[Concepts] Constraint Enforcement & Diagnostics

Part of the C++20 concepts implementation effort.
- Associated constraints (requires clauses, currently) are now enforced when instantiating/specializing templates and when considering partial specializations and function overloads.
- Elaborated diagnostics give helpful insight as to why the constraints were not satisfied.
Phabricator: D41569

Author: saarraz

clang/include/clang/AST/ASTConcept.h

@@ -0,0 +1,80 @@
+//===--- ASTConcept.h - Concepts Related AST Data Structures ----*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// \brief This file provides AST data structures related to concepts.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_ASTCONCEPT_H
+#define LLVM_CLANG_AST_ASTCONCEPT_H
+#include "clang/AST/Expr.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/PointerUnion.h"
+#include "llvm/ADT/SmallVector.h"
+#include <string>
+#include <utility>
+namespace clang {
+
+/// \brief The result of a constraint satisfaction check, containing the
+/// necessary information to diagnose an unsatisfied constraint.
+struct ConstraintSatisfaction {
+  using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
+  using Detail = llvm::PointerUnion<Expr *, SubstitutionDiagnostic *>;
+
+  bool IsSatisfied = false;
+
+  /// \brief Pairs of unsatisfied atomic constraint expressions along with the
+  /// substituted constraint expr, if the template arguments could be
+  /// substituted into them, or a diagnostic if substitution resulted in an
+  /// invalid expression.
+  llvm::SmallVector<std::pair<const Expr *, Detail>, 4> Details;
+
+  // This can leak if used in an AST node, use ASTConstraintSatisfaction
+  // instead.
+  void *operator new(size_t bytes, ASTContext &C) = delete;
+};
+
+/// Pairs of unsatisfied atomic constraint expressions along with the
+/// substituted constraint expr, if the template arguments could be
+/// substituted into them, or a diagnostic if substitution resulted in
+/// an invalid expression.
+using UnsatisfiedConstraintRecord =
+    std::pair<const Expr *,
+              llvm::PointerUnion<Expr *,
+                                 std::pair<SourceLocation, StringRef> *>>;
+
+/// \brief The result of a constraint satisfaction check, containing the
+/// necessary information to diagnose an unsatisfied constraint.
+///
+/// This is safe to store in an AST node, as opposed to ConstraintSatisfaction.
+struct ASTConstraintSatisfaction final :
+    llvm::TrailingObjects<ASTConstraintSatisfaction,
+                          UnsatisfiedConstraintRecord> {
+  std::size_t NumRecords;
+  bool IsSatisfied : 1;
+
+  const UnsatisfiedConstraintRecord *begin() const {
+    return getTrailingObjects<UnsatisfiedConstraintRecord>();
+  }
+
+  const UnsatisfiedConstraintRecord *end() const {
+    return getTrailingObjects<UnsatisfiedConstraintRecord>() + NumRecords;
+  }
+
+  ASTConstraintSatisfaction(const ASTContext &C,
+                            const ConstraintSatisfaction &Satisfaction);
+
+  static ASTConstraintSatisfaction *
+  Create(const ASTContext &C, const ConstraintSatisfaction &Satisfaction);
+};
+
+} // clang
+
+#endif // LLVM_CLANG_AST_ASTCONCEPT_H

clang/include/clang/AST/ExprCXX.h

@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_AST_EXPRCXX_H
 #define LLVM_CLANG_AST_EXPRCXX_H
 
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
@@ -4851,6 +4852,10 @@ class ConceptSpecializationExpr final : public Expr,
                                     TemplateArgument> {
   friend class ASTStmtReader;
   friend TrailingObjects;
+public:
+  using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
+
+protected:
 
   // \brief The optional nested name specifier used when naming the concept.
   NestedNameSpecifierLoc NestedNameSpec;
@@ -4868,11 +4873,8 @@ class ConceptSpecializationExpr final : public Expr,
   /// through a UsingShadowDecl.
   NamedDecl *FoundDecl;
 
-  /// \brief The concept named, and whether or not the concept with the given
-  /// arguments was satisfied when the expression was created.
-  /// If any of the template arguments are dependent (this expr would then be
-  /// isValueDependent()), this bit is to be ignored.
-  llvm::PointerIntPair<ConceptDecl *, 1, bool> NamedConcept;
+  /// \brief The concept named.
+  ConceptDecl *NamedConcept;
 
   /// \brief The template argument list source info used to specialize the
   /// concept.
@@ -4882,13 +4884,18 @@ class ConceptSpecializationExpr final : public Expr,
   /// converted template arguments.
   unsigned NumTemplateArgs;
 
