From 3bf6197803e2bca33a3d651c3a1b525b5fda49ce Mon Sep 17 00:00:00 2001
From: Christoph Zwerschke <cito@online.de>
Date: Tue, 28 Sep 2021 22:21:51 +0200
Subject: [PATCH] lexer: fix expression to decode surrogate pairs

---
 src/language/__tests__/lexer-test.ts | 15 +++++++++++++++
 src/language/lexer.ts                |  2 +-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/language/__tests__/lexer-test.ts b/src/language/__tests__/lexer-test.ts
index 173d7176cb1..1e925f73cc3 100644
--- a/src/language/__tests__/lexer-test.ts
+++ b/src/language/__tests__/lexer-test.ts
@@ -1040,6 +1040,21 @@ describe('Lexer', () => {
       locations: [{ line: 1, column: 1 }],
     });
 
+    expectSyntaxError('\uD83D\uDE00').to.deep.equal({
+      message: 'Syntax Error: Unexpected character: U+1F600.',
+      locations: [{ line: 1, column: 1 }],
+    });
+
+    expectSyntaxError('\uD800\uDC00').to.deep.equal({
+      message: 'Syntax Error: Unexpected character: U+10000.',
+      locations: [{ line: 1, column: 1 }],
+    });
+
+    expectSyntaxError('\uDBFF\uDFFF').to.deep.equal({
+      message: 'Syntax Error: Unexpected character: U+10FFFF.',
+      locations: [{ line: 1, column: 1 }],
+    });
+
     expectSyntaxError('\uDEAD').to.deep.equal({
       message: 'Syntax Error: Invalid character: U+DEAD.',
       locations: [{ line: 1, column: 1 }],
diff --git a/src/language/lexer.ts b/src/language/lexer.ts
index c6f24092e5d..4821ed765c7 100644
--- a/src/language/lexer.ts
+++ b/src/language/lexer.ts
@@ -152,7 +152,7 @@ function encodeSurrogatePair(point: number): string {
 }
 
 function decodeSurrogatePair(leading: number, trailing: number): number {
-  return 0x10000 | ((leading & 0x03ff) << 10) | (trailing & 0x03ff);
+  return 0x10000 + ((leading & 0x03ff) << 10) | (trailing & 0x03ff);
 }
 
 /**