[clang-tools-extra] [clang-tidy] Fix infinite loop in misc-multiple-inheritance (PR #220223)

[Purna Chandra via cfe-commits]

https://github.com/Purna-Chandra-4706 updated https://github.com/llvm/llvm-project/pull/220223

>From f4e08f4bbaee53ba1c6d7b34a40b1e6456c70538 Mon Sep 17 00:00:00 2001
From: Purna-Chandra-4706 <vennelapurnachandra2006 at gmail.com>
Date: Tue, 1 Sep 2026 16:15:44 +0530
Subject: [PATCH] [clang-tidy] Fix infinite loop in misc-multiple-inheritance

---
 .../clang-tidy/misc/MultipleInheritanceCheck.cpp          | 8 ++++----
 clang-tools-extra/docs/ReleaseNotes.md                    | 4 ++++
 .../clang-tidy/checkers/misc/multiple-inheritance.cpp     | 6 ++++++
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp b/clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp
index 72e6aa6ac0b47..633842f3f6b22 100644
--- a/clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/MultipleInheritanceCheck.cpp
@@ -23,16 +23,16 @@ AST_MATCHER(CXXRecordDecl, hasBases) {
 
 bool MultipleInheritanceCheck::isInterface(const CXXBaseSpecifier &Base) {
   const CXXRecordDecl *const Node = Base.getType()->getAsCXXRecordDecl();
-  if (!Node)
+  if (!Node || !Node->hasDefinition())
     return true;
 
-  assert(Node->isCompleteDefinition());
-
   // Short circuit the lookup if we have analyzed this record before.
   if (const auto CachedValue = InterfaceMap.find(Node);
       CachedValue != InterfaceMap.end())
     return CachedValue->second;
 
+  InterfaceMap.try_emplace(Node, false);
+
   // To be an interface, a class must have...
   const bool CurrentClassIsInterface =
       // ...no bases that aren't interfaces...
@@ -47,7 +47,7 @@ bool MultipleInheritanceCheck::isInterface(const CXXBaseSpecifier &Base) {
         return M->isUserProvided() && !M->isPureVirtual() && !M->isStatic();
       });
 
-  InterfaceMap.try_emplace(Node, CurrentClassIsInterface);
+  InterfaceMap[Node] = CurrentClassIsInterface;
   return CurrentClassIsInterface;
 }
 
diff --git a/clang-tools-extra/docs/ReleaseNotes.md b/clang-tools-extra/docs/ReleaseNotes.md
index 420b7ddce20e6..6719b598c5de7 100644
--- a/clang-tools-extra/docs/ReleaseNotes.md
+++ b/clang-tools-extra/docs/ReleaseNotes.md
@@ -164,6 +164,10 @@ infrastructure are described first, followed by tool-specific sections.
   - Fixed false positives when the pointee is written through a pointer
     assignment, such as `*(p = q) = 0`.
 
+- Fixed an infinite loop in {doc}`misc-multiple-inheritance
+  <clang-tidy/checks/misc/multiple-inheritance>` when checking a class that
+  inherits from itself or has a circular inheritance graph.
+
 - Improved {doc}`misc-redundant-expression
   <clang-tidy/checks/misc/redundant-expression>` by fixing false positives in
   nested expressions involving different macros or a mix of macro and
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 257e16ab18f2f..15aa5d4d11228 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
@@ -182,3 +182,9 @@ struct VI : virtual VA { virtual void h() = 0; };
 struct VD : VI, VB {};
 
 } // namespace M
+
+template<class T> struct X {
+  struct B;
+  struct A : public B { virtual void foo() {} };
+};
+template<class T> struct X<T>::B : public A { virtual void foo() {} };