[clang] [Clang][Sema] Fix crash when MS dependent base class lookup occurs in an incomplete context (PR #83024)

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian created https://github.com/llvm/llvm-project/pull/83024

When compiling the following with `-fms-compatibility`:
```cpp
template<typename T> struct C;

// Test lookup with incomplete lookup context
template<typename T>
auto C<T>::f() -> decltype(x) { }
```
An assert fails because `CXXRecordDecl::hasAnyDependentBases` is called on an incomplete class. This patch ensures we don't perform unqualified lookup into dependent base classes when the lookup context is incomplete.


>From c125aa46c61a0d019eccd5f64f8f3f03aa571196 Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian at gmail.com>
Date: Mon, 26 Feb 2024 11:03:06 -0500
Subject: [PATCH] [Clang][Sema] Fix crash when MS dependent base class lookup
 occurs in an incomplete context

---
 clang/lib/Sema/SemaExpr.cpp                                | 2 +-
 .../test/SemaTemplate/ms-lookup-template-base-classes.cpp  | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 816ee9e281359a..403839f77a2b7c 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -2653,7 +2653,7 @@ recoverFromMSUnqualifiedLookup(Sema &S, ASTContext &Context,
     RD = ThisType->getPointeeType()->getAsCXXRecordDecl();
   else if (auto *MD = dyn_cast<CXXMethodDecl>(S.CurContext))
     RD = MD->getParent();
-  if (!RD || !RD->hasAnyDependentBases())
+  if (!RD || !RD->hasDefinition() || !RD->hasAnyDependentBases())
     return nullptr;
 
   // Diagnose this as unqualified lookup into a dependent base class.  If 'this'
diff --git a/clang/test/SemaTemplate/ms-lookup-template-base-classes.cpp b/clang/test/SemaTemplate/ms-lookup-template-base-classes.cpp
index 7856a0a16307be..534a5dc9ddc10d 100644
--- a/clang/test/SemaTemplate/ms-lookup-template-base-classes.cpp
+++ b/clang/test/SemaTemplate/ms-lookup-template-base-classes.cpp
@@ -71,6 +71,13 @@ class B : public A<T> {
 
 template class B<int>;
 
+template<typename T> struct C;
+
+// Test lookup with incomplete lookup context
+template<typename T>
+auto C<T>::f() -> decltype(x) { } // expected-error {{use of undeclared identifier 'x'}}
+                                  // expected-error at -1 {{out-of-line definition of 'f' from class 'C<T>' without definition}}
+
 }