Clamp calculated sourcemap positions rather than throwing (#28583)

* Clamp calculated sourcemap positions rather than throwing, to allow the underlying file to drift out of date with the sourcemap without a crash

* Clamp line as well

Author: weswigham

src/compiler/scanner.ts

@@ -342,12 +342,29 @@ namespace ts {
     }
 
     /* @internal */
-    export function computePositionOfLineAndCharacter(lineStarts: ReadonlyArray<number>, line: number, character: number, debugText?: string): number {
+    export function getPositionOfLineAndCharacterWithEdits(sourceFile: SourceFileLike, line: number, character: number): number {
+        return computePositionOfLineAndCharacter(getLineStarts(sourceFile), line, character, sourceFile.text, /*allowEdits*/ true);
+    }
+
+    /* @internal */
+    export function computePositionOfLineAndCharacter(lineStarts: ReadonlyArray<number>, line: number, character: number, debugText?: string, allowEdits?: true): number {
         if (line < 0 || line >= lineStarts.length) {
-            Debug.fail(`Bad line number. Line: ${line}, lineStarts.length: ${lineStarts.length} , line map is correct? ${debugText !== undefined ? arraysEqual(lineStarts, computeLineStarts(debugText)) : "unknown"}`);
+            if (allowEdits) {
+                // Clamp line to nearest allowable value
+                line = line < 0 ? 0 : line >= lineStarts.length ? lineStarts.length - 1 : line;
+            }
+            else {
+                Debug.fail(`Bad line number. Line: ${line}, lineStarts.length: ${lineStarts.length} , line map is correct? ${debugText !== undefined ? arraysEqual(lineStarts, computeLineStarts(debugText)) : "unknown"}`);
+            }
         }
 
         const res = lineStarts[line] + character;
+        if (allowEdits) {
+            // Clamp to nearest allowable values to allow the underlying to be edited without crashing (accuracy is lost, instead)
+            // TODO: Somehow track edits between file as it was during the creation of sourcemap we have and the current file and
+            // apply them to the computed position to improve accuracy
+            return res > lineStarts[line + 1] ? lineStarts[line + 1] : typeof debugText === "string" && res > debugText.length ? debugText.length : res;
+        }
         if (line < lineStarts.length - 1) {
             Debug.assert(res < lineStarts[line + 1]);
         }

src/compiler/sourcemap.ts

@@ -608,7 +608,7 @@ namespace ts {
 
         function processMapping(mapping: Mapping): MappedPosition {
             const generatedPosition = generatedFile !== undefined
-                ? getPositionOfLineAndCharacter(generatedFile, mapping.generatedLine, mapping.generatedCharacter)
+                ? getPositionOfLineAndCharacterWithEdits(generatedFile, mapping.generatedLine, mapping.generatedCharacter)
                 : -1;
             let source: string | undefined;
             let sourcePosition: number | undefined;
@@ -617,7 +617,7 @@ namespace ts {
                 const sourceFile = host.getSourceFileLike(sourceFilePath);
                 source = map.sources[mapping.sourceIndex];
                 sourcePosition = sourceFile !== undefined
-                    ? getPositionOfLineAndCharacter(sourceFile, mapping.sourceLine, mapping.sourceCharacter)
+                    ? getPositionOfLineAndCharacterWithEdits(sourceFile, mapping.sourceLine, mapping.sourceCharacter)
                     : -1;
             }
             return {

tests/cases/fourslash/server/declarationMapsOutOfDateMapping.ts

@@ -0,0 +1,32 @@
+/// <reference path="../fourslash.ts" />
+
+// @Filename: /node_modules/a/dist/index.d.ts
+////export declare class Foo {
+////    bar: any;
+////}
+//////# sourceMappingURL=index.d.ts.map
+
+// @Filename: /node_modules/a/dist/index.d.ts.map
+////{"version":3,"file":"index.d.ts","sourceRoot":"","sources":["../src/index.ts"],"names":[],"mappings":"AAAA,qBAAa,GAAG;IACZ,GAAG,MAAC;CACP"}
+
+// @Filename: /node_modules/a/src/index.ts
+////export class /*2*/Foo {
+////}
+////
+
+// @Filename: /node_modules/a/package.json
+////{
+////    "name": "a",
+////    "version": "0.0.0",
+////    "private": true,
+////    "main": "dist",
+////    "types": "dist"
+////}
+
+// @Filename: /index.ts
+////import { Foo/*1*/ } from "a";
+
+goTo.file("/index.ts");
+
+goTo.marker("1");
+verify.goToDefinitionIs("2"); // getDefinitionAndBoundSpan