From 0baa7bac708350fce440614be335103b968da2aa Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian@gmail.com>
Date: Fri, 13 Mar 2026 09:26:36 -0400
Subject: [PATCH] [Clang][Sema] Only call PerformDependentDiagnostics for
 dependent contexts (#177452)

---
 clang/docs/ReleaseNotes.rst                   |  1 +
 .../lib/Sema/SemaTemplateInstantiateDecl.cpp  |  3 ++-
 clang/test/SemaTemplate/GH176155.cpp          | 26 +++++++++++++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaTemplate/GH176155.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 658461b13fd4..031d7405e7f5 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -326,6 +326,7 @@ Bug Fixes in This Version
 - Fixed a crash when parsing ``#pragma clang attribute`` arguments for attributes that forbid arguments. (#GH182122)
 - Fixed a bug with multiple-include optimization (MIOpt) state not being preserved in some cases during lexing, which could suppress header-guard mismatch diagnostics and interfere with include-guard optimization. (#GH180155)
 - Fixed a crash when normalizing constraints involving concept template parameters whose index coincided with non-concept template parameters in the same parameter mapping.
+- Fixed a crash caused by accessing dependent diagnostics of a non-dependent context.
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 972981d7c11f..cc24e03e77c0 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -5969,7 +5969,8 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
 
     checkReferenceToTULocalFromOtherTU(Function, PointOfInstantiation);
 
-    PerformDependentDiagnostics(PatternDecl, TemplateArgs);
+    if (PatternDecl->isDependentContext())
+      PerformDependentDiagnostics(PatternDecl, TemplateArgs);
 
     if (auto *Listener = getASTMutationListener())
       Listener->FunctionDefinitionInstantiated(Function);
diff --git a/clang/test/SemaTemplate/GH176155.cpp b/clang/test/SemaTemplate/GH176155.cpp
new file mode 100644
index 000000000000..12a9de2ae2d4
--- /dev/null
+++ b/clang/test/SemaTemplate/GH176155.cpp
@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+template <int> struct bad {
+  template <class T, auto =
+                         [] { // #lambda
+                           // expected-note@#lambda {{while substituting into a lambda expression here}}
+                           // expected-note@#lambda 2{{capture 'i' by value}}
+                           // expected-note@#lambda 2{{capture 'i' by reference}}
+                           // expected-note@#lambda 2{{default capture by value}}
+                           // expected-note@#lambda 2{{default capture by reference}}
+                           for (int i = 0; i < 100; ++i) { // #i
+                             // expected-error@-1 {{variable 'i' cannot be implicitly captured in a lambda with no capture-default specified}}
+                             // expected-note@#i {{'i' declared here}}
+                             // expected-note@#lambda {{lambda expression begins here}}
+                             // expected-error@-4 {{variable 'i' cannot be implicitly captured in a lambda with no capture-default specified}}
+                             // expected-note@#i {{'i' declared here}}
+                             // expected-note@#lambda {{lambda expression begins here}}
+                             struct LoopHelper {
+                               static constexpr void process() {}
+                             };
+                           }
+                         }>
+  static void f(T) {} // expected-note {{in instantiation of default argument for 'f<int>' required here}}
+};
+
+int main() { bad<0>::f(0); } // expected-note {{while substituting deduced template arguments into function template 'f'}}