[Clang] Add detailed notes explaining why is_constructible evaluates to false (Revert 16d5db7) (#151935)

Adds explanation why `is_constructible` evaluates to false.

This reapplies as-is e476f96.
This was reverted in 16d5db7 because of
a test failure in libc++.

The test failure in libc++ is interesting in that, in the absence of
nested diagnostics a bunch of diagnostics are emitted as error instead
of notes, which we cannot silence with `-verify-ignore-unexpected`.

The fix here is to prevent the diagnostics to be emitted in the first
place.
However this is clearly not ideal and we should make sure to deploy a
better solution in the clang 22 time frame, in the lines of
https://discourse.llvm.org/t/rfc-add-a-new-text-diagnostics-format-that-supports-nested-diagnostics/87641/12

Fixes #150601

---------

Co-authored-by: Shamshura Egor <[email protected]>

Author: cor3ntin

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1812,7 +1812,10 @@ def note_unsatisfied_trait_reason
            "%DeletedAssign{has a deleted %select{copy|move}1 "
            "assignment operator}|"
            "%UnionWithUserDeclaredSMF{is a union with a user-declared "
-           "%sub{select_special_member_kind}1}"
+           "%sub{select_special_member_kind}1}|"
+           "%FunctionType{is a function type}|"
+           "%CVVoidType{is a cv void type}|"
+           "%IncompleteArrayType{is an incomplete array type}"
            "}0">;
 
 def warn_consteval_if_always_true : Warning<

clang/lib/Sema/SemaTypeTraits.cpp

@@ -11,7 +11,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/DeclCXX.h"
+#include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
+#include "clang/Basic/DiagnosticIDs.h"
 #include "clang/Basic/DiagnosticParse.h"
 #include "clang/Basic/DiagnosticSema.h"
 #include "clang/Basic/TypeTraits.h"
@@ -1963,6 +1965,7 @@ static std::optional<TypeTrait> StdNameToTypeTrait(StringRef Name) {
       .Case("is_assignable", TypeTrait::BTT_IsAssignable)
       .Case("is_empty", TypeTrait::UTT_IsEmpty)
       .Case("is_standard_layout", TypeTrait::UTT_IsStandardLayout)
+      .Case("is_constructible", TypeTrait::TT_IsConstructible)
       .Default(std::nullopt);
 }
 
@@ -1999,8 +2002,16 @@ static ExtractedTypeTraitInfo ExtractTypeTraitFromExpression(const Expr *E) {
     Trait = StdNameToTypeTrait(Name);
     if (!Trait)
       return std::nullopt;
-    for (const auto &Arg : VD->getTemplateArgs().asArray())
-      Args.push_back(Arg.getAsType());
+    for (const auto &Arg : VD->getTemplateArgs().asArray()) {
+      if (Arg.getKind() == TemplateArgument::ArgKind::Pack) {
+        for (const auto &InnerArg : Arg.pack_elements())
+          Args.push_back(InnerArg.getAsType());
+      } else if (Arg.getKind() == TemplateArgument::ArgKind::Type) {
+        Args.push_back(Arg.getAsType());
+      } else {
+        llvm_unreachable("Unexpected kind");
+      }
+    }
     return {{Trait.value(), std::move(Args)}};
   }
 
@@ -2273,6 +2284,60 @@ static void DiagnoseNonTriviallyCopyableReason(Sema &SemaRef,
   }
 }
 
