[Clang] Diagnose unsatisfied std::is_assignable.

Part of the work for #141911.

Checking `is_assignable<T, U>` boils down to checking the well-formedness of `declval<T>() = declval<U>()`; this PR recycles logic from EvaluateBinaryTypeTrait in order to produce the relevant diagnostics.

Author: rkirsling

clang/lib/Sema/SemaTypeTraits.cpp

@@ -1949,6 +1949,7 @@ static std::optional<TypeTrait> StdNameToTypeTrait(StringRef Name) {
       .Case("is_replaceable", TypeTrait::UTT_IsReplaceable)
       .Case("is_trivially_copyable", TypeTrait::UTT_IsTriviallyCopyable)
       .Case("is_constructible", TypeTrait::TT_IsConstructible)
+      .Case("is_assignable", TypeTrait::BTT_IsAssignable)
       .Default(std::nullopt);
 }
 
@@ -2340,6 +2341,31 @@ static void DiagnoseNonTriviallyCopyableReason(Sema &SemaRef,
   SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
 }
 
+static void DiagnoseNonAssignableReason(Sema &SemaRef, SourceLocation Loc,
+                                        QualType T, QualType U) {
+  const CXXRecordDecl *D = T->getAsCXXRecordDecl();
+
+  if (T->isObjectType() || T->isFunctionType())
+    T = SemaRef.Context.getRValueReferenceType(T);
+  if (U->isObjectType() || U->isFunctionType())
+    U = SemaRef.Context.getRValueReferenceType(U);
+  OpaqueValueExpr LHS(Loc, T.getNonLValueExprType(SemaRef.Context),
+                      Expr::getValueKindForType(T));
+  OpaqueValueExpr RHS(Loc, U.getNonLValueExprType(SemaRef.Context),
+                      Expr::getValueKindForType(U));
+
+  EnterExpressionEvaluationContext Unevaluated(
+      SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
+  Sema::ContextRAII TUContext(SemaRef,
+                              SemaRef.Context.getTranslationUnitDecl());
+  SemaRef.BuildBinOp(/*S=*/nullptr, Loc, BO_Assign, &LHS, &RHS);
+
+  if (!D || D->isInvalidDecl())
+    return;
+
+  SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
+}
+
 void Sema::DiagnoseTypeTraitDetails(const Expr *E) {
   E = E->IgnoreParenImpCasts();
   if (E->containsErrors())
@@ -2363,6 +2389,9 @@ void Sema::DiagnoseTypeTraitDetails(const Expr *E) {
   case TT_IsConstructible:
     DiagnoseNonConstructibleReason(*this, E->getBeginLoc(), Args);
     break;
+  case BTT_IsAssignable:
+    DiagnoseNonAssignableReason(*this, E->getBeginLoc(), Args[0], Args[1]);
+    break;
   default:
     break;
   }

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

@@ -28,6 +28,14 @@ struct is_constructible {
 
 template <typename... Args>
 constexpr bool is_constructible_v = __is_constructible(Args...);
+
+template <typename T, typename U>
+struct is_assignable {
+    static constexpr bool value = __is_assignable(T, U);
+};
+
+template <typename T, typename U>
+constexpr bool is_assignable_v = __is_assignable(T, U);
 #endif
 
 #ifdef STD2
@@ -63,6 +71,17 @@ using is_constructible  = __details_is_constructible<Args...>;
 
 template <typename... Args>
 constexpr bool is_constructible_v = __is_constructible(Args...);
+
+template <typename T, typename U>
+struct __details_is_assignable {
+    static constexpr bool value = __is_assignable(T, U);
+};
+
+template <typename T, typename U>
+using is_assignable = __details_is_assignable<T, U>;
+
+template <typename T, typename U>
+constexpr bool is_assignable_v = __is_assignable(T, U);
 #endif
 
 
@@ -101,6 +120,15 @@ using is_constructible  = __details_is_constructible<Args...>;
 
 template <typename... Args>
 constexpr bool is_constructible_v = is_constructible<Args...>::value;
+
+template <typename T, typename U>
+struct __details_is_assignable : bool_constant<__is_assignable(T, U)> {};
+
+template <typename T, typename U>
+using is_assignable  = __details_is_assignable<T, U>;
+
+template <typename T, typename U>
+constexpr bool is_assignable_v = is_assignable<T, U>::value;
 #endif
 
 }
@@ -137,6 +165,15 @@ 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}}
 
+static_assert(std::is_assignable<int&, int>::value);
+
+static_assert(std::is_assignable<int&, void>::value);
+// expected-error-re@-1 {{static assertion failed due to requirement 'std::{{.*}}is_assignable<int &, void>::value'}} \
+// expected-error@-1 {{assigning to 'int' from incompatible type 'void'}}
+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'}}
+
 namespace test_namespace {
     using namespace std;
     static_assert(is_trivially_relocatable<int&>::value);
@@ -163,6 +200,13 @@ namespace test_namespace {
     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}}
+
+    static_assert(is_assignable<int&, void>::value);
+    // expected-error-re@-1 {{static assertion failed due to requirement '{{.*}}is_assignable<int &, void>::value'}} \
+    // expected-error@-1 {{assigning to 'int' from incompatible type 'void'}}
+    static_assert(is_assignable_v<int&, void>);
+    // expected-error@-1 {{static assertion failed due to requirement 'is_assignable_v<int &, void>'}} \
+    // expected-error@-1 {{assigning to 'int' from incompatible type 'void'}}
 }
 
