[analysis] assume expr is not mutated after analysis to avoid recursi…

…ve (#90581)

Fixes: #89376.

clang-tools-extra/docs/ReleaseNotes.rst

@@ -261,7 +261,8 @@ Changes in existing checks
 
 - Improved :doc:`misc-const-correctness
   <clang-tidy/checks/misc/const-correctness>` check by avoiding infinite recursion
-  for recursive forwarding reference.
+  for recursive functions with forwarding reference parameters and reference
+  variables which refer to themselves.
 
 - Improved :doc:`misc-definitions-in-headers
   <clang-tidy/checks/misc/definitions-in-headers>` check by replacing the local

clang/lib/Analysis/ExprMutationAnalyzer.cpp

@@ -235,15 +235,17 @@ const Stmt *ExprMutationAnalyzer::Analyzer::findMutationMemoized(
   if (Memoized != MemoizedResults.end())
     return Memoized->second;
 
+  // Assume Exp is not mutated before analyzing Exp.
+  MemoizedResults[Exp] = nullptr;
   if (isUnevaluated(Exp))
-    return MemoizedResults[Exp] = nullptr;
+    return nullptr;
 
   for (const auto &Finder : Finders) {
     if (const Stmt *S = (this->*Finder)(Exp))
       return MemoizedResults[Exp] = S;
   }
 
-  return MemoizedResults[Exp] = nullptr;
+  return nullptr;
 }
 
 const Stmt *

clang/unittests/Analysis/ExprMutationAnalyzerTest.cpp

@@ -10,8 +10,8 @@
 #include "clang/AST/TypeLoc.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Frontend/ASTUnit.h"
 #include "clang/Tooling/Tooling.h"
-#include "llvm/ADT/SmallString.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include <cctype>
@@ -43,7 +43,7 @@ std::unique_ptr<ASTUnit> buildASTFromCode(const Twine &Code) {
 }
 
 ExprMatcher declRefTo(StringRef Name) {
-  return declRefExpr(to(namedDecl(hasName(Name))));
+  return declRefExpr(to(namedDecl(hasName(Name)).bind("decl")));
 }
 
 StmtMatcher withEnclosingCompound(ExprMatcher Matcher) {
@@ -57,6 +57,13 @@ bool isMutated(const SmallVectorImpl<BoundNodes> &Results, ASTUnit *AST) {
   return ExprMutationAnalyzer(*S, AST->getASTContext()).isMutated(E);
 }
 
+bool isDeclMutated(const SmallVectorImpl<BoundNodes> &Results, ASTUnit *AST) {
+  const auto *const S = selectFirst<Stmt>("stmt", Results);
+  const auto *const D = selectFirst<Decl>("decl", Results);
+  TraversalKindScope RAII(AST->getASTContext(), TK_AsIs);
+  return ExprMutationAnalyzer(*S, AST->getASTContext()).isMutated(D);
+}
+
 SmallVector<std::string, 1>
 mutatedBy(const SmallVectorImpl<BoundNodes> &Results, ASTUnit *AST) {
   const auto *const S = selectFirst<Stmt>("stmt", Results);
@@ -1552,6 +1559,21 @@ TEST(ExprMutationAnalyzerTest, UniquePtr) {
 
 // section: complex problems detected on real code
 
+TEST(ExprMutationAnalyzerTest, SelfRef) {
+  std::unique_ptr<ASTUnit> AST{};
+  SmallVector<BoundNodes, 1> Results{};
+
+  AST = buildASTFromCodeWithArgs("void f() { int &x = x; }",
+                                 {"-Wno-unused-value", "-Wno-uninitialized"});
+  Results = match(withEnclosingCompound(declRefTo("x")), AST->getASTContext());
+  EXPECT_FALSE(isDeclMutated(Results, AST.get()));
+
+  AST = buildASTFromCodeWithArgs("void f() { int &x = x; x = 1; }",
+                                 {"-Wno-unused-value", "-Wno-uninitialized"});
+  Results = match(withEnclosingCompound(declRefTo("x")), AST->getASTContext());
+  EXPECT_TRUE(isDeclMutated(Results, AST.get()));
+}
+
 TEST(ExprMutationAnalyzerTest, UnevaluatedContext) {
   const std::string Example =
       "template <typename T>"