+static void DiagnoseNonConstructibleReason(
+    Sema &SemaRef, SourceLocation Loc,
+    const llvm::SmallVector<clang::QualType, 1> &Ts) {
+  if (Ts.empty()) {
+    return;
+  }
+
+  bool ContainsVoid = false;
+  for (const QualType &ArgTy : Ts) {
+    ContainsVoid |= ArgTy->isVoidType();
+  }
+
+  if (ContainsVoid)
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::CVVoidType;
+
+  QualType T = Ts[0];
+  if (T->isFunctionType())
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::FunctionType;
+
+  if (T->isIncompleteArrayType())
+    SemaRef.Diag(Loc, diag::note_unsatisfied_trait_reason)
+        << diag::TraitNotSatisfiedReason::IncompleteArrayType;
+
+  const CXXRecordDecl *D = T->getAsCXXRecordDecl();
+  if (!D || D->isInvalidDecl() || !D->hasDefinition())
+    return;
+
+  llvm::BumpPtrAllocator OpaqueExprAllocator;
+  SmallVector<Expr *, 2> ArgExprs;
+  ArgExprs.reserve(Ts.size() - 1);
+  for (unsigned I = 1, N = Ts.size(); I != N; ++I) {
+    QualType ArgTy = Ts[I];
+    if (ArgTy->isObjectType() || ArgTy->isFunctionType())
+      ArgTy = SemaRef.Context.getRValueReferenceType(ArgTy);
+    ArgExprs.push_back(
+        new (OpaqueExprAllocator.Allocate<OpaqueValueExpr>())
+            OpaqueValueExpr(Loc, ArgTy.getNonLValueExprType(SemaRef.Context),
+                            Expr::getValueKindForType(ArgTy)));
+  }
+
+  EnterExpressionEvaluationContext Unevaluated(
+      SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
+  Sema::ContextRAII TUContext(SemaRef,
+                              SemaRef.Context.getTranslationUnitDecl());
+  InitializedEntity To(InitializedEntity::InitializeTemporary(T));
+  InitializationKind InitKind(InitializationKind::CreateDirect(Loc, Loc, Loc));
+  InitializationSequence Init(SemaRef, To, InitKind, ArgExprs);
+
+  Init.Diagnose(SemaRef, To, InitKind, ArgExprs);
+  SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
+}
+
 static void DiagnoseNonTriviallyCopyableReason(Sema &SemaRef,
                                                SourceLocation Loc, QualType T) {
   SemaRef.Diag(Loc, diag::note_unsatisfied_trait)
@@ -2559,6 +2624,9 @@ void Sema::DiagnoseTypeTraitDetails(const Expr *E) {
   case UTT_IsStandardLayout:
     DiagnoseNonStandardLayoutReason(*this, E->getBeginLoc(), Args[0]);
     break;
+  case TT_IsConstructible:
+    DiagnoseNonConstructibleReason(*this, E->getBeginLoc(), Args);
+    break;
   default:
     break;
   }

clang/test/CXX/drs/cwg18xx.cpp

@@ -564,11 +564,12 @@ struct A {
 namespace ex2 {
 #if __cplusplus >= 201103L
 struct Bar {
-  struct Baz {
+  struct Baz { // #cwg1890-Baz
     int a = 0;
   };
   static_assert(__is_constructible(Baz), "");
   // since-cxx11-error@-1 {{static assertion failed due to requirement '__is_constructible(cwg1890::ex2::Bar::Baz)'}}
+  // since-cxx11-note@#cwg1890-Baz {{'Baz' defined here}}
 };
 #endif
 } // namespace ex2

clang/test/SemaCXX/overload-resolution-deferred-templates.cpp

@@ -80,21 +80,30 @@ struct ImplicitlyCopyable {
 static_assert(__is_constructible(ImplicitlyCopyable, const ImplicitlyCopyable&));
 
 
-struct Movable {
+struct Movable { // #Movable
   template <typename T>
   requires __is_constructible(Movable, T) // #err-self-constraint-1
-  explicit Movable(T op) noexcept; // #1
-  Movable(Movable&&) noexcept = default; // #2
+  explicit Movable(T op) noexcept; // #Movable1
+  Movable(Movable&&) noexcept = default; // #Movable2
 };
 static_assert(__is_constructible(Movable, Movable&&));
 static_assert(__is_constructible(Movable, const Movable&));
-// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(Movable, const Movable &)'}}
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(Movable, const Movable &)'}} \
+// expected-error@-1 {{call to implicitly-deleted copy constructor of 'Movable'}} \
+// expected-note@#Movable  {{'Movable' defined here}} \
+// expected-note@#Movable  {{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const Movable' for 1st argument}} \
+// expected-note@#Movable2  {{copy constructor is implicitly deleted because 'Movable' has a user-declared move constructor}} \
+// expected-note@#Movable2  {{candidate constructor not viable: no known conversion from 'int' to 'Movable' for 1st argument}} \
+// expected-note@#Movable1  {{candidate template ignored: constraints not satisfied [with T = int]}}
+
 
 static_assert(__is_constructible(Movable, int));
-// expected-error@-1{{static assertion failed due to requirement '__is_constructible(Movable, int)'}} \
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(Movable, int)'}} \
+// expected-error@-1 {{no matching constructor for initialization of 'Movable'}} \
 // expected-note@-1 2{{}}
 // expected-error@#err-self-constraint-1{{satisfaction of constraint '__is_constructible(Movable, T)' depends on itself}}
 // expected-note@#err-self-constraint-1 4{{}}
+// expected-note@#Movable  {{'Movable' defined here}}
 
 template <typename T>
 struct Members {

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

@@ -42,6 +42,14 @@ static constexpr bool value = __is_standard_layout(T);
 };
 template <typename T>
 constexpr bool is_standard_layout_v = __is_standard_layout(T);
+
+template <typename... Args>
+struct is_constructible {
+    static constexpr bool value = __is_constructible(Args...);
+};
+
+template <typename... Args>
+constexpr bool is_constructible_v = __is_constructible(Args...);
 #endif
 
