[Clang] Explain why a type trait evaluated to false.

`static_assert(std::is_xx_v<MyType>);` is a common pattern
to check that a type meets a requirement.

This patch produces diagnostics notes when such assertion fails.
The first type trait for which we provide detailed explanation
is std::is_trivially_relocatable.

We employ the same mechanisn when a type trait appears an
an unsatisfied atomic constraint.

I plan to also support `std::is_trivially_replaceable` in a follow up PR,
and hopefully, over time we can support more type traits.

Author: cor3ntin

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1762,6 +1762,29 @@ def err_user_defined_msg_constexpr : Error<
   "%sub{subst_user_defined_msg}0 must be produced by a "
   "constant expression">;
 
+// Type traits explanations
+def note_unsatisfied_trait : Note<"%0 is not %enum_select<TraitName>{"
+                                  "%TriviallyRelocatable{trivially relocatable}"
+                                  "}1">;
+
+def note_unsatisfied_trait_reason
+    : Note<"because it "
+           "%enum_select<TraitNotSatisfiedReason>{"
+           "%Ref{is a reference type}|"
+           "%HasArcLifetime{has an ARC lifetime qualifier}|"
+           "%VLA{is a variably-modified type}|"
+           "%VBase{has a virtual base %1}|"
+           "%NRBase{has a non-trivially-relocatable base %1}|"
+           "%NRField{has a non-trivially-relocatable member %1 of type %2}|"
+           "%DeletedDtr{has a %select{deleted|user-provided}1 destructor}|"
+           "%UserProvidedCtr{has a user provided %select{copy|move}1 "
+           "constructor}|"
+           "%UserProvidedAssign{has a user provided %select{copy|move}1 "
+           "assignment operator}|"
+           "%UnionWithUserDeclaredSMF{is a union with a user-declared "
+           "%sub{select_special_member_kind}1}"
+           "}0">;
+
 def warn_consteval_if_always_true : Warning<
   "consteval if is always true in an %select{unevaluated|immediate}0 context">,
   InGroup<DiagGroup<"redundant-consteval-if">>;

clang/include/clang/Sema/Sema.h

@@ -5910,6 +5910,11 @@ class Sema final : public SemaBase {
   /// with expression \E
   void DiagnoseStaticAssertDetails(const Expr *E);
 
+  /// If E represents a built-in type trait, or a known standard type trait,
+  /// try to print more information about why the type type-trait failed.
+  /// This assumes we already evaluated the expression to a false boolean value.
+  void DiagnoseTypeTraitDetails(const Expr *E);
+
   /// Handle a friend type declaration.  This works in tandem with
   /// ActOnTag.
   ///

clang/lib/Sema/SemaConcept.cpp

@@ -1320,6 +1320,7 @@ static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
   S.Diag(SubstExpr->getSourceRange().getBegin(),
          diag::note_atomic_constraint_evaluated_to_false)
       << (int)First << SubstExpr;
+  S.DiagnoseTypeTraitDetails(SubstExpr);
 }
 
 template <typename SubstitutionDiagnostic>

clang/lib/Sema/SemaDeclCXX.cpp

@@ -17694,6 +17694,8 @@ void Sema::DiagnoseStaticAssertDetails(const Expr *E) {
           << DiagSide[0].ValueString << Op->getOpcodeStr()
           << DiagSide[1].ValueString << Op->getSourceRange();
     }
+  } else {
+    DiagnoseTypeTraitDetails(E);
   }
 }
 

