From 572ddf1365b40b213ed4275949ceacb6ba95523d Mon Sep 17 00:00:00 2001
From: PaulyBearCoding <PaulyBearCoding@users.noreply.github.com>
Date: Thu, 6 Nov 2025 23:30:02 -0800
Subject: [PATCH] Reject using and await using declarations in switch
 case/default clauses
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Fixes #62708

This change implements parser validation to reject `using` and `await using`
declarations when directly nested within `case` or `default` clauses of switch
statements, as per the updated ECMAScript spec.

The spec was updated to disallow this pattern (see rbuckton/ecma262#14) because:
- It makes the number of resource declarations statically knowable
- All major JS engines agreed to this restriction
- The pattern is rarely used in practice

Changes:
- Add new diagnostic (TS95198) for the error message
- Add validation in parseVariableDeclarationList() to check parsingContext
- Add error baseline for existing test case that now produces errors

Valid workaround: Wrap using declarations in a block statement:
  case 0: {
    using x = ...;  // OK
  }

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>
---
 src/compiler/diagnosticMessages.json          |  4 +++
 src/compiler/parser.ts                        |  6 ++++
 ...ngDeclarations.1(target=esnext).errors.txt | 28 +++++++++++++++++++
 3 files changed, 38 insertions(+)
 create mode 100644 tests/baselines/reference/usingDeclarations.1(target=esnext).errors.txt

diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 5731936e4978e..b12931c9a7684 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -8332,6 +8332,10 @@
         "category": "Message",
         "code": 95197
     },
+    "A '{0}' declaration cannot be placed within a 'case' or 'default' clause.": {
+        "category": "Error",
+        "code": 95198
+    },
 
     "No value exists in scope for the shorthand property '{0}'. Either declare one or provide an initializer.": {
         "category": "Error",
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 68133ac5f1e40..6053a12717662 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -7678,12 +7678,18 @@ namespace Parser {
                 break;
             case SyntaxKind.UsingKeyword:
                 flags |= NodeFlags.Using;
+                if (parsingContext === ParsingContext.SwitchClauseStatements) {
+                    parseErrorAtCurrentToken(Diagnostics.A_0_declaration_cannot_be_placed_within_a_case_or_default_clause, "using");
+                }
                 break;
             case SyntaxKind.AwaitKeyword:
                 if (!isAwaitUsingDeclaration()) {
                     break;
                 }
                 flags |= NodeFlags.AwaitUsing;
+                if (parsingContext === ParsingContext.SwitchClauseStatements) {
+                    parseErrorAtCurrentToken(Diagnostics.A_0_declaration_cannot_be_placed_within_a_case_or_default_clause, "await using");
+                }
                 nextToken();
                 break;
             default:
diff --git a/tests/baselines/reference/usingDeclarations.1(target=esnext).errors.txt b/tests/baselines/reference/usingDeclarations.1(target=esnext).errors.txt
new file mode 100644
index 0000000000000..7a043110be79c
--- /dev/null
+++ b/tests/baselines/reference/usingDeclarations.1(target=esnext).errors.txt
@@ -0,0 +1,28 @@
+usingDeclarations.1.ts(100,9): error TS95198: A 'using' declaration cannot be placed within a 'case' or 'default' clause.
+usingDeclarations.1.ts(104,9): error TS95198: A 'using' declaration cannot be placed within a 'case' or 'default' clause.
+usingDeclarations.1.ts(111,13): error TS95198: A 'using' declaration cannot be placed within a 'case' or 'default' clause.
+
+
+==== usingDeclarations.1.ts (3 errors) ====
+    switch (Math.random()) {
+        case 0:
+            using d20 = { [Symbol.dispose]() {} };
+            ~~~~~
+!!! error TS95198: A 'using' declaration cannot be placed within a 'case' or 'default' clause.
+            break;
+
+        case 1:
+            using d21 = { [Symbol.dispose]() {} };
+            ~~~~~
+!!! error TS95198: A 'using' declaration cannot be placed within a 'case' or 'default' clause.
+            break;
+    }
+
+    if (true)
+        switch (0) {
+            case 0:
+                using d22 = { [Symbol.dispose]() {} };
+                ~~~~~
+!!! error TS95198: A 'using' declaration cannot be placed within a 'case' or 'default' clause.
+                break;
+        }