[clang-tools-extra] b73a64f - [clang-tidy] Fix virtual inheritance FP in misc-multiple-inheritance (#186103)

[via cfe-commits]

Author: Zeyi Xu
Date: 2026-03-14T16:34:41+08:00
New Revision: b73a64fc3eb01635deb115b8c836cbca0cab1856

URL: https://github.com/llvm/llvm-project/commit/b73a64fc3eb01635deb115b8c836cbca0cab1856
DIFF: https://github.com/llvm/llvm-project/commit/b73a64fc3eb01635deb115b8c836cbca0cab1856.diff

LOG: [clang-tidy] Fix virtual inheritance FP in misc-multiple-inheritance (#186103)

Avoid double-counting concrete bases introduced through virtual
inheritance in `misc-multiple-inheritance`.

As of AI-Usage: Gemini 3 is used for pre-commit reviewing.

Closes https://github.com/llvm/llvm-project/issues/186059

Added: 
    

Modified: 
    clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp
    clang-tools-extra/docs/ReleaseNotes.rst
    clang-tools-extra/test/clang-tidy/checkers/misc/multiple-inheritance.cpp

Removed: 
    


################################################################################
diff  --git a/clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp b/clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp
index 557b4559697b9..72e6aa6ac0b47 100644
--- a/clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp
@@ -58,16 +58,35 @@ void MultipleInheritanceCheck::registerMatchers(MatchFinder *Finder) {
 
 void MultipleInheritanceCheck::check(const MatchFinder::MatchResult &Result) {
   const auto &D = *Result.Nodes.getNodeAs<CXXRecordDecl>("decl");
-  // Check to see if the class inherits from multiple concrete classes.
-  unsigned NumConcrete =
-      llvm::count_if(D.bases(), [&](const CXXBaseSpecifier &I) {
-        return !I.isVirtual() && !isInterface(I);
-      });
+  // Collect the direct and virtual concrete bases of the class.
+  SmallVector<const CXXRecordDecl *> DirectConcreteBases;
+  for (const CXXBaseSpecifier &Base : D.bases())
+    if (!Base.isVirtual() && !isInterface(Base))
+      DirectConcreteBases.push_back(Base.getType()->getAsCXXRecordDecl());
+
+  SmallVector<const CXXRecordDecl *> VirtualConcreteBases;
+  for (const CXXBaseSpecifier &VBase : D.vbases())
+    if (!isInterface(VBase))
+      VirtualConcreteBases.push_back(VBase.getType()->getAsCXXRecordDecl());
 
-  // Check virtual bases to see if there is more than one concrete
-  // non-virtual base.
+  unsigned NumConcrete = DirectConcreteBases.size();
+
+  // Count only virtual concrete bases that introduce an additional
+  // implementation base, skipping those already represented by a more derived
+  // concrete base.
   NumConcrete += llvm::count_if(
-      D.vbases(), [&](const CXXBaseSpecifier &V) { return !isInterface(V); });
+      VirtualConcreteBases, [&](const CXXRecordDecl *VirtualBase) {
+        const bool HiddenByMoreDerivedVirtualBase = llvm::any_of(
+            VirtualConcreteBases, [&](const CXXRecordDecl *OtherVirtualBase) {
+              return VirtualBase != OtherVirtualBase &&
+                     OtherVirtualBase->isVirtuallyDerivedFrom(VirtualBase);
+            });
+        const bool HiddenByDirectConcreteBase = llvm::any_of(
+            DirectConcreteBases, [&](const CXXRecordDecl *DirectBase) {
+              return DirectBase->isVirtuallyDerivedFrom(VirtualBase);
+            });
+        return !HiddenByMoreDerivedVirtualBase && !HiddenByDirectConcreteBase;
+      });
 
   if (NumConcrete > 1)
     diag(D.getBeginLoc(), "inheriting multiple classes that aren't "

diff  --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index f0607996c5ff2..4b207609d598d 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -258,6 +258,10 @@ Changes in existing checks
   - Fixed false positive where an array of pointers to ``const`` was
     incorrectly diagnosed as allowing the pointee to be made ``const``.
 
+- Improved :doc:`misc-multiple-inheritance
+  <clang-tidy/checks/misc/multiple-inheritance>` by avoiding false positives when
+  virtual inheritance causes concrete bases to be counted more than once.
+
 - Improved :doc:`misc-throw-by-value-catch-by-reference
   <clang-tidy/checks/misc/throw-by-value-catch-by-reference>` check:
 

diff  --git a/clang-tools-extra/test/clang-tidy/checkers/misc/multiple-inheritance.cpp b/clang-tools-extra/test/clang-tidy/checkers/misc/multiple-inheritance.cpp
index 6004ab3d812ea..257e16ab18f2f 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/misc/multiple-inheritance.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/misc/multiple-inheritance.cpp
@@ -163,3 +163,22 @@ struct S2 { int i; };
 struct S3 : S1, S2 {};
 
 } // namespace N
+
+namespace M {
+
+class basic_ios { int state; };
+class ostream : virtual public basic_ios { int more_state; };
+class OStringStream final : public ostream {};
+
+struct A { int x; };
+struct B : A {};
+// CHECK-MESSAGES: [[@LINE+1]]:1: warning: inheriting multiple classes that aren't pure virtual is discouraged [misc-multiple-inheritance]
+struct C : A, B {};
+
+struct VA { virtual void f(); };
+struct VB : VA { virtual void g(); };
+struct VI : virtual VA { virtual void h() = 0; };
+// CHECK-MESSAGES: [[@LINE+1]]:1: warning: inheriting multiple classes that aren't pure virtual is discouraged [misc-multiple-inheritance]
+struct VD : VI, VB {};
+
+} // namespace M