clang/lib/Sema/SemaTypeTraits.cpp

@@ -1917,3 +1917,190 @@ ExprResult Sema::BuildExpressionTrait(ExpressionTrait ET, SourceLocation KWLoc,
   return new (Context)
       ExpressionTraitExpr(KWLoc, ET, Queried, Value, RParen, Context.BoolTy);
 }
+
+static std::optional<TypeTrait> StdNameToTypeTrait(StringRef Name) {
+  return llvm::StringSwitch<std::optional<TypeTrait>>(Name)
+      .Case("is_trivially_relocatable",
+            TypeTrait::UTT_IsCppTriviallyRelocatable)
+      .Default(std::nullopt);
+}
+
+using ExtractedTypeTraitInfo =
+    std::optional<std::pair<TypeTrait, llvm::SmallVector<QualType, 1>>>;
+
+// Recognize type traits that are builting type traits, or known standard
+// type traits in <type_traits>. Note that at this point we assume the
+// trait evaluated to false, so we need only to recognize the shape of the
+// outer-most symbol.
+static ExtractedTypeTraitInfo ExtractTypeTraitFromExpression(const Expr *E) {
+  llvm::SmallVector<QualType, 1> Args;
+  std::optional<TypeTrait> Trait;
+
+  // builtins
+  if (const auto *TraitExpr = dyn_cast<TypeTraitExpr>(E)) {
+    Trait = TraitExpr->getTrait();
+    for (const auto *Arg : TraitExpr->getArgs())
+      Args.push_back(Arg->getType());
+    return {{Trait.value(), std::move(Args)}};
+  }
+  const auto *Ref = dyn_cast<DeclRefExpr>(E);
+  if (!Ref)
+    return std::nullopt;
+
+  // std::is_xxx_v<>
+  if (const auto *VD =
+          dyn_cast<VarTemplateSpecializationDecl>(Ref->getDecl())) {
+    if (!VD->isInStdNamespace())
+      return std::nullopt;
+    StringRef Name = VD->getIdentifier()->getName();
+    if (!Name.consume_back("_v"))
+      return std::nullopt;
+    Trait = StdNameToTypeTrait(Name);
+    if (!Trait)
+      return std::nullopt;
+    for (const auto &Arg : VD->getTemplateArgs().asArray())
+      Args.push_back(Arg.getAsType());
+    return {{Trait.value(), std::move(Args)}};
+  }
+
+  // std::is_xxx<>::value
+  if (const auto *VD = dyn_cast<VarDecl>(Ref->getDecl());
+      Ref->hasQualifier() && VD && VD->getIdentifier()->isStr("value")) {
+    const Type *T = Ref->getQualifier()->getAsType();
+    if (!T)
+      return std::nullopt;
+    const TemplateSpecializationType *Ts =
+        T->getAs<TemplateSpecializationType>();
+    if (!Ts)
+      return std::nullopt;
+    const TemplateDecl *D = Ts->getTemplateName().getAsTemplateDecl();
+    if (!D || !D->isInStdNamespace())
+      return std::nullopt;
+    Trait = StdNameToTypeTrait(D->getIdentifier()->getName());
+    if (!Trait)
+      return std::nullopt;
+    for (const auto &Arg : Ts->template_arguments())
+      Args.push_back(Arg.getAsType());
+    return {{Trait.value(), std::move(Args)}};
+  }
+  return std::nullopt;
+}
+
+static void DiagnoseNonTriviallyRelocatableReason(Sema &SemaRef,
+                                                  SourceLocation Loc,
+                                                  const CXXRecordDecl *D) {
+  for (const CXXBaseSpecifier &B : D->bases()) {
+    const auto *BaseDecl = B.getType()->getAsCXXRecordDecl();
+    if (!BaseDecl)
+      continue;
+    if (B.isVirtual())
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::VBase << B.getType()
+          << B.getSourceRange();
+    if (!SemaRef.IsCXXTriviallyRelocatableType(B.getType()))
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::NRBase << B.getType()
+          << B.getSourceRange();
+  }
+  for (const FieldDecl *Field : D->fields()) {
+    if (Field->getType()->isReferenceType())