 #ifdef STD2
@@ -97,6 +105,17 @@ template <typename T>
 using is_standard_layout = __details_is_standard_layout<T>;
 template <typename T>
 constexpr bool is_standard_layout_v = __is_standard_layout(T);
+
+template <typename... Args>
+struct __details_is_constructible{
+    static constexpr bool value = __is_constructible(Args...);
+};
+
+template <typename... Args>
+using is_constructible  = __details_is_constructible<Args...>;
+
+template <typename... Args>
+constexpr bool is_constructible_v = __is_constructible(Args...);
 #endif
 
 
@@ -149,6 +168,15 @@ template <typename T>
 using is_standard_layout = __details_is_standard_layout<T>;
 template <typename T>
 constexpr bool is_standard_layout_v = is_standard_layout<T>::value;
+
+template <typename... Args>
+struct __details_is_constructible : bool_constant<__is_constructible(Args...)> {};
+
+template <typename... Args>
+using is_constructible  = __details_is_constructible<Args...>;
+
+template <typename... Args>
+constexpr bool is_constructible_v = is_constructible<Args...>::value;
 #endif
 
 }
@@ -211,6 +239,15 @@ static_assert(std::is_assignable_v<int&, void>);
 // expected-error@-1 {{static assertion failed due to requirement 'std::is_assignable_v<int &, void>'}} \
 // expected-error@-1 {{assigning to 'int' from incompatible type 'void'}}
 
+static_assert(std::is_constructible<int, int>::value);
+
+static_assert(std::is_constructible<void>::value);
+// expected-error-re@-1 {{static assertion failed due to requirement 'std::{{.*}}is_constructible<void>::value'}} \
+// expected-note@-1 {{because it is a cv void type}}
+static_assert(std::is_constructible_v<void>);
+// expected-error@-1 {{static assertion failed due to requirement 'std::is_constructible_v<void>'}} \
+// expected-note@-1 {{because it is a cv void type}}
+
 namespace test_namespace {
     using namespace std;
     static_assert(is_trivially_relocatable<int&>::value);
@@ -256,6 +293,13 @@ namespace test_namespace {
     // expected-error@-1 {{static assertion failed due to requirement 'is_empty_v<int &>'}} \
     // expected-note@-1 {{'int &' is not empty}} \
     // expected-note@-1 {{because it is a reference type}}
+
+    static_assert(is_constructible<void>::value);
+    // expected-error-re@-1 {{static assertion failed due to requirement '{{.*}}is_constructible<void>::value'}} \
+    // expected-note@-1 {{because it is a cv void type}}
+    static_assert(is_constructible_v<void>);
+    // expected-error@-1 {{static assertion failed due to requirement 'is_constructible_v<void>'}} \
+    // expected-note@-1 {{because it is a cv void type}}
 }
 
 
@@ -284,6 +328,15 @@ concept C4 = std::is_assignable_v<T, U>; // #concept8
 
 template <C4<void> T> void g4();  // #cand8
 
+template <typename... Args>
+requires std::is_constructible<Args...>::value void f3();  // #cand5
+
+template <typename... Args>
+concept C3 = std::is_constructible_v<Args...>; // #concept6
+
+template <C3 T> void g3();  // #cand6
+
+
 void test() {
     f<int&>();
     // expected-error@-1 {{no matching function for call to 'f'}} \
@@ -327,6 +380,19 @@ void test() {
     // expected-note@#cand8 {{because 'C4<int &, void>' evaluated to false}} \
     // expected-note@#concept8 {{because 'std::is_assignable_v<int &, void>' evaluated to false}} \
     // expected-error@#concept8 {{assigning to 'int' from incompatible type 'void'}}
+
+    f3<void>();
+    // expected-error@-1 {{no matching function for call to 'f3'}} \
+    // expected-note@#cand5 {{candidate template ignored: constraints not satisfied [with Args = <void>]}} \
+    // expected-note-re@#cand5 {{because '{{.*}}is_constructible<void>::value' evaluated to false}} \
+    // expected-note@#cand5 {{because it is a cv void type}}
+
+    g3<void>();
+    // expected-error@-1 {{no matching function for call to 'g3'}} \
+    // expected-note@#cand6 {{candidate template ignored: constraints not satisfied [with T = void]}} \
+    // expected-note@#cand6 {{because 'void' does not satisfy 'C3'}} \
+    // expected-note@#concept6 {{because 'std::is_constructible_v<void>' evaluated to false}} \
+    // expected-note@#concept6 {{because it is a cv void type}}
 }
 }
 

