From ee709cffe520df3cf082dc7923609c42dd14cabc Mon Sep 17 00:00:00 2001
From: Yong He <yonghe@outlook.com>
Date: Wed, 23 Oct 2024 20:45:59 -0700
Subject: [PATCH] Fix language server crash. (#5395)

---
 source/slang/slang-check-decl.cpp        | 7 +++++++
 tests/language-server/robustness-8.slang | 8 ++++++++
 2 files changed, 15 insertions(+)
 create mode 100644 tests/language-server/robustness-8.slang

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 781a3a237f..8030bcf974 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -7861,6 +7861,13 @@ namespace Slang
                 if (semantics)
                     semantics->ensureDecl(genericTypeConstraintDecl, DeclCheckState::ReadyForReference);
                 auto constraintDeclRef = astBuilder->getDirectDeclRef<GenericTypeConstraintDecl>(genericTypeConstraintDecl);
+                auto supType = getSup(astBuilder, constraintDeclRef);
+                if (!supType)
+                {
+                    args.add(astBuilder->getErrorType());
+                    shouldCache = false;
+                    continue;
+                }
                 auto witness =
                     astBuilder->getDeclaredSubtypeWitness(
                         getSub(astBuilder, constraintDeclRef),
diff --git a/tests/language-server/robustness-8.slang b/tests/language-server/robustness-8.slang
new file mode 100644
index 0000000000..84d6c8945a
--- /dev/null
+++ b/tests/language-server/robustness-8.slang
@@ -0,0 +1,8 @@
+//TEST:LANG_SERVER(filecheck=CHECK):
+//
+__generic < T : struct> extension T
+{
+// CHECK: null
+//HOVER:7,32
+    __init(StructuredBuffer<T>) {}
+}
\ No newline at end of file