[clang] Sema: filter out invalid base-specifiers before attaching (PR #147213)

[Shashi Shankar via cfe-commits]

https://github.com/shashi1687 updated https://github.com/llvm/llvm-project/pull/147213

>From 05211dac479bf25aaf6d8ecc3a53871a51f7ffdd Mon Sep 17 00:00:00 2001
From: Shashi Shankar <shashishankar1687 at gmail.com>
Date: Sun, 6 Jul 2025 22:25:48 +0200
Subject: [PATCH] Sema: filter out invalid base-specifiers before attaching
 ActOnBaseSpecifiers now skips nullptr entries returned for invalid bases,
 avoiding ICE on invalid inheritance. Added regression test in
 SemaCXX/invalid-inheritance.cpp.

Fixes #147186

Signed-off-by: Shashi Shankar <shashishankar1687 at gmail.com>
---
 clang/lib/Sema/SemaDeclCXX.cpp                  | 17 ++++++++++++++++-
 clang/test/SemaCXX/invalid-base-inheritance.cpp | 13 +++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/invalid-base-inheritance.cpp

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index f0247f865ba40..231610241d8ee 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -3031,8 +3031,23 @@ void Sema::ActOnBaseSpecifiers(Decl *ClassDecl,
   if (!ClassDecl || Bases.empty())
     return;
 
+  // --- drop any bases already diagnosed invalid ---
+  SmallVector<CXXBaseSpecifier *, 4> ValidBases;
+  ValidBases.reserve(Bases.size());
+  for (auto *BS : Bases)
+       if (BS)
+          ValidBases.push_back(BS);
+  if (ValidBases.empty())
+       return;
+
+  if (ValidBases.empty())
+    return; // nothing valid to attach
+
   AdjustDeclIfTemplate(ClassDecl);
-  AttachBaseSpecifiers(cast<CXXRecordDecl>(ClassDecl), Bases);
+  // Attach only the valid bases so downstream never ICEs
+  AttachBaseSpecifiers(cast<CXXRecordDecl>(ClassDecl),
+                       llvm::MutableArrayRef<CXXBaseSpecifier *>(
+                       ValidBases.data(), ValidBases.size()));
 }
 
 bool Sema::IsDerivedFrom(SourceLocation Loc, CXXRecordDecl *Derived,
diff --git a/clang/test/SemaCXX/invalid-base-inheritance.cpp b/clang/test/SemaCXX/invalid-base-inheritance.cpp
new file mode 100644
index 0000000000000..21b3eb57ef914
--- /dev/null
+++ b/clang/test/SemaCXX/invalid-base-inheritance.cpp
@@ -0,0 +1,13 @@
+// Tests that invalid base-specifiers no longer crash the compiler.
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+class X; // expected-note {{forward declaration of 'X'}} expected-note {{forward declaration of 'X'}}
+
+class A : X { // expected-error {{base class has incomplete type}}
+};
+
+class Y : int { // expected-error {{expected class name}}
+};
+
+class Z : X*, virtual int { // expected-error {{base class has incomplete type}} expected-error {{expected class name}}
+};