fix(22682): Fail on class declaration used in statement position for ES2015 or higher

Author: nicdard

src/compiler/checker.ts

@@ -43614,6 +43614,9 @@ namespace ts {
         }
 
         function checkGrammarClassLikeDeclaration(node: ClassLikeDeclaration): boolean {
+            if (node.parent && isIfStatement(node.parent)) {
+                grammarErrorOnNode(node, Diagnostics.A_class_declaration_can_only_be_used_in_a_scope_where_other_statements_can_reference_it);
+            }
             const file = getSourceFileOfNode(node);
             return checkGrammarClassDeclarationHeritageClauses(node) ||
                 checkGrammarTypeParameterList(node.typeParameters, file);

src/compiler/diagnosticMessages.json

@@ -1469,7 +1469,10 @@
         "category": "Message",
         "code": 1476
     },
-
+    "A class declaration can only be used in a scope where other statements can reference it." : {
+        "category": "Error",
+        "code": 1477
+    },
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",
         "code": 2200

tests/baselines/reference/scopeClassDeclarations.errors.txt

@@ -0,0 +1,13 @@
+tests/cases/compiler/scopeClassDeclarations.ts(4,11): error TS1477: A class declaration can only be used in a scope where other statements can reference it.
+
+
+==== tests/cases/compiler/scopeClassDeclarations.ts (1 errors) ====
+    let condition = false as boolean;
+    
+    if (condition)
+        class C {} // runtime error if transpiled to ES2015 or above: Unexpected token class
+              ~
+!!! error TS1477: A class declaration can only be used in a scope where other statements can reference it.
+    
+    new C(); // runtime error if transpiled to ES5, because 'C' is undefined
+    

tests/baselines/reference/scopeClassDeclarations.js

@@ -0,0 +1,19 @@
+//// [scopeClassDeclarations.ts]
+let condition = false as boolean;
+
+if (condition)
+    class C {} // runtime error if transpiled to ES2015 or above: Unexpected token class
+
+new C(); // runtime error if transpiled to ES5, because 'C' is undefined
+
+
+//// [scopeClassDeclarations.js]
+var condition = false;
+if (condition) {
+    var C = /** @class */ (function () {
+        function C() {
+        }
+        return C;
+    }());
+} // runtime error if transpiled to ES2015 or above: Unexpected token class
+new C(); // runtime error if transpiled to ES5, because 'C' is undefined

tests/baselines/reference/scopeClassDeclarations.symbols

@@ -0,0 +1,13 @@
+=== tests/cases/compiler/scopeClassDeclarations.ts ===
+let condition = false as boolean;
+>condition : Symbol(condition, Decl(scopeClassDeclarations.ts, 0, 3))
+
+if (condition)
+>condition : Symbol(condition, Decl(scopeClassDeclarations.ts, 0, 3))
+
+    class C {} // runtime error if transpiled to ES2015 or above: Unexpected token class
+>C : Symbol(C, Decl(scopeClassDeclarations.ts, 2, 14))
+
+new C(); // runtime error if transpiled to ES5, because 'C' is undefined
+>C : Symbol(C, Decl(scopeClassDeclarations.ts, 2, 14))
+

tests/baselines/reference/scopeClassDeclarations.types

@@ -0,0 +1,16 @@
+=== tests/cases/compiler/scopeClassDeclarations.ts ===
+let condition = false as boolean;
+>condition : boolean
+>false as boolean : boolean
+>false : false
+
+if (condition)
+>condition : boolean
+
+    class C {} // runtime error if transpiled to ES2015 or above: Unexpected token class
+>C : C
+
+new C(); // runtime error if transpiled to ES5, because 'C' is undefined
+>new C() : C
+>C : typeof C
+

tests/cases/compiler/scopeClassDeclarations.ts

@@ -0,0 +1,6 @@
+let condition = false as boolean;
+
+if (condition)
+    class C {} // runtime error if transpiled to ES2015 or above: Unexpected token class
+
+new C(); // runtime error if transpiled to ES5, because 'C' is undefined