 
@@ -191,6 +235,14 @@ concept C3 = std::is_constructible_v<Args...>; // #concept6
 
 template <C3 T> void g3();  // #cand6
 
+template <typename T, typename U>
+requires std::is_assignable<T, U>::value void f4();  // #cand7
+
+template <typename T, typename U>
+concept C4 = std::is_assignable_v<T, U>; // #concept8
+
+template <C4<void> T> void g4();  // #cand8
+
 
 void test() {
     f<int&>();
@@ -235,6 +287,19 @@ void test() {
     // 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}}
+
+    f4<int&, void>();
+    // expected-error@-1 {{no matching function for call to 'f4'}} \
+    // expected-note@#cand7 {{candidate template ignored: constraints not satisfied [with T = int &, U = void]}} \
+    // expected-note-re@#cand7 {{because '{{.*}}is_assignable<int &, void>::value' evaluated to false}} \
+    // expected-error@#cand7 {{assigning to 'int' from incompatible type 'void'}}
+
+    g4<int&>();
+    // expected-error@-1 {{no matching function for call to 'g4'}} \
+    // expected-note@#cand8 {{candidate template ignored: constraints not satisfied [with T = int &]}} \
+    // 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'}}
 }
 }
 

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

@@ -550,3 +550,74 @@ 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&));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(assignable::S1 &, const assignable::S1 &)'}} \
+// expected-error@-1 {{no viable overloaded '='}} \
+// expected-note@-1 {{type 'S1' is incomplete}}
+
+static_assert(__is_assignable(void, int));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(void, int)'}} \
+// expected-error@-1 {{expression is not assignable}}
+
+static_assert(__is_assignable(int, int));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(int, int)'}} \
+// expected-error@-1 {{expression is not assignable}}
+
+static_assert(__is_assignable(int*, int));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(int *, int)'}} \
+// expected-error@-1 {{expression is not assignable}}
+
+static_assert(__is_assignable(int[], int));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(int[], int)'}} \
+// expected-error@-1 {{expression is not assignable}}
+
+static_assert(__is_assignable(int&, void));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(int &, void)'}} \
+// expected-error@-1 {{assigning to 'int' from incompatible type 'void'}}
+
+static_assert(__is_assignable(int*&, float*));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(int *&, float *)'}} \
+// expected-error@-1 {{incompatible pointer types assigning to 'int *' from 'float *'}}
+
+static_assert(__is_assignable(const int&, int));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(const int &, int)'}} \
+// expected-error@-1 {{read-only variable is not assignable}}
+
+struct S2 {}; // #a-S2
+static_assert(__is_assignable(const S2, S2));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(const assignable::S2, assignable::S2)'}} \
+// expected-error@-1 {{no viable overloaded '='}} \
+// expected-note@#a-S2 {{candidate function (the implicit copy assignment operator) not viable: 'this' argument has type 'const S2', but method is not marked const}} \
+// expected-note@#a-S2 {{candidate function (the implicit move assignment operator) not viable: 'this' argument has type 'const S2', but method is not marked const}} \
+// expected-note@#a-S2 {{'S2' defined here}}
+
+struct S3 { // #a-S3
+    S3& operator=(const S3&) = delete; // #aca-S3
+    S3& operator=(S3&&) = delete;  // #ama-S3
+};
+static_assert(__is_assignable(S3, const S3&));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(assignable::S3, const assignable::S3 &)'}} \
+// expected-error@-1 {{overload resolution selected deleted operator '='}} \
+// expected-note@#aca-S3 {{candidate function has been explicitly deleted}} \
+// expected-note@#ama-S3 {{candidate function not viable: 1st argument ('const S3') would lose const qualifier}} \
+// expected-note@#a-S3 {{'S3' defined here}}
+static_assert(__is_assignable(S3, S3&&));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(assignable::S3, assignable::S3 &&)'}} \
+// expected-error@-1 {{overload resolution selected deleted operator '='}} \
+// expected-note@#aca-S3 {{candidate function has been explicitly deleted}} \
+// expected-note@#ama-S3 {{candidate function has been explicitly deleted}} \
+// expected-note@#a-S3 {{'S3' defined here}}
+
+class C1 { // #a-C1
+  C1& operator=(const C1&) = default;
+  C1& operator=(C1&&) = default; // #ama-C1
+};
+static_assert(__is_assignable(C1, C1));
+// expected-error@-1 {{static assertion failed due to requirement '__is_assignable(assignable::C1, assignable::C1)'}} \
+// expected-error@-1 {{'operator=' is a private member of 'assignable::C1'}} \
+// expected-note@#ama-C1 {{implicitly declared private here}} \
+// expected-note@#a-C1 {{'C1' defined here}}
+}