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

@@ -489,6 +489,68 @@ static_assert(__is_trivially_copyable(S12));
 // expected-note@#tc-S12 {{'S12' defined here}}
 }
 
+namespace constructible {
+
+struct S1 {  // #c-S1
+    S1(int); // #cc-S1
+};
+static_assert(__is_constructible(S1, char*));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(constructible::S1, char *)'}} \
+// expected-error@-1 {{no matching constructor for initialization of 'S1'}} \
+// expected-note@#c-S1 {{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'char *' to 'const S1' for 1st argument}} \
+// expected-note@#c-S1 {{candidate constructor (the implicit move constructor) not viable: no known conversion from 'char *' to 'S1' for 1st argument}} \
+// expected-note@#cc-S1 {{candidate constructor not viable: no known conversion from 'char *' to 'int' for 1st argument; dereference the argument with *}} \
+// expected-note@#c-S1 {{'S1' defined here}}
+
+struct S2 { // #c-S2
+    S2(int, float, double); // #cc-S2
+};
+static_assert(__is_constructible(S2, float));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(constructible::S2, float)'}} \
+// expected-note@#c-S2 {{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'float' to 'const S2' for 1st argument}} \
+// expected-note@#c-S2 {{candidate constructor (the implicit move constructor) not viable: no known conversion from 'float' to 'S2' for 1st argument}} \
+// expected-error@-1 {{no matching constructor for initialization of 'S2'}} \
+// expected-note@#cc-S2 {{candidate constructor not viable: requires 3 arguments, but 1 was provided}} \
+// expected-note@#c-S2 {{'S2' defined here}}
+
+static_assert(__is_constructible(S2, float, void));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(constructible::S2, float, void)'}} \
+// expected-note@#c-S2 {{candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 2 were provided}} \
+// expected-note@#c-S2 {{candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 2 were provided}} \
+// expected-note@-1{{because it is a cv void type}} \
+// expected-error@-1 {{no matching constructor for initialization of 'S2'}} \
+// expected-note@#cc-S2 {{candidate constructor not viable: requires 3 arguments, but 2 were provided}} \
+// expected-note@#c-S2 {{'S2' defined here}}
+
+static_assert(__is_constructible(int[]));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(int[])'}} \
+// expected-note@-1 {{because it is an incomplete array type}}
+
+static_assert(__is_constructible(void));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(void)'}} \
+// expected-note@-1 {{because it is a cv void type}}
+
+static_assert(__is_constructible(void, void));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(void, void)'}} \
+// expected-note@-1 {{because it is a cv void type}}
+
+static_assert(__is_constructible(const void));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(const void)'}} \
+// expected-note@-1 {{because it is a cv void type}}
+
+static_assert(__is_constructible(volatile void));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(volatile void)'}} \
+// expected-note@-1 {{because it is a cv void type}}
+
+static_assert(__is_constructible(int ()));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(int ())'}} \
+// expected-note@-1 {{because it is a function type}}
+
+static_assert(__is_constructible(void (int, float)));
+// expected-error@-1 {{static assertion failed due to requirement '__is_constructible(void (int, float))'}} \
+// expected-note@-1 {{because it is a function type}}
+}
+
 namespace assignable {
 struct S1;
 static_assert(__is_assignable(S1&, const S1&));

libcxx/test/libcxx/utilities/expected/expected.expected/and_then.mandates.verify.cpp

@@ -55,6 +55,7 @@ void test() {
       // expected-error-re@*:* {{static assertion failed {{.*}}The result of f(value()) must be a specialization of std::expected}}
       // expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
       // expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
+      // expected-error@*:* 0-1{{excess elements in struct initializer}}
     }
 
     // !std::is_same_v<U:error_type, E>
@@ -74,6 +75,7 @@ void test() {
       // expected-error-re@*:* {{static assertion failed {{.*}}The result of f(value()) must be a specialization of std::expected}}
       // expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
       // expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
+      // expected-error@*:* 0-1{{excess elements in struct initializer}}
     }
 
     // !std::is_same_v<U:error_type, E>
@@ -94,6 +96,7 @@ void test() {
       // expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(value())) must be a specialization of std::expected}}
       // expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
       // expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
+      // expected-error@*:* 0-1{{excess elements in struct initializer}}
     }
 
     // !std::is_same_v<U:error_type, E>
@@ -113,6 +116,7 @@ void test() {
       // expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(value())) must be a specialization of std::expected}}
       // expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
       // expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
+      // expected-error@*:* 0-1{{excess elements in struct initializer}}
     }
 
     // !std::is_same_v<U:error_type, E>