+  /// \brief Information about the satisfaction of the named concept with the
+  /// given arguments. If this expression is value dependent, this is to be
+  /// ignored.
+  ASTConstraintSatisfaction *Satisfaction;
+
   ConceptSpecializationExpr(ASTContext &C, NestedNameSpecifierLoc NNS,
                             SourceLocation TemplateKWLoc,
                             SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
                             ConceptDecl *NamedConcept,
                             const ASTTemplateArgumentListInfo *ArgsAsWritten,
                             ArrayRef<TemplateArgument> ConvertedArgs,
-                            Optional<bool> IsSatisfied);
+                            const ConstraintSatisfaction *Satisfaction);
 
   ConceptSpecializationExpr(EmptyShell Empty, unsigned NumTemplateArgs);
 
@@ -4899,7 +4906,8 @@ class ConceptSpecializationExpr final : public Expr,
          SourceLocation TemplateKWLoc, SourceLocation ConceptNameLoc,
          NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
          const ASTTemplateArgumentListInfo *ArgsAsWritten,
-         ArrayRef<TemplateArgument> ConvertedArgs, Optional<bool> IsSatisfied);
+         ArrayRef<TemplateArgument> ConvertedArgs,
+         const ConstraintSatisfaction *Satisfaction);
 
   static ConceptSpecializationExpr *
   Create(ASTContext &C, EmptyShell Empty, unsigned NumTemplateArgs);
@@ -4913,7 +4921,7 @@ class ConceptSpecializationExpr final : public Expr,
   }
 
   ConceptDecl *getNamedConcept() const {
-    return NamedConcept.getPointer();
+    return NamedConcept;
   }
 
   ArrayRef<TemplateArgument> getTemplateArguments() const {
@@ -4930,12 +4938,21 @@ class ConceptSpecializationExpr final : public Expr,
                             ArrayRef<TemplateArgument> Converted);
 
   /// \brief Whether or not the concept with the given arguments was satisfied
-  /// when the expression was created. This method assumes that the expression
-  /// is not dependent!
+  /// when the expression was created.
+  /// The expression must not be dependent.
   bool isSatisfied() const {
     assert(!isValueDependent()
            && "isSatisfied called on a dependent ConceptSpecializationExpr");
-    return NamedConcept.getInt();
+    return Satisfaction->IsSatisfied;
+  }
+
+  /// \brief Get elaborated satisfaction info about the template arguments'
+  /// satisfaction of the named concept.
+  /// The expression must not be dependent.
+  const ASTConstraintSatisfaction &getSatisfaction() const {
+    assert(!isValueDependent()
+           && "getSatisfaction called on dependent ConceptSpecializationExpr");
+    return *Satisfaction;
   }
 
   SourceLocation getConceptNameLoc() const { return ConceptNameLoc; }

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2542,6 +2542,26 @@ def err_non_constant_constraint_expression : Error<
   "expression">;
 def err_non_bool_atomic_constraint : Error<
   "atomic constraint must be of type 'bool' (found %0)">;
+def err_template_arg_list_constraints_not_satisfied : Error<
+  "constraints not satisfied for %select{class template|function template|variable template|alias template|"
+  "template template parameter|template}0 %1%2">;
+def note_constraints_not_satisfied : Note<
+  "constraints not satisfied">;
+def note_substituted_constraint_expr_is_ill_formed : Note<
+  "because substituted constraint expression is ill-formed%0">;
+def note_atomic_constraint_evaluated_to_false : Note<
+  "%select{and |because }0'%1' evaluated to false">;
+def note_concept_specialization_constraint_evaluated_to_false : Note<
+  "%select{and |because }0'%1' evaluated to false">;
+def note_single_arg_concept_specialization_constraint_evaluated_to_false : Note<
+  "%select{and |because }0%1 does not satisfy %2">;
+def note_atomic_constraint_evaluated_to_false_elaborated : Note<
+  "%select{and |because }0'%1' (%2 %3 %4) evaluated to false">;
+def err_could_not_normalize_ill_formed_constraint : Error<
+  "required expansion of concept specialization %0 failed, substituted "
+  "expression would be illegal">;
+def note_could_not_normalize_ill_formed_constraint_reason : Note<
+  "because: %0">;
 
