Fix: Handle Negative VLQ Offsets in Source Maps for VSCode Jump to Definition

internal/fourslash/tests/statedeclarationmaps_test.go

@@ -385,3 +385,47 @@ fn5();
 		})
 	}
 }
+
+// TestDeclarationMapsNonMonotonicMappings verifies that getMappedLocation clamps
+// inverted ranges caused by non-monotonic source map mappings.
+//
+// The baseline comparison catches regressions: without the fix, the output shows
+// inverted markers (e.g., "|]|>" appearing before "<|"), while with the fix,
+// the ranges are clamped to valid zero-length ranges (e.g., "<||>").
+func TestDeclarationMapsNonMonotonicMappings(t *testing.T) {
+	t.Parallel()
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	// The source map creates a non-monotonic mapping:
+	// - .d.ts line 0 col 24 ('b' identifier) -> source line 1, col 16
+	// - .d.ts line 0 col 25 (right after 'b') -> source line 0, col 0 (EARLIER!)
+	//
+	// When looking up 'b' identifier [24, 25), start maps to ~byte 39,
+	// but end maps to byte 0, creating an inverted range.
+	// The fix in getMappedLocation clamps this to prevent negative ranges.
+	const content = `
+// @Filename: /src/index.ts
+export function a() {}
+export function b() {}
+// @Filename: /src/indexdef.d.ts.map
+{
+	"version": 3,
+	"file": "indexdef.d.ts",
+	"sourceRoot": "",
+	"sources": ["index.ts"],
+	"names": [],
+	"mappings": "AACA,wBAAgB,CADhB;AAAA,wBAAgB"
+}
+// @Filename: /src/indexdef.d.ts
+export declare function b(): void;
+export declare function a(): void;
+//# sourceMappingURL=indexdef.d.ts.map
+// @Filename: /src/user.ts
+import { a, b } from "./indexdef";
+/*1*/a();
+/*2*/b();
+// @Filename: /src/tsconfig.json
+{}`
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+	f.VerifyBaselineGoToDefinition(t, true, "1", "2")
+}

internal/ls/source_map.go

@@ -27,7 +27,17 @@ func (l *LanguageService) getMappedLocation(fileName string, fileRange core.Text
 		}
 	}
 	debug.Assert(endPos.FileName == startPos.FileName, "start and end should be in same file")
-	newRange := core.NewTextRange(startPos.Pos, endPos.Pos)
+
+	// Validate that the end position is valid (same file and after start position).
+	// This handles non-monotonic source map mappings where the next mapping entry
+	// might have an original position that's earlier in the source file.
+	// If end is before start, clamp to start position to avoid inverted ranges.
+	endPosValue := endPos.Pos
+	if endPos.FileName != startPos.FileName || endPos.Pos < startPos.Pos {
+		endPosValue = startPos.Pos
+	}
+
+	newRange := core.NewTextRange(startPos.Pos, endPosValue)
 	lspRange := l.createLspRangeFromRange(newRange, l.getScript(startPos.FileName))
 	return lsproto.Location{
 		Uri:   lsconv.FileNameToDocumentURI(startPos.FileName),

testdata/baselines/reference/fourslash/goToDefinition/declarationMapsNonMonotonicMappings.baseline.jsonc

@@ -0,0 +1,21 @@
+// === goToDefinition ===
+// === /src/index.ts ===
+// <|export function [|a|]() {}
+// export func|>tion b() {}
+
+// === /src/user.ts ===
+// import { a, b } from "./indexdef";
+// /*GOTO DEF*/[|a|]();
+// b();
+
+
+
+// === goToDefinition ===
+// === /src/index.ts ===
+// export function a() {}
+// <||>export function [|{ contextId: 0 |}|]b() {}
+
+// === /src/user.ts ===
+// import { a, b } from "./indexdef";
+// a();
+// /*GOTO DEF*/[|b|]();