[clang] adding explicit(bool) from c++2a

this patch adds support for the explicit bool specifier.

Changes:
- The parsing for the explicit(bool) specifier was added in ParseDecl.cpp.
- The storage of the explicit specifier was changed. the explicit specifier was stored as a boolean value in the FunctionDeclBitfields and in the DeclSpec class. now it is stored as a PointerIntPair<Expr*, 2> with a flag and a potential expression in CXXConstructorDecl, CXXDeductionGuideDecl, CXXConversionDecl and in the DeclSpec class.
- Following the AST change, Serialization, ASTMatchers, ASTComparator and ASTPrinter were adapted.
- Template instantiation was adapted to instantiate the potential expressions of the explicit(bool) specifier When instantiating their associated declaration.
- The Add*Candidate functions were adapted, they now take a Boolean indicating if the context allowing explicit constructor or conversion function and this boolean is used to remove invalid overloads that required template instantiation to be detected.
- Test for Semantic and Serialization were added.

This patch is not yet complete. I still need to check that interaction with CTAD and deduction guides is correct. and add more tests for AST operations. But I wanted first feedback.
Perhaps this patch should be spited in smaller patches, but making each patch testable as a standalone may be tricky.

Patch by Tyker

Differential Revision: https://reviews.llvm.org/D60934

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@359949 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 110 insertions, 63 deletions

diff --git a/lib/Sema/SemaOverload.cpp b/lib/Sema/SemaOverload.cpp
index df979b68b6..1ef932e05e 100644
--- a/lib/Sema/SemaOverload.cpp
+++ b/lib/Sema/SemaOverload.cpp
@@ -3242,10 +3242,13 @@ IsInitializerListConstructorConversion(Sema &S, Expr *From, QualType ToType,
       if (Info.ConstructorTmpl)
         S.AddTemplateOverloadCandidate(Info.ConstructorTmpl, Info.FoundDecl,
                                        /*ExplicitArgs*/ nullptr, From,
-                                       CandidateSet, SuppressUserConversions);
+                                       CandidateSet, SuppressUserConversions,
+                                       /*PartialOverloading*/ false,
+                                       AllowExplicit);
       else
         S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl, From,
-                               CandidateSet, SuppressUserConversions);
+                               CandidateSet, SuppressUserConversions,
+                               /*PartialOverloading*/ false, AllowExplicit);
     }
   }
 
@@ -3372,13 +3375,15 @@ IsUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
             S.AddTemplateOverloadCandidate(
                 Info.ConstructorTmpl, Info.FoundDecl,
                 /*ExplicitArgs*/ nullptr, llvm::makeArrayRef(Args, NumArgs),
-                CandidateSet, SuppressUserConversions);
+                CandidateSet, SuppressUserConversions,
+                /*PartialOverloading*/ false, AllowExplicit);
           else
             // Allow one user-defined conversion when user specifies a
             // From->ToType conversion via an static cast (c-style, etc).
             S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl,
                                    llvm::makeArrayRef(Args, NumArgs),
-                                   CandidateSet, SuppressUserConversions);
+                                   CandidateSet, SuppressUserConversions,
+                                   /*PartialOverloading*/ false, AllowExplicit);
         }
       }
     }
@@ -3410,14 +3415,13 @@ IsUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
 
         if (AllowExplicit || !Conv->isExplicit()) {
           if (ConvTemplate)
-            S.AddTemplateConversionCandidate(ConvTemplate, FoundDecl,
-                                             ActingContext, From, ToType,
-                                             CandidateSet,
-                                             AllowObjCConversionOnExplicit);
+            S.AddTemplateConversionCandidate(
+                ConvTemplate, FoundDecl, ActingContext, From, ToType,
+                CandidateSet, AllowObjCConversionOnExplicit, AllowExplicit);
           else
-            S.AddConversionCandidate(Conv, FoundDecl, ActingContext,
-                                     From, ToType, CandidateSet,
-                                     AllowObjCConversionOnExplicit);
+            S.AddConversionCandidate(
+                Conv, FoundDecl, ActingContext, From, ToType, CandidateSet,
+                AllowObjCConversionOnExplicit, AllowExplicit);
         }
       }
     }
@@ -4445,13 +4449,13 @@ FindConversionForRefInit(Sema &S, ImplicitConversionSequence &ICS,
     }
 
