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Sema: filter out invalid base-specifiers before attaching #147213

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@shashi1687 shashi1687 commented Jul 6, 2025

Summary

Briefly what this PR does, in one or two sentences.

Filters out invalid (null) base-specifiers in ActOnBaseSpecifiers(), preventing ICEs on incomplete or non-class bases.

Motivation

Why this change is needed.

When you inherit from a forward‐declared class or non-class type, Sema used to emit an error and still hand the bad pointer to later code, causing an internal compiler error. We now drop those invalid entries immediately.

Root Cause Analysis

A very short bulleted “root cause.”

  • ActOnBaseSpecifier returns nullptr for invalid bases.
  • ActOnBaseSpecifiers passed all pointers (including null) to AttachBaseSpecifiers.
  • Later layout or special‐member lookup on null caused the ICE.

Fixes #147186

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@llvmbot llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Jul 6, 2025
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llvmbot commented Jul 6, 2025

@llvm/pr-subscribers-clang

Author: Shashi Shankar (shashi1687)

Changes

Summary

Briefly what this PR does, in one or two sentences.
> Filters out invalid (null) base-specifiers in ActOnBaseSpecifiers(), preventing ICEs on incomplete or non-class bases.

Motivation

Why this change is needed.
> When you inherit from a forward‐declared class or non-class type, Sema used to emit an error and still hand the bad pointer to later code, causing an internal compiler error. We now drop those invalid entries immediately.

Root Cause Analysis

A very short bulleted “root cause.”

  • ActOnBaseSpecifier returns nullptr for invalid bases.
  • ActOnBaseSpecifiers passed all pointers (including null) to AttachBaseSpecifiers.
  • Later layout or special‐member lookup on null caused the ICE.

Changes in this PR

List the key files and rough diff summary.

  • clang/lib/Sema/SemaDeclCXX.cpp
    • In ActOnBaseSpecifiers(), gather Bases into a SmallVector, skipping null entries.
  • clang/test/SemaCXX/invalid-base-inheritance.cpp
    • New regression test checking for the proper diagnostics and that no ICE occurs.

Test Plan

How you verified the change.

  1. Local build/test
    mkdir build && cd build
    cmake -G Ninja ../llvm \
      -DLLVM_ENABLE_PROJECTS="clang" \
      -DCMAKE_BUILD_TYPE=Release
    ninja clang
    ninja check-clang
    
    

Full diff: https://github.com/llvm/llvm-project/pull/147213.diff

2 Files Affected:

  • (modified) clang/lib/Sema/SemaDeclCXX.cpp (+16-1)
  • (added) clang/test/SemaCXX/invalid-base-inheritance.cpp (+17)
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..e37b77bcd3fea
--- /dev/null
+++ b/clang/test/SemaCXX/invalid-base-inheritance.cpp
@@ -0,0 +1,17 @@
+// Tests that invalid base-specifiers no longer crash the compiler.
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+class X;              // forward declaration
+
+class A : X {         // incomplete base → error, but no ICE
+    // expected-error@-1 {{base class has incomplete type}}
+};
+
+class Y : int {       // base not a class → error, but no ICE
+  // expected-error@-1 {{expected class name}}
+};
+
+class Z : X*, virtual int { // mixed invalid specifiers
+    // expected-error@-2 {{base class has incomplete type}}
+    // expected-error@-1 {{expected class name}}
+};

@shashi1687 shashi1687 force-pushed the fix-invalid-base-ice branch 3 times, most recently from 6184a8e to 8e0ec03 Compare July 6, 2025 20:39
ActOnBaseSpecifiers now skips nullptr entries returned for invalid bases,
avoiding ICE on invalid inheritance. Added regression test in SemaCXX/invalid-inheritance.cpp.

Fixes llvm#147186

Signed-off-by: Shashi Shankar <[email protected]>
@shashi1687 shashi1687 force-pushed the fix-invalid-base-ice branch from 8e0ec03 to 05211da Compare July 7, 2025 00:03
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@cor3ntin cor3ntin left a comment

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Thanks for working on this

Did you try to use !isUsable instead of IsInvalid in
ParseBaseSpecifier ? I think that might be a simpler fix.


This change needs a release note.
Please add an entry to clang/docs/ReleaseNotes.rst in the section the most adapted to the change, and referencing any Github issue this change fixes. Thanks!

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[clang] ICE on invalid inheritance since version 6
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