[debug] Only apply TDZ 'value unavailable' logic for let/const

This CL refines https://crrev.com/c/3829539 to only apply to let and
const declared variables. `var`s should stay `undefined`.

R=jarin@chromium.org

Bug: chromium:1328681
Change-Id: I35778c89fb04439348a4f6aebcdeb2db6234f9d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3848960Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82640}

src/debug/debug-scopes.cc

@@ -878,7 +878,8 @@ bool ScopeIterator::VisitLocals(const Visitor& visitor, Mode mode,
                 current_scope_->AsDeclarationScope()->arguments() == var) {
               continue;
             }
-          } else if (value->IsUndefined(isolate_) &&
+          } else if (IsLexicalVariableMode(var->mode()) &&
+                     value->IsUndefined(isolate_) &&
                      GetSourcePosition() != kNoSourcePosition &&
                      GetSourcePosition() <= var->initializer_position()) {
             // Variables that are `undefined` could also mean an elided hole

test/inspector/debugger/value-unavailable-scopes-expected.txt

@@ -8,3 +8,6 @@ variable y correctly reported as <value_unavailable>
 
 Running test: testUnusedValueInTdz
 variable y correctly reported as <value_unavailable>
+
+Running test: testVarStaysUndefined
+variable y correctly reported as <undefined>

test/inspector/debugger/value-unavailable-scopes.js

@@ -38,23 +38,34 @@ function unusedValInTdz() {
   debugger;
   let y = 1;
 }
+
+function varStaysUndefined() {
+  debugger;
+  var y = 42;
+}
 `);
 
-Protocol.Debugger.onPaused(async ({params: {callFrames: [{scopeChain}]}}) => {
+async function findLocalVariable(name, scopeChain) {
   for (const scope of scopeChain) {
     if (scope.type !== 'local') continue;
     const {result: {result: variables}} =
         await Protocol.Runtime.getProperties({objectId: scope.object.objectId});
-    for (const variable of variables) {
-      if (variable.name !== 'y') continue;
-      if ('value' in variable || 'get' in variable || 'set' in variable) {
-        InspectorTest.log(
-            'FAIL: variable y was expected to be reported as <value_unavailable>');
-      } else {
-        InspectorTest.log(
-            'variable y correctly reported as <value_unavailable>');
-      }
+    const variable = variables.find(variable => variable.name === name);
+    if (!variable) {
+      InspectorTest.log(`FAIL: variable ${name} not found in local scope`);
     }
+    return variable;
+  }
+}
+
+Protocol.Debugger.onPaused(async ({ params: { callFrames: [{ scopeChain }] } }) => {
+  const variable = await findLocalVariable('y', scopeChain);
+  if ('value' in variable || 'get' in variable || 'set' in variable) {
+    InspectorTest.log(
+        'FAIL: variable y was expected to be reported as <value_unavailable>');
+  } else {
+    InspectorTest.log(
+        'variable y correctly reported as <value_unavailable>');
   }
   await Protocol.Debugger.resume();
 });
@@ -95,4 +106,24 @@ InspectorTest.runAsyncTestSuite([
       Protocol.Debugger.disable(),
     ]);
   },
+
+  async function testVarStaysUndefined() {
+    await Promise.all([
+      Protocol.Runtime.enable(),
+      Protocol.Debugger.enable(),
+    ]);
+    Protocol.Runtime.evaluate({ expression: 'varStaysUndefined()' });
+    const { params: { callFrames: [{ scopeChain }] } } = await Protocol.Debugger.oncePaused();
+    const variable = await findLocalVariable('y', scopeChain);
+    if ('value' in variable && variable.value.type === 'undefined') {
+      InspectorTest.log('variable y correctly reported as <undefined>');
+    } else {
+      InspectorTest.log('FAIL: variable y was expected to be reported as <undefined>');
+    }
+    await Protocol.Debugger.resume();
+    await Promise.all([
+      Protocol.Runtime.disable(),
+      Protocol.Debugger.disable(),
+    ]);
+  },
 ]);