     if (ConvTemplate)
-      S.AddTemplateConversionCandidate(ConvTemplate, I.getPair(), ActingDC,
-                                       Init, DeclType, CandidateSet,
-                                       /*AllowObjCConversionOnExplicit=*/false);
+      S.AddTemplateConversionCandidate(
+          ConvTemplate, I.getPair(), ActingDC, Init, DeclType, CandidateSet,
+          /*AllowObjCConversionOnExplicit=*/false, AllowExplicit);
     else
-      S.AddConversionCandidate(Conv, I.getPair(), ActingDC, Init,
-                               DeclType, CandidateSet,
-                               /*AllowObjCConversionOnExplicit=*/false);
+      S.AddConversionCandidate(
+          Conv, I.getPair(), ActingDC, Init, DeclType, CandidateSet,
+          /*AllowObjCConversionOnExplicit=*/false, AllowExplicit);
   }
 
   bool HadMultipleCandidates = (CandidateSet.size() > 1);
@@ -5414,7 +5418,7 @@ static ExprResult CheckConvertedConstantExpression(Sema &S, Expr *From,
   //  condition shall be a contextually converted constant expression of type
   //  bool.
   ImplicitConversionSequence ICS =
-      CCE == Sema::CCEK_ConstexprIf
+      CCE == Sema::CCEK_ConstexprIf || CCE == Sema::CCEK_ExplicitBool
           ? TryContextuallyConvertToBool(S, From)
           : TryCopyInitialization(S, From, T,
                                   /*SuppressUserConversions=*/false,
@@ -5730,12 +5734,13 @@ collectViableConversionCandidates(Sema &SemaRef, Expr *From, QualType ToType,
 
     if (ConvTemplate)
       SemaRef.AddTemplateConversionCandidate(
-        ConvTemplate, FoundDecl, ActingContext, From, ToType, CandidateSet,
-        /*AllowObjCConversionOnExplicit=*/false);
+          ConvTemplate, FoundDecl, ActingContext, From, ToType, CandidateSet,
+          /*AllowObjCConversionOnExplicit=*/false, /*AllowExplicit*/ true);
     else
       SemaRef.AddConversionCandidate(Conv, FoundDecl, ActingContext, From,
                                      ToType, CandidateSet,
-                                     /*AllowObjCConversionOnExplicit=*/false);
+                                     /*AllowObjCConversionOnExplicit=*/false,
+                                     /*AllowExplicit*/ true);
   }
 }
 
@@ -5987,13 +5992,11 @@ static bool IsAcceptableNonMemberOperatorCandidate(ASTContext &Context,
 /// \param PartialOverloading true if we are performing "partial" overloading
 /// based on an incomplete set of function arguments. This feature is used by
 /// code completion.
-void Sema::AddOverloadCandidate(FunctionDecl *Function,
-                                DeclAccessPair FoundDecl, ArrayRef<Expr *> Args,
-                                OverloadCandidateSet &CandidateSet,
-                                bool SuppressUserConversions,
-                                bool PartialOverloading, bool AllowExplicit,
-                                ADLCallKind IsADLCandidate,
-                                ConversionSequenceList EarlyConversions) {
+void Sema::AddOverloadCandidate(
+    FunctionDecl *Function, DeclAccessPair FoundDecl, ArrayRef<Expr *> Args,
+    OverloadCandidateSet &CandidateSet, bool SuppressUserConversions,
+    bool PartialOverloading, bool AllowExplicit, bool AllowExplicitConversions,
+    ADLCallKind IsADLCandidate, ConversionSequenceList EarlyConversions) {
   const FunctionProtoType *Proto
     = dyn_cast<FunctionProtoType>(Function->getType()->getAs<FunctionType>());
   assert(Proto && "Functions without a prototype cannot be overloaded");
@@ -6150,13 +6153,11 @@ void Sema::AddOverloadCandidate(FunctionDecl *Function,
       // (13.3.3.1) that converts that argument to the corresponding
       // parameter of F.
       QualType ParamType = Proto->getParamType(ArgIdx);
-      Candidate.Conversions[ArgIdx]
-        = TryCopyInitialization(*this, Args[ArgIdx], ParamType,
-                                SuppressUserConversions,
-                                /*InOverloadResolution=*/true,
-                                /*AllowObjCWritebackConversion=*/
-                                  getLangOpts().ObjCAutoRefCount,
-                                AllowExplicit);
+      Candidate.Conversions[ArgIdx] = TryCopyInitialization(
+          *this, Args[ArgIdx], ParamType, SuppressUserConversions,
+          /*InOverloadResolution=*/true,
+          /*AllowObjCWritebackConversion=*/
+          getLangOpts().ObjCAutoRefCount, AllowExplicitConversions);
       if (Candidate.Conversions[ArgIdx].isBad()) {
         Candidate.Viable = false;
         Candidate.FailureKind = ovl_fail_bad_conversion;
@@ -6170,6 +6171,15 @@ void Sema::AddOverloadCandidate(FunctionDecl *Function,
     }
   }
 
