[clang] __is_trivially_equality_comparable for types containing lambdas (#68506)

Lambdas (closure types) are trivially equality-comparable iff they are
non-capturing, because non-capturing lambdas are convertible to function
pointers: if (lam1 == lam2) compiles, then lam1 and lam2 must have
the same type, and be always-equal, and be empty.

Author: AMP999

clang/include/clang/AST/DeclCXX.h

@@ -1052,6 +1052,12 @@ class CXXRecordDecl : public RecordDecl {
     return static_cast<LambdaCaptureDefault>(getLambdaData().CaptureDefault);
   }
 
+  bool isCapturelessLambda() const {
+    if (!isLambda())
+      return false;
+    return getLambdaCaptureDefault() == LCD_None && capture_size() == 0;
+  }
+
   /// Set the captures for this lambda closure type.
   void setCaptures(ASTContext &Context, ArrayRef<LambdaCapture> Captures);
 

clang/lib/AST/DeclCXX.cpp

@@ -686,7 +686,7 @@ bool CXXRecordDecl::lambdaIsDefaultConstructibleAndAssignable() const {
   // C++17 [expr.prim.lambda]p21:
   //   The closure type associated with a lambda-expression has no default
   //   constructor and a deleted copy assignment operator.
-  if (getLambdaCaptureDefault() != LCD_None || capture_size() != 0)
+  if (!isCapturelessLambda())
     return false;
   return getASTContext().getLangOpts().CPlusPlus20;
 }

clang/lib/AST/Type.cpp

@@ -2663,6 +2663,8 @@ static bool
 HasNonDeletedDefaultedEqualityComparison(const CXXRecordDecl *Decl) {
   if (Decl->isUnion())
     return false;
+  if (Decl->isLambda())
+    return Decl->isCapturelessLambda();
 
   auto IsDefaultedOperatorEqualEqual = [&](const FunctionDecl *Function) {
     return Function->getOverloadedOperator() ==

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -1216,11 +1216,10 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
       SkippedChecks.set(SanitizerKind::ObjectSize, true);
       QualType ThisTy = MD->getThisType();
 
-      // If this is the call operator of a lambda with no capture-default, it
+      // If this is the call operator of a lambda with no captures, it
       // may have a static invoker function, which may call this operator with
       // a null 'this' pointer.
-      if (isLambdaCallOperator(MD) &&
-          MD->getParent()->getLambdaCaptureDefault() == LCD_None)
+      if (isLambdaCallOperator(MD) && MD->getParent()->isCapturelessLambda())
         SkippedChecks.set(SanitizerKind::Null, true);
 
       EmitTypeCheck(

clang/lib/Sema/SemaLambda.cpp

@@ -393,8 +393,7 @@ void Sema::DiagnoseInvalidExplicitObjectParameterInLambda(
   CXXRecordDecl *RD = Method->getParent();
   if (Method->getType()->isDependentType())
     return;
-  if (RD->getLambdaCaptureDefault() == LambdaCaptureDefault::LCD_None &&
-      RD->capture_size() == 0)
+  if (RD->isCapturelessLambda())
     return;
   QualType ExplicitObjectParameterType = Method->getParamDecl(0)
                                              ->getType()

clang/test/SemaCXX/type-traits.cpp

@@ -3160,11 +3160,18 @@ static_assert(!__is_trivially_equality_comparable(float), "");
 static_assert(!__is_trivially_equality_comparable(double), "");
 static_assert(!__is_trivially_equality_comparable(long double), "");
 
-struct TriviallyEqualityComparableNoDefaultedComparator {
+struct NonTriviallyEqualityComparableNoComparator {
   int i;
   int j;
 };
-static_assert(!__is_trivially_equality_comparable(TriviallyEqualityComparableNoDefaultedComparator), "");
+static_assert(!__is_trivially_equality_comparable(NonTriviallyEqualityComparableNoComparator), "");
+
+struct NonTriviallyEqualityComparableNonDefaultedComparator {
+  int i;
+  int j;
+  bool operator==(const NonTriviallyEqualityComparableNonDefaultedComparator&);
+};
+static_assert(!__is_trivially_equality_comparable(NonTriviallyEqualityComparableNonDefaultedComparator), "");
 
 #if __cplusplus >= 202002L
 
@@ -3177,7 +3184,7 @@ struct TriviallyEqualityComparable {
 
   bool operator==(const TriviallyEqualityComparable&) const = default;
 };
-static_assert(__is_trivially_equality_comparable(TriviallyEqualityComparable), "");
+static_assert(__is_trivially_equality_comparable(TriviallyEqualityComparable));
 
 struct TriviallyEqualityComparableContainsArray {
   int a[4];
@@ -3193,6 +3200,17 @@ struct TriviallyEqualityComparableContainsMultiDimensionArray {
 };
 static_assert(__is_trivially_equality_comparable(TriviallyEqualityComparableContainsMultiDimensionArray));
 
+auto GetNonCapturingLambda() { return [](){ return 42; }; }
+
+struct TriviallyEqualityComparableContainsLambda {
+  [[no_unique_address]] decltype(GetNonCapturingLambda()) l;
+  int i;
+
+  bool operator==(const TriviallyEqualityComparableContainsLambda&) const = default;
+};
+static_assert(!__is_trivially_equality_comparable(decltype(GetNonCapturingLambda()))); // padding
+static_assert(__is_trivially_equality_comparable(TriviallyEqualityComparableContainsLambda));
+
 struct TriviallyEqualityComparableNonTriviallyCopyable {
   TriviallyEqualityComparableNonTriviallyCopyable(const TriviallyEqualityComparableNonTriviallyCopyable&);
   ~TriviallyEqualityComparableNonTriviallyCopyable();