handle ins/del in completion-record typechecks (#610)

Author: bakkot

src/typechecker.ts

@@ -347,17 +347,19 @@ function previousText(expr: Expr, path: PathItem[]): string | null {
 
 function textFromPreviousPart(seq: Seq, index: number): string | null {
   let prevIndex = index - 1;
+  let text = null;
   let prev;
   while (isProsePart((prev = seq.items[prevIndex]))) {
     if (prev.name === 'text') {
-      return prev.contents;
+      text = prev.contents + (text ?? '');
+      --prevIndex;
     } else if (prev.name === 'tag') {
       --prevIndex;
     } else {
       break;
     }
   }
-  return null;
+  return text;
 }
 
 function stripWhitespace(items: NonSeq[]) {

test/typecheck.js

@@ -203,6 +203,8 @@ describe('typechecking completions', () => {
             1. Set _a_ to Completion(<emu-meta suppress-effects="user-code">ExampleAlg()</emu-meta>).
             1. Set _a_ to ! ExampleAlg().
             1. Return ? ExampleAlg().
+            1. Return <ins>?</ins> ExampleAlg().
+            1. Return <del>?</del> <ins>!</ins> ExampleAlg().
             1. Let _foo_ be 0.
             1. Set _a_ to Completion(ExampleSDO of _foo_).
             1. Set _a_ to Completion(ExampleSDO of _foo_ with argument 0).
@@ -284,6 +286,7 @@ describe('typechecking completions', () => {
         </dl>
         <emu-alg>
           1. Return ExampleAlg().
+          1. Return <del>!</del> ExampleAlg().
         </emu-alg>
         </emu-clause>
       `);