+  if (!AllowExplicit) {
+    ExplicitSpecifier ES = ExplicitSpecifier::getFromDecl(Function);
+    if (ES.getKind() != ExplicitSpecKind::ResolvedFalse) {
+      Candidate.Viable = false;
+      Candidate.FailureKind = ovl_fail_explicit_resolved;
+      return;
+    }
+  }
+
   if (EnableIfAttr *FailedAttr = CheckEnableIf(Function, Args)) {
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_enable_if;
@@ -6759,7 +6769,7 @@ void Sema::AddTemplateOverloadCandidate(
     FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
     TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
     OverloadCandidateSet &CandidateSet, bool SuppressUserConversions,
-    bool PartialOverloading, ADLCallKind IsADLCandidate) {
+    bool PartialOverloading, bool AllowExplicit, ADLCallKind IsADLCandidate) {
   if (!CandidateSet.isNewCandidate(FunctionTemplate))
     return;
 
@@ -6808,9 +6818,10 @@ void Sema::AddTemplateOverloadCandidate(
   // Add the function template specialization produced by template argument
   // deduction as a candidate.
   assert(Specialization && "Missing function template specialization?");
-  AddOverloadCandidate(Specialization, FoundDecl, Args, CandidateSet,
-                       SuppressUserConversions, PartialOverloading,
-                       /*AllowExplicit*/ false, IsADLCandidate, Conversions);
+  AddOverloadCandidate(
+      Specialization, FoundDecl, Args, CandidateSet, SuppressUserConversions,
+      PartialOverloading, AllowExplicit,
+      /*AllowExplicitConversions*/ false, IsADLCandidate, Conversions);
 }
 
 /// Check that implicit conversion sequences can be formed for each argument
@@ -6915,14 +6926,11 @@ static bool isAllowableExplicitConversion(Sema &S,
 /// and ToType is the type that we're eventually trying to convert to
 /// (which may or may not be the same type as the type that the
 /// conversion function produces).
-void
-Sema::AddConversionCandidate(CXXConversionDecl *Conversion,
-                             DeclAccessPair FoundDecl,
-                             CXXRecordDecl *ActingContext,
-                             Expr *From, QualType ToType,
-                             OverloadCandidateSet& CandidateSet,
-                             bool AllowObjCConversionOnExplicit,
-                             bool AllowResultConversion) {
+void Sema::AddConversionCandidate(
+    CXXConversionDecl *Conversion, DeclAccessPair FoundDecl,
+    CXXRecordDecl *ActingContext, Expr *From, QualType ToType,
+    OverloadCandidateSet &CandidateSet, bool AllowObjCConversionOnExplicit,
+    bool AllowExplicit, bool AllowResultConversion) {
   assert(!Conversion->getDescribedFunctionTemplate() &&
          "Conversion function templates use AddTemplateConversionCandidate");
   QualType ConvType = Conversion->getConversionType().getNonReferenceType();
@@ -7081,6 +7089,13 @@ Sema::AddConversionCandidate(CXXConversionDecl *Conversion,
            "Can only end up with a standard conversion sequence or failure");
   }
 
+  if (!AllowExplicit && Conversion->getExplicitSpecifier().getKind() !=
+                            ExplicitSpecKind::ResolvedFalse) {
+    Candidate.Viable = false;
+    Candidate.FailureKind = ovl_fail_explicit_resolved;
+    return;
+  }
+
   if (EnableIfAttr *FailedAttr = CheckEnableIf(Conversion, None)) {
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_enable_if;
@@ -7100,14 +7115,11 @@ Sema::AddConversionCandidate(CXXConversionDecl *Conversion,
 /// to deduce the template arguments of the conversion function
 /// template from the type that we are converting to (C++
 /// [temp.deduct.conv]).
-void
-Sema::AddTemplateConversionCandidate(FunctionTemplateDecl *FunctionTemplate,
-                                     DeclAccessPair FoundDecl,
-                                     CXXRecordDecl *ActingDC,
-                                     Expr *From, QualType ToType,
-                                     OverloadCandidateSet &CandidateSet,
-                                     bool AllowObjCConversionOnExplicit,
-                                     bool AllowResultConversion) {
+void Sema::AddTemplateConversionCandidate(
+    FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
+    CXXRecordDecl *ActingDC, Expr *From, QualType ToType,
+    OverloadCandidateSet &CandidateSet, bool AllowObjCConversionOnExplicit,
+    bool AllowExplicit, bool AllowResultConversion) {
   assert(isa<CXXConversionDecl>(FunctionTemplate->getTemplatedDecl()) &&
          "Only conversion function templates permitted here");
 
@@ -7137,7 +7149,7 @@ Sema::AddTemplateConversionCandidate(FunctionTemplateDecl *FunctionTemplate,
   assert(Specialization && "Missing function template specialization?");
   AddConversionCandidate(Specialization, FoundDecl, ActingDC, From, ToType,
                          CandidateSet, AllowObjCConversionOnExplicit,
-                         AllowResultConversion);
+                         AllowExplicit, AllowResultConversion);
 }
 
 /// AddSurrogateCandidate - Adds a "surrogate" candidate function that
@@ -8991,12 +9003,14 @@ Sema::AddArgumentDependentLookupCandidates(DeclarationName Name,
 
       AddOverloadCandidate(FD, FoundDecl, Args, CandidateSet,
                            /*SupressUserConversions=*/false, PartialOverloading,
-                           /*AllowExplicit=*/false, ADLCallKind::UsesADL);
+                           /*AllowExplicitConversions*/ false,
+                           /*AllowExplicit*/ false, ADLCallKind::UsesADL);
     } else {
-      AddTemplateOverloadCandidate(cast<FunctionTemplateDecl>(*I), FoundDecl,
-                                   ExplicitTemplateArgs, Args, CandidateSet,
-                                   /*SupressUserConversions=*/false,
-                                   PartialOverloading, ADLCallKind::UsesADL);
+      AddTemplateOverloadCandidate(
+          cast<FunctionTemplateDecl>(*I), FoundDecl, ExplicitTemplateArgs, Args,
+          CandidateSet,
+          /*SupressUserConversions=*/false, PartialOverloading,
+          /*AllowExplicit*/ false, ADLCallKind::UsesADL);
     }
   }
 }
@@ -10327,6 +10341,33 @@ static void DiagnoseFailedEnableIfAttr(Sema &S, OverloadCandidate *Cand) {
       << Attr->getCond()->getSourceRange() << Attr->getMessage();
 }
 
+static void DiagnoseFailedExplicitSpec(Sema &S, OverloadCandidate *Cand) {
+  ExplicitSpecifier ES;
+  const char *DeclName;
+  switch (Cand->Function->getDeclKind()) {
+  case Decl::Kind::CXXConstructor:
+    ES = cast<CXXConstructorDecl>(Cand->Function)->getExplicitSpecifier();
+    DeclName = "constructor";
+    break;
+  case Decl::Kind::CXXConversion:
+    ES = cast<CXXConversionDecl>(Cand->Function)->getExplicitSpecifier();
+    DeclName = "conversion operator";
+    break;
+  case Decl::Kind::CXXDeductionGuide:
+    ES = cast<CXXDeductionGuideDecl>(Cand->Function)->getExplicitSpecifier();
+    DeclName = "deductiong guide";
+    break;
+  default:
+    llvm_unreachable("invalid Decl");
+  }
+  assert(ES.getExpr() && "null expression should be handled before");
+  S.Diag(Cand->Function->getLocation(),
+         diag::note_ovl_candidate_explicit_forbidden)
+      << DeclName;
+  S.Diag(ES.getExpr()->getBeginLoc(),
+         diag::note_explicit_bool_resolved_to_true);
+}
+
 static void DiagnoseOpenCLExtensionDisabled(Sema &S, OverloadCandidate *Cand) {
   FunctionDecl *Callee = Cand->Function;
 
@@ -10411,6 +10452,9 @@ static void NoteFunctionCandidate(Sema &S, OverloadCandidate *Cand,
   case ovl_fail_enable_if:
     return DiagnoseFailedEnableIfAttr(S, Cand);
 
+  case ovl_fail_explicit_resolved:
+    return DiagnoseFailedExplicitSpec(S, Cand);
+
   case ovl_fail_ext_disabled:
     return DiagnoseOpenCLExtensionDisabled(S, Cand);
 
@@ -12981,8 +13025,11 @@ Sema::BuildCallToMemberFunction(Scope *S, Expr *MemExprE,
 
       // Microsoft supports direct constructor calls.
       if (getLangOpts().MicrosoftExt && isa<CXXConstructorDecl>(Func)) {
-        AddOverloadCandidate(cast<CXXConstructorDecl>(Func), I.getPair(),
-                             Args, CandidateSet);
+        AddOverloadCandidate(cast<CXXConstructorDecl>(Func), I.getPair(), Args,
+                             CandidateSet,
+                             /*SuppressUserConversions*/ false,
+                             /*PartialOverloading*/ false,
+                             /*AllowExplicit*/ true);
       } else if ((Method = dyn_cast<CXXMethodDecl>(Func))) {
         // If explicit template arguments were provided, we can't call a
         // non-template member function.