 def err_template_different_requires_clause : Error<
   "requires clause differs in template redeclaration">;
@@ -3820,6 +3840,8 @@ def note_ovl_candidate_inconsistent_deduction_types : Note<
 def note_ovl_candidate_explicit_arg_mismatch_named : Note<
     "candidate template ignored: invalid explicitly-specified argument "
     "for template parameter %0">;
+def note_ovl_candidate_unsatisfied_constraints : Note<
+    "candidate template ignored: constraints not satisfied%0">;
 def note_ovl_candidate_explicit_arg_mismatch_unnamed : Note<
     "candidate template ignored: invalid explicitly-specified argument "
     "for %ordinal0 template parameter">;
@@ -4512,6 +4534,14 @@ def note_template_default_arg_checking : Note<
   "while checking a default template argument used here">;
 def note_concept_specialization_here : Note<
   "while checking the satisfaction of concept '%0' requested here">;
+def note_checking_constraints_for_template_id_here : Note<
+  "while checking constraint satisfaction for template '%0' required here">;
+def note_checking_constraints_for_var_spec_id_here : Note<
+  "while checking constraint satisfaction for variable template "
+  "partial specialization '%0' required here">;
+def note_checking_constraints_for_class_spec_id_here : Note<
+  "while checking constraint satisfaction for class template partial "
+  "specialization '%0' required here">;
 def note_constraint_substitution_here : Note<
   "while substituting template arguments into constraint expression here">;
 def note_instantiation_contexts_suppressed : Note<

clang/include/clang/Sema/Sema.h

@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_SEMA_SEMA_H
 #define LLVM_CLANG_SEMA_SEMA_H
 
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/Availability.h"
 #include "clang/AST/ComparisonCategories.h"
@@ -6131,17 +6132,84 @@ class Sema {
   /// A diagnostic is emitted if it is not, and false is returned.
   bool CheckConstraintExpression(Expr *CE);
 
-  bool CalculateConstraintSatisfaction(ConceptDecl *NamedConcept,
-                                       MultiLevelTemplateArgumentList &MLTAL,
-                                       Expr *ConstraintExpr,
-                                       bool &IsSatisfied);
+  /// \brief Check whether the given list of constraint expressions are
+  /// satisfied (as if in a 'conjunction') given template arguments.
+  /// \param ConstraintExprs a list of constraint expressions, treated as if
+  /// they were 'AND'ed together.
+  /// \param TemplateArgs the list of template arguments to substitute into the
+  /// constraint expression.
+  /// \param TemplateIDRange The source range of the template id that
+  /// caused the constraints check.
+  /// \param Satisfaction if true is returned, will contain details of the
+  /// satisfaction, with enough information to diagnose an unsatisfied
+  /// expression.
+  /// \returns true if an error occurred and satisfaction could not be checked,
+  /// false otherwise.
+  bool CheckConstraintSatisfaction(TemplateDecl *Template,
+                                   ArrayRef<const Expr *> ConstraintExprs,
+                                   ArrayRef<TemplateArgument> TemplateArgs,
+                                   SourceRange TemplateIDRange,
+                                   ConstraintSatisfaction &Satisfaction);
+
+  bool CheckConstraintSatisfaction(ClassTemplatePartialSpecializationDecl *TD,
+                                   ArrayRef<const Expr *> ConstraintExprs,
+                                   ArrayRef<TemplateArgument> TemplateArgs,
+                                   SourceRange TemplateIDRange,
+                                   ConstraintSatisfaction &Satisfaction);
+
+  bool CheckConstraintSatisfaction(VarTemplatePartialSpecializationDecl *TD,
+                                   ArrayRef<const Expr *> ConstraintExprs,
+                                   ArrayRef<TemplateArgument> TemplateArgs,
+                                   SourceRange TemplateIDRange,
+                                   ConstraintSatisfaction &Satisfaction);
+
+  /// \brief Check whether the given non-dependent constraint expression is
+  /// satisfied. Returns false and updates Satisfaction with the satisfaction
+  /// verdict if successful, emits a diagnostic and returns true if an error
+  /// occured and satisfaction could not be determined.
+  ///
+  /// \returns true if an error occurred, false otherwise.
+  bool CheckConstraintSatisfaction(const Expr *ConstraintExpr,
+                                   ConstraintSatisfaction &Satisfaction);
 
   /// Check that the associated constraints of a template declaration match the
   /// associated constraints of an older declaration of which it is a
   /// redeclaration.
   bool CheckRedeclarationConstraintMatch(TemplateParameterList *Old,
                                          TemplateParameterList *New);
 