+      continue;
+    if (!SemaRef.IsCXXTriviallyRelocatableType(Field->getType()))
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::NRField << Field << Field->getType()
+          << Field->getSourceRange();
+  }
+  if (D->hasDeletedDestructor())
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::DeletedDtr << /*Deleted*/ 0
+        << D->getDestructor()->getSourceRange();
+
+  if (D->hasAttr<TriviallyRelocatableAttr>())
+    return;
+
+  if (D->isUnion()) {
+    auto DiagSPM = [&](CXXSpecialMemberKind K, bool Has) {
+      if (Has)
+        SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+            << diag::TraitNotSatisfiedReason::UnionWithUserDeclaredSMF << K;
+    };
+    DiagSPM(CXXSpecialMemberKind::CopyConstructor,
+            D->hasUserDeclaredCopyConstructor());
+    DiagSPM(CXXSpecialMemberKind::CopyAssignment,
+            D->hasUserDeclaredCopyAssignment());
+    DiagSPM(CXXSpecialMemberKind::MoveConstructor,
+            D->hasUserDeclaredMoveConstructor());
+    DiagSPM(CXXSpecialMemberKind::MoveAssignment,
+            D->hasUserDeclaredMoveAssignment());
+    return;
+  }
+
+  if (!D->hasSimpleMoveConstructor() && !D->hasSimpleCopyConstructor()) {
+    const auto *Decl = cast<CXXConstructorDecl>(
+        LookupSpecialMemberFromXValue(SemaRef, D, /*Assign=*/false));
+    if (Decl && Decl->isUserProvided())
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::UserProvidedCtr
+          << Decl->isMoveConstructor() << Decl->getSourceRange();
+  }
+  if (!D->hasSimpleMoveAssignment() && !D->hasSimpleCopyAssignment()) {
+    CXXMethodDecl *Decl =
+        LookupSpecialMemberFromXValue(SemaRef, D, /*Assign=*/true);
+    if (Decl && Decl->isUserProvided())
+      SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+          << diag::TraitNotSatisfiedReason::UserProvidedAssign
+          << Decl->isMoveAssignmentOperator() << Decl->getSourceRange();
+  }
+  CXXDestructorDecl *Dtr = D->getDestructor();
+  if (Dtr && Dtr->isUserProvided() && !Dtr->isDefaulted())
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::DeletedDtr << /*User Provided*/ 1
+        << Dtr->getSourceRange();
+}
+
+static void DiagnoseNonTriviallyRelocatableReason(Sema &SemaRef,
+                                                  SourceLocation Loc,
+                                                  QualType T) {
+  SemaRef.Diag(Loc, diag::note_unsatisfied_trait)
+      << T << diag::TraitName::TriviallyRelocatable;
+  if (T->isVariablyModifiedType())
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::VLA;
+
+  if (T->isReferenceType())
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::Ref;
+  T = T.getNonReferenceType();
+
+  if (T.hasNonTrivialObjCLifetime())
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::HasArcLifetime;
+
+  const CXXRecordDecl *D = T->getAsCXXRecordDecl();
+  if (!D)
+    return;
+
+  if (D->hasDefinition())
+    DiagnoseNonTriviallyRelocatableReason(SemaRef, Loc, D);
+
+  SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
+}
+
+void Sema::DiagnoseTypeTraitDetails(const Expr *E) {
+  E = E->IgnoreParenImpCasts();
+  if (E->containsErrors())
+    return;
+
+  ExtractedTypeTraitInfo TraitInfo = ExtractTypeTraitFromExpression(E);
+  if (!TraitInfo)
+    return;
+
+  const auto &[Trait, Args] = TraitInfo.value();
+  switch (Trait) {
+  case UTT_IsCppTriviallyRelocatable:
+    DiagnoseNonTriviallyRelocatableReason(*this, E->getBeginLoc(), Args[0]);
+    break;
+  default:
+    break;
+  }
+}

