[clang-tidy] Fix a crash in modernize-loop-convert around conversion operators

modernize-loop-convert checks and fixes when a loop that iterates over the
elements of a container can be rewritten from a for(...; ...; ...) style into
the "new" C++11 for-range format. For that, it needs to parse the elements of
that loop, like its init-statement, such as ItType it = cont.begin().
modernize-loop-convert checks whether the loop variable is initialized by a
begin() member function.

When an iterator is initialized with a conversion operator (e.g. for
(const_iterator it = non_const_container.begin(); ...), attempts to retrieve the
name of the initializer expression resulted in an assert, as conversion
operators don't have a valid IdentifierInfo.

I fixed this by making digThroughConstructors dig through conversion operators
as well.

Differential Revision: https://reviews.llvm.org/D113201

clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp

@@ -304,8 +304,8 @@ StatementMatcher makePseudoArrayLoopMatcher() {
 static const Expr *getContainerFromBeginEndCall(const Expr *Init, bool IsBegin,
                                                 bool *IsArrow, bool IsReverse) {
   // FIXME: Maybe allow declaration/initialization outside of the for loop.
-  const auto *TheCall =
-      dyn_cast_or_null<CXXMemberCallExpr>(digThroughConstructors(Init));
+  const auto *TheCall = dyn_cast_or_null<CXXMemberCallExpr>(
+      digThroughConstructorsConversions(Init));
   if (!TheCall || TheCall->getNumArgs() != 0)
     return nullptr;
 

clang-tools-extra/clang-tidy/modernize/LoopConvertUtils.cpp

@@ -152,20 +152,21 @@ bool DeclFinderASTVisitor::VisitTypeLoc(TypeLoc TL) {
   return true;
 }
 
-/// Look through conversion/copy constructors to find the explicit
-/// initialization expression, returning it is found.
+/// Look through conversion/copy constructors and member functions to find the
+/// explicit initialization expression, returning it is found.
 ///
 /// The main idea is that given
 ///   vector<int> v;
 /// we consider either of these initializations
 ///   vector<int>::iterator it = v.begin();
 ///   vector<int>::iterator it(v.begin());
+///   vector<int>::const_iterator it(v.begin());
 /// and retrieve `v.begin()` as the expression used to initialize `it` but do
 /// not include
 ///   vector<int>::iterator it;
 ///   vector<int>::iterator it(v.begin(), 0); // if this constructor existed
 /// as being initialized from `v.begin()`
-const Expr *digThroughConstructors(const Expr *E) {
+const Expr *digThroughConstructorsConversions(const Expr *E) {
   if (!E)
     return nullptr;
   E = E->IgnoreImplicit();
@@ -178,8 +179,13 @@ const Expr *digThroughConstructors(const Expr *E) {
     E = ConstructExpr->getArg(0);
     if (const auto *Temp = dyn_cast<MaterializeTemporaryExpr>(E))
       E = Temp->getSubExpr();
-    return digThroughConstructors(E);
+    return digThroughConstructorsConversions(E);
   }
+  // If this is a conversion (as iterators commonly convert into their const
+  // iterator counterparts), dig through that as well.
+  if (const auto *ME = dyn_cast<CXXMemberCallExpr>(E))
+    if (const auto *D = dyn_cast<CXXConversionDecl>(ME->getMethodDecl()))
+      return digThroughConstructorsConversions(ME->getImplicitObjectArgument());
   return E;
 }
 
@@ -357,7 +363,7 @@ static bool isAliasDecl(ASTContext *Context, const Decl *TheDecl,
   bool OnlyCasts = true;
   const Expr *Init = VDecl->getInit()->IgnoreParenImpCasts();
   if (isa_and_nonnull<CXXConstructExpr>(Init)) {
-    Init = digThroughConstructors(Init);
+    Init = digThroughConstructorsConversions(Init);
     OnlyCasts = false;
   }
   if (!Init)

clang-tools-extra/clang-tidy/modernize/LoopConvertUtils.h

@@ -275,7 +275,7 @@ private:
 typedef llvm::SmallVector<Usage, 8> UsageResult;
 
 // General functions used by ForLoopIndexUseVisitor and LoopConvertCheck.
-const Expr *digThroughConstructors(const Expr *E);
+const Expr *digThroughConstructorsConversions(const Expr *E);
 bool areSameExpr(ASTContext *Context, const Expr *First, const Expr *Second);
 const DeclRefExpr *getDeclRef(const Expr *E);
 bool areSameVariable(const ValueDecl *First, const ValueDecl *Second);

clang-tools-extra/test/clang-tidy/checkers/Inputs/modernize-loop-convert/structures.h

@@ -53,6 +53,23 @@ struct T {
   iterator end();
 };
 
+struct Q {
+  typedef int value_type;
+  struct const_iterator {
+    value_type &operator*();
+    const value_type &operator*() const;
+    const_iterator &operator++();
+    bool operator!=(const const_iterator &other);
+    void insert(value_type);
+    value_type X;
+  };
+  struct iterator {
+    operator const_iterator() const;
+  };
+  iterator begin();
+  iterator end();
+};
+
 struct U {
   struct iterator {
     Val& operator*();

clang-tools-extra/test/clang-tidy/checkers/modernize-loop-convert-basic.cpp

@@ -273,6 +273,16 @@ void f() {
   // CHECK-FIXES: for (int & It : Tt)
   // CHECK-FIXES-NEXT: printf("I found %d\n", It);
 
+  // Do not crash because of Qq.begin() converting. Q::iterator converts with a
+  // conversion operator, which has no name, to Q::const_iterator.
+  Q Qq;
+  for (Q::const_iterator It = Qq.begin(), E = Qq.end(); It != E; ++It) {
+    printf("I found %d\n", *It);
+  }
+  // CHECK-MESSAGES: :[[@LINE-3]]:3: warning: use range-based for loop instead
+  // CHECK-FIXES: for (int & It : Qq)
+  // CHECK-FIXES-NEXT: printf("I found %d\n", It);
+
   T *Pt;
   for (T::iterator It = Pt->begin(), E = Pt->end(); It != E; ++It) {
     printf("I found %d\n", *It);