[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

Re-commit, after fixing some memory bugs.

1 files changed, 47 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index a97847461e3..d733563c98d 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -591,6 +591,12 @@ namespace {
     TemplateArgumentList *TemplateArgs;
     unsigned CallArgIndex;
   };
+  // Structure used by DeductionFailureInfo to store information about
+  // unsatisfied constraints.
+  struct CNSInfo {
+    TemplateArgumentList *TemplateArgs;
+    ConstraintSatisfaction Satisfaction;
+  };
 }
 
 /// Convert from Sema's representation of template deduction information
@@ -661,6 +667,14 @@ clang::MakeDeductionFailureInfo(ASTContext &Context,
     }
     break;
 
+  case Sema::TDK_ConstraintsNotSatisfied: {
+    CNSInfo *Saved = new (Context) CNSInfo;
+    Saved->TemplateArgs = Info.take();
+    Saved->Satisfaction = Info.AssociatedConstraintsSatisfaction;
+    Result.Data = Saved;
+    break;
+  }
+
   case Sema::TDK_Success:
   case Sema::TDK_NonDependentConversionFailure:
     llvm_unreachable("not a deduction failure");
@@ -701,6 +715,15 @@ void DeductionFailureInfo::Destroy() {
     }
     break;
 
+  case Sema::TDK_ConstraintsNotSatisfied:
+    // FIXME: Destroy the template argument list?
+    Data = nullptr;
+    if (PartialDiagnosticAt *Diag = getSFINAEDiagnostic()) {
+      Diag->~PartialDiagnosticAt();
+      HasDiagnostic = false;
+    }
+    break;
+
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
     break;
@@ -726,6 +749,7 @@ TemplateParameter DeductionFailureInfo::getTemplateParameter() {
   case Sema::TDK_NonDeducedMismatch:
   case Sema::TDK_CUDATargetMismatch:
   case Sema::TDK_NonDependentConversionFailure:
+  case Sema::TDK_ConstraintsNotSatisfied:
     return TemplateParameter();
 
   case Sema::TDK_Incomplete:
@@ -769,6 +793,9 @@ TemplateArgumentList *DeductionFailureInfo::getTemplateArgumentList() {
   case Sema::TDK_SubstitutionFailure:
     return static_cast<TemplateArgumentList*>(Data);
 
+  case Sema::TDK_ConstraintsNotSatisfied:
+    return static_cast<CNSInfo*>(Data)->TemplateArgs;
+
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
     break;
@@ -789,6 +816,7 @@ const TemplateArgument *DeductionFailureInfo::getFirstArg() {
   case Sema::TDK_SubstitutionFailure:
   case Sema::TDK_CUDATargetMismatch:
   case Sema::TDK_NonDependentConversionFailure:
+  case Sema::TDK_ConstraintsNotSatisfied:
     return nullptr;
 
   case Sema::TDK_IncompletePack:
@@ -820,6 +848,7 @@ const TemplateArgument *DeductionFailureInfo::getSecondArg() {
   case Sema::TDK_SubstitutionFailure:
   case Sema::TDK_CUDATargetMismatch:
   case Sema::TDK_NonDependentConversionFailure:
+  case Sema::TDK_ConstraintsNotSatisfied:
     return nullptr;
 
   case Sema::TDK_Inconsistent:
@@ -1255,6 +1284,8 @@ bool Sema::IsOverload(FunctionDecl *New, FunctionDecl *Old,
     return NewTarget != OldTarget;
   }
 
+  // TODO: Concepts: Check function trailing requires clauses here.
+
   // The signatures match; this is not an overload.
   return false;
 }
@@ -10352,6 +10383,21 @@ static void DiagnoseBadDeduction(Sema &S, NamedDecl *Found, Decl *Templated,
     MaybeEmitInheritedConstructorNote(S, Found);
     return;
 
+  case Sema::TDK_ConstraintsNotSatisfied: {
+    // Format the template argument list into the argument string.
+    SmallString<128> TemplateArgString;
+    TemplateArgumentList *Args = DeductionFailure.getTemplateArgumentList();
+    TemplateArgString = " ";
+    TemplateArgString += S.getTemplateArgumentBindingsText(
+        getDescribedTemplate(Templated)->getTemplateParameters(), *Args);
+    S.Diag(Templated->getLocation(),
+           diag::note_ovl_candidate_unsatisfied_constraints)
+        << TemplateArgString;
+
+    S.DiagnoseUnsatisfiedConstraint(
+        static_cast<CNSInfo*>(DeductionFailure.Data)->Satisfaction);
+    return;
+  }
   case Sema::TDK_TooManyArguments:
   case Sema::TDK_TooFewArguments:
     DiagnoseArityMismatch(S, Found, Templated, NumArgs);
@@ -10804,6 +10850,7 @@ static unsigned RankDeductionFailure(const DeductionFailureInfo &DFI) {
 
   case Sema::TDK_SubstitutionFailure:
   case Sema::TDK_DeducedMismatch:
+  case Sema::TDK_ConstraintsNotSatisfied:
   case Sema::TDK_DeducedMismatchNested:
   case Sema::TDK_NonDeducedMismatch:
   case Sema::TDK_MiscellaneousDeductionFailure: