/kmc-ldml/src/compiler/vars.ts

@@ -68,9 +68,16 @@ export class VarsCompiler extends SectionCompiler {
       // Strings
       for (const { id, value } of variables.string) {
         addId(id);
-        allStrings.add(id);
         const stringrefs = VariableParser.allStringReferences(value);
+        for(const ref of stringrefs) {
+          if(!allStrings.has(ref)) {
+            valid = false;
+            this.callbacks.reportMessage(LdmlCompilerMessages.Error_MissingStringVariable({id: ref}));
+            allStrings.add(ref); // avoids multiple reports of same missing variable
+          }
+        }
         st.string.add(SubstitutionUse.variable, stringrefs);
+        allStrings.add(id);
       }
       // Sets
       for (const { id, value } of variables.set) {

developer/src/kmc-ldml/test/fixtures/sections/vars/fail-badref-7.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<keyboard3 xmlns="https://schemas.unicode.org/cldr/45/keyboard3" locale="mt" conformsTo="45">
+  <info name="vars-fail"/>
+
+  <keys />
+
+  <!-- from spec -->
+  <variables>
+    <string id="y" value="${usedBeforeDefinition}" /> <!-- FAIL: reference before definition -->
+    <string id="usedBeforeDefinition" value="yes" />
+  </variables>
+
+
+</keyboard3>

developer/src/kmc-ldml/test/test-vars.ts

@@ -187,7 +187,14 @@ describe('vars', function () {
         LdmlCompilerMessages.Error_MissingStringVariable({id: 'missingStringInSet'})
       ],
     },
-  ], varsDependencies);
+    {
+      subpath: 'sections/vars/fail-badref-7.xml',
+      errors: [
+        LdmlCompilerMessages.Error_MissingStringVariable({id: 'usedBeforeDefinition'})
+      ],
+      strictErrors: true
+    },
+], varsDependencies);
   describe('should match some marker constants', () => {
     // neither of these live here, but, common/web/types does not import ldml-keyboard-constants otherwise.
 

developer/src/kmc-ldml/test/test-visual-keyboard-compiler.ts

@@ -153,7 +153,7 @@ describe('visual-keyboard-compiler', function() {
     assert.equal(vk.keys[1].text, '\u{0e81}');
   });
 
-  it.skip('should read string variables in key.output', async function() {
+  it('should read string variables in key.output', async function() {
     const xml = stripIndent`
       <?xml version="1.0" encoding="UTF-8"?>
       <keyboard3 xmlns="https://schemas.unicode.org/cldr/45/keyboard3" locale="mt" conformsTo="45">
@@ -165,8 +165,8 @@ describe('visual-keyboard-compiler', function() {
           <layer modifiers="none"><row keys="x" /></layer>
         </layers>
         <variables>
-          <string id="one" value="\${two}" />
           <string id="two" value="2" />
+          <string id="one" value="\${two}" />
         </variables>
       </keyboard3>
     `;
@@ -177,7 +177,7 @@ describe('visual-keyboard-compiler', function() {
     assert.equal(vk.keys[0].text, '2');
   });
 
-  it.skip('should read string variables in display.display', async function() {
+  it('should read string variables in display.display', async function() {
     const xml = stripIndent`
       <?xml version="1.0" encoding="UTF-8"?>
       <keyboard3 xmlns="https://schemas.unicode.org/cldr/45/keyboard3" locale="mt" conformsTo="45">
@@ -192,8 +192,8 @@ describe('visual-keyboard-compiler', function() {
           <layer modifiers="none"><row keys="x" /></layer>
         </layers>
         <variables>
-          <string id="one" value="\${two}" />
           <string id="two" value="2" />
+          <string id="one" value="\${two}" />
         </variables>
       </keyboard3>
     `;