Assert that enclosing declaration is not synthesized

Author: amcasey

src/compiler/checker.ts

@@ -2444,18 +2444,21 @@ namespace ts {
         function createNodeBuilder() {
             return {
                 typeToTypeNode: (type: Type, enclosingDeclaration?: Node, flags?: NodeBuilderFlags) => {
+                    Debug.assert(enclosingDeclaration === undefined || (enclosingDeclaration.flags & NodeFlags.Synthesized) === 0);
                     const context = createNodeBuilderContext(enclosingDeclaration, flags);
                     const resultingNode = typeToTypeNodeHelper(type, context);
                     const result = context.encounteredError ? undefined : resultingNode;
                     return result;
                 },
                 indexInfoToIndexSignatureDeclaration: (indexInfo: IndexInfo, kind: IndexKind, enclosingDeclaration?: Node, flags?: NodeBuilderFlags) => {
+                    Debug.assert(enclosingDeclaration === undefined || (enclosingDeclaration.flags & NodeFlags.Synthesized) === 0);
                     const context = createNodeBuilderContext(enclosingDeclaration, flags);
                     const resultingNode = indexInfoToIndexSignatureDeclarationHelper(indexInfo, kind, context);
                     const result = context.encounteredError ? undefined : resultingNode;
                     return result;
                 },
                 signatureToSignatureDeclaration: (signature: Signature, kind: SyntaxKind, enclosingDeclaration?: Node, flags?: NodeBuilderFlags) => {
+                    Debug.assert(enclosingDeclaration === undefined || (enclosingDeclaration.flags & NodeFlags.Synthesized) === 0);
                     const context = createNodeBuilderContext(enclosingDeclaration, flags);
                     const resultingNode = signatureToSignatureDeclarationHelper(signature, kind, context);
                     const result = context.encounteredError ? undefined : resultingNode;