Implement most of N3638 (return type deduction for normal functions).

Missing (somewhat ironically) is support for the new deduction rules
in lambda functions, plus PCH support for return type patching.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181108 91177308-0d34-0410-b5e6-96231b3b80d8

8 files changed, 276 insertions, 46 deletions

diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index f4e53e6105..b827dc1080 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -2036,6 +2036,16 @@ const FunctionType *ASTContext::adjustFunctionType(const FunctionType *T,
   return cast<FunctionType>(Result.getTypePtr());
 }
 
+void ASTContext::adjustDeducedFunctionResultType(FunctionDecl *FD,
+                                                 QualType ResultType) {
+  // FIXME: Need to inform serialization code about this!
+  for (FD = FD->getMostRecentDecl(); FD; FD = FD->getPreviousDecl()) {
+    const FunctionProtoType *FPT = FD->getType()->castAs<FunctionProtoType>();
+    FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
+    FD->setType(getFunctionType(