Fix bad line number assertion in ScriptInfo.positionToLineOffset

This is the line number side of ecddf84 (from #21924, fixing #21818).
But the code is slightly improved for both cases: instead of testing
that `leaf` is defined, check whether `lineCount` is zero, and if it is,
return `〈1,0〉` for the one-based line and zero-based column numbers.
(The requirement of `lineCount > 0` is also seen in the fact
that `lineNumberToInfo` expects a "*One*BasedLine" argument.)

I've stared at this code way too much, since I think that there is
something more fundamentally wrong here.  E.g., `EditWalker` only
`push`es to `startPath` but never pops even a `children`-less node that
is left after deleting the whole contents.  But I can't figure out the
overall structure, which is also why the test that I added is not
great (see the comment there; also, #21924 is dealing with the same
problem and didn't add a test).

Fixes #44518.

Author: elibarzilay

src/server/scriptVersionCache.ts

@@ -67,10 +67,8 @@ namespace ts.server {
             }
             const lm = LineIndex.linesFromText(insertedText);
             const lines = lm.lines;
-            if (lines.length > 1) {
-                if (lines[lines.length - 1] === "") {
-                    lines.pop();
-                }
+            if (lines.length > 1 && lines[lines.length - 1] === "") {
+                lines.pop();
             }
             let branchParent: LineNode | undefined;
             let lastZeroCount: LineCollection | undefined;
@@ -683,8 +681,12 @@ namespace ts.server {
             }
 
             // Skipped all children
-            const { leaf } = this.lineNumberToInfo(this.lineCount(), 0);
-            return { oneBasedLine: this.lineCount(), zeroBasedColumn: leaf ? leaf.charCount() : 0, lineText: undefined };
+            const lineCount = this.lineCount();
+            if (lineCount === 0) { // it's empty! (and lineNumberToInfo expects a one-based line)
+                return { oneBasedLine: 1, zeroBasedColumn: 0, lineText: undefined };
+            }
+            const leaf = Debug.checkDefined(this.lineNumberToInfo(lineCount, 0).leaf);
+            return { oneBasedLine: lineCount, zeroBasedColumn: leaf.charCount(), lineText: undefined };
         }
 
         /**

src/testRunner/unittests/tsserver/versionCache.ts

@@ -52,6 +52,18 @@ var q:Point=<Point>p;`;
             assert.deepEqual(lineIndex.positionToLineOffset(0), { line: 1, offset: 1 });
         });
 
+        it("handles emptying whole file (GH#44518)", () => {
+            // See below for the main thing that this tests; it would be better to have a test
+            // that uses `ScriptInfo.positionToLineOffset` but I couldn't find away to do that
+            const { lines } = server.LineIndex.linesFromText("function foo() {\n\ndsa\n\n}\n\nfo(dsa\n\n\n    ");
+            const lineIndex = new server.LineIndex();
+            lineIndex.load(lines);
+            const snapshot = lineIndex.edit(0, 39);
+            assert.equal(snapshot.getText(0, snapshot.getLength()), "");
+            // line must always be >=1, otherwise the failIfInvalidLocation(location) assertion in ScriptInfo.positionToLineOffset will fail
+            assert.deepEqual(snapshot.positionToLineOffset(0), { line: 1, offset: 1 });
+        });
+
         it(`change 9 1 0 1 {"y"}`, () => {
             validateEditAtLineCharIndex(9, 1, 0, "y");
         });