Fix checker crash with new block scoped bindings

Signed-off-by: Joey Watts <[email protected]>

Author: Joey Watts

src/compiler/checker.ts

@@ -17698,18 +17698,20 @@ namespace ts {
                 if (usedInFunction) {
                     // mark iteration statement as containing block-scoped binding captured in some function
                     let capturesBlockScopeBindingInLoopBody = true;
-                    if (isForStatement(container) &&
-                        getAncestor(symbol.valueDeclaration, SyntaxKind.VariableDeclarationList)!.parent === container) {
-                        const part = getPartOfForStatementContainingNode(node.parent, container);
-                        if (part) {
-                            const links = getNodeLinks(part);
-                            links.flags |= NodeCheckFlags.ContainsCapturedBlockScopeBinding;
-
-                            const capturedBindings = links.capturedBlockScopeBindings || (links.capturedBlockScopeBindings = []);
-                            pushIfUnique(capturedBindings, symbol);
-
-                            if (part === container.initializer) {
-                                capturesBlockScopeBindingInLoopBody = false; // Initializer is outside of loop body
+                    if (isForStatement(container)) {
+                        const varDeclList = getAncestor(symbol.valueDeclaration, SyntaxKind.VariableDeclarationList);
+                        if (varDeclList && varDeclList.parent === container) {
+                            const part = getPartOfForStatementContainingNode(node.parent, container);
+                            if (part) {
+                                const links = getNodeLinks(part);
+                                links.flags |= NodeCheckFlags.ContainsCapturedBlockScopeBinding;
+
+                                const capturedBindings = links.capturedBlockScopeBindings || (links.capturedBlockScopeBindings = []);
+                                pushIfUnique(capturedBindings, symbol);
+
+                                if (part === container.initializer) {
+                                    capturesBlockScopeBindingInLoopBody = false; // Initializer is outside of loop body
+                                }
                             }
                         }
                     }
@@ -17720,10 +17722,11 @@ namespace ts {
 
                 // mark variables that are declared in loop initializer and reassigned inside the body of ForStatement.
                 // if body of ForStatement will be converted to function then we'll need a extra machinery to propagate reassigned values back.
-                if (container.kind === SyntaxKind.ForStatement &&
-                    getAncestor(symbol.valueDeclaration, SyntaxKind.VariableDeclarationList)!.parent === container &&
-                    isAssignedInBodyOfForStatement(node, <ForStatement>container)) {
-                    getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.NeedsLoopOutParameter;
+                if (isForStatement(container)) {
+                    const varDeclList = getAncestor(symbol.valueDeclaration, SyntaxKind.VariableDeclarationList);
+                    if (varDeclList && varDeclList.parent === container && isAssignedInBodyOfForStatement(node, container)) {
+                        getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.NeedsLoopOutParameter;
+                    }
                 }
 
                 // set 'declared inside loop' bit on the block-scoped binding