clang/test/SemaCXX/type-traits-unsatisfied-diags-std.cpp

@@ -0,0 +1,101 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 -DSTD1 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 -DSTD2 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 -DSTD3 %s
+
+namespace std {
+
+#ifdef STD1
+template <typename T>
+struct is_trivially_relocatable {
+    static constexpr bool value = __builtin_is_cpp_trivially_relocatable(T);
+};
+
+template <typename T>
+constexpr bool is_trivially_relocatable_v = __builtin_is_cpp_trivially_relocatable(T);
+#endif
+
+#ifdef STD2
+template <typename T>
+struct __details_is_trivially_relocatable {
+    static constexpr bool value = __builtin_is_cpp_trivially_relocatable(T);
+};
+
+template <typename T>
+using is_trivially_relocatable  = __details_is_trivially_relocatable<T>;
+
+template <typename T>
+constexpr bool is_trivially_relocatable_v = __builtin_is_cpp_trivially_relocatable(T);
+#endif
+
+
+#ifdef STD3
+template< class T, T v >
+struct integral_constant {
+    static constexpr T value = v;
+};
+
+template< bool B >
+using bool_constant = integral_constant<bool, B>;
+
+template <typename T>
+struct __details_is_trivially_relocatable : bool_constant<__builtin_is_cpp_trivially_relocatable(T)> {};
+
+template <typename T>
+using is_trivially_relocatable  = __details_is_trivially_relocatable<T>;
+
+template <typename T>
+constexpr bool is_trivially_relocatable_v = is_trivially_relocatable<T>::value;
+#endif
+
+}
+
+static_assert(std::is_trivially_relocatable<int>::value);
+
+static_assert(std::is_trivially_relocatable<int&>::value);
+// expected-error-re@-1 {{static assertion failed due to requirement 'std::{{.*}}is_trivially_relocatable<int &>::value'}} \
+// expected-note@-1 {{'int &' is not trivially relocatable}} \
+// expected-note@-1 {{because it is a reference type}}
+static_assert(std::is_trivially_relocatable_v<int&>);
+// expected-error@-1 {{static assertion failed due to requirement 'std::is_trivially_relocatable_v<int &>'}} \
+// expected-note@-1 {{'int &' is not trivially relocatable}} \
+// expected-note@-1 {{because it is a reference type}}
+
+namespace test_namespace {
+    using namespace std;
+    static_assert(is_trivially_relocatable<int&>::value);
+    // expected-error-re@-1 {{static assertion failed due to requirement '{{.*}}is_trivially_relocatable<int &>::value'}} \
+    // expected-note@-1 {{'int &' is not trivially relocatable}} \
+    // expected-note@-1 {{because it is a reference type}}
+    static_assert(is_trivially_relocatable_v<int&>);
+    // expected-error@-1 {{static assertion failed due to requirement 'is_trivially_relocatable_v<int &>'}} \
+    // expected-note@-1 {{'int &' is not trivially relocatable}} \
+    // expected-note@-1 {{because it is a reference type}}
+}
+
+
+namespace concepts {
+template <typename T>
+requires std::is_trivially_relocatable<T>::value void f();  // #cand1
+
+template <typename T>
+concept C = std::is_trivially_relocatable_v<T>; // #concept2
+
+template <C T> void g();  // #cand2
+
+void test() {
+    f<int&>();
+    // expected-error@-1 {{no matching function for call to 'f'}} \
+    // expected-note@#cand1 {{candidate template ignored: constraints not satisfied [with T = int &]}} \
+    // expected-note-re@#cand1 {{because '{{.*}}is_trivially_relocatable<int &>::value' evaluated to false}} \
+    // expected-note@#cand1 {{'int &' is not trivially relocatable}} \
+    // expected-note@#cand1 {{because it is a reference type}}
+
+    g<int&>();
+    // expected-error@-1 {{no matching function for call to 'g'}} \
+    // expected-note@#cand2 {{candidate template ignored: constraints not satisfied [with T = int &]}} \
+    // expected-note@#cand2 {{because 'int &' does not satisfy 'C'}} \
+    // expected-note@#concept2 {{because 'std::is_trivially_relocatable_v<int &>' evaluated to false}} \
+    // expected-note@#concept2 {{'int &' is not trivially relocatable}} \
+    // expected-note@#concept2 {{because it is a reference type}}
+}
+}