+  /// \brief Ensure that the given template arguments satisfy the constraints
+  /// associated with the given template, emitting a diagnostic if they do not.
+  ///
+  /// \param Template The template to which the template arguments are being
+  /// provided.
+  ///
+  /// \param TemplateArgs The converted, canonicalized template arguments.
+  ///
+  /// \param TemplateIDRange The source range of the template id that
+  /// caused the constraints check.
+  ///
+  /// \returns true if the constrains are not satisfied or could not be checked
+  /// for satisfaction, false if the constraints are satisfied.
+  bool EnsureTemplateArgumentListConstraints(TemplateDecl *Template,
+                                       ArrayRef<TemplateArgument> TemplateArgs,
+                                             SourceRange TemplateIDRange);
+
+  /// \brief Emit diagnostics explaining why a constraint expression was deemed
+  /// unsatisfied.
+  void
+  DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction& Satisfaction);
+
+  /// \brief Emit diagnostics explaining why a constraint expression was deemed
+  /// unsatisfied.
+  void
+  DiagnoseUnsatisfiedConstraint(const ASTConstraintSatisfaction& Satisfaction);
+
+  /// \brief Emit diagnostics explaining why a constraint expression was deemed
+  /// unsatisfied because it was ill-formed.
+  void DiagnoseUnsatisfiedIllFormedConstraint(SourceLocation DiagnosticLocation,
+                                              StringRef Diagnostic);
+
   // ParseObjCStringLiteral - Parse Objective-C string literals.
   ExprResult ParseObjCStringLiteral(SourceLocation *AtLocs,
                                     ArrayRef<Expr *> Strings);
@@ -6957,13 +7025,18 @@ class Sema {
   /// contain the converted forms of the template arguments as written.
   /// Otherwise, \p TemplateArgs will not be modified.
   ///
+  /// \param ConstraintsNotSatisfied If provided, and an error occured, will
+  /// receive true if the cause for the error is the associated constraints of
+  /// the template not being satisfied by the template arguments.
+  ///
   /// \returns true if an error occurred, false otherwise.
   bool CheckTemplateArgumentList(TemplateDecl *Template,
                                  SourceLocation TemplateLoc,
                                  TemplateArgumentListInfo &TemplateArgs,
                                  bool PartialTemplateArgs,
                                  SmallVectorImpl<TemplateArgument> &Converted,
-                                 bool UpdateArgsWithConversions = true);
+                                 bool UpdateArgsWithConversions = true,
+                                 bool *ConstraintsNotSatisfied = nullptr);
 
   bool CheckTemplateTypeArgument(TemplateTypeParmDecl *Param,
                                  TemplateArgumentLoc &Arg,
@@ -7505,6 +7578,9 @@ class Sema {
     TDK_InvalidExplicitArguments,
     /// Checking non-dependent argument conversions failed.
     TDK_NonDependentConversionFailure,
+    /// The deduced arguments did not satisfy the constraints associated
+    /// with the template.
+    TDK_ConstraintsNotSatisfied,
     /// Deduction failed; that's all we know.
     TDK_MiscellaneousDeductionFailure,
     /// CUDA Target attributes do not match.
@@ -8017,7 +8093,7 @@ class Sema {
     /// constrained entity (a concept declaration or a template with associated
     /// constraints).
     InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
-                          ConstraintsCheck, TemplateDecl *Template,
+                          ConstraintsCheck, NamedDecl *Template,
                           ArrayRef<TemplateArgument> TemplateArgs,
                           SourceRange InstantiationRange);
 
@@ -8026,7 +8102,7 @@ class Sema {
     /// with a template declaration or as part of the satisfaction check of a
     /// concept.
     InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
-                          ConstraintSubstitution, TemplateDecl *Template,
+                          ConstraintSubstitution, NamedDecl *Template,
                           sema::TemplateDeductionInfo &DeductionInfo,
                           SourceRange InstantiationRange);
 

clang/include/clang/Sema/TemplateDeduction.h

@@ -14,6 +14,8 @@
 #ifndef LLVM_CLANG_SEMA_TEMPLATEDEDUCTION_H
 #define LLVM_CLANG_SEMA_TEMPLATEDEDUCTION_H
 
+#include "clang/Sema/Ownership.h"
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/DeclAccessPair.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/TemplateBase.h"
@@ -218,6 +220,10 @@ class TemplateDeductionInfo {
   ///
   /// FIXME: This should be kept internal to SemaTemplateDeduction.
   SmallVector<DeducedPack *, 8> PendingDeducedPacks;
+
+  /// \brief The constraint satisfaction details resulting from the associated
+  /// constraints satisfaction tests.
+  ConstraintSatisfaction AssociatedConstraintsSatisfaction;
 };
 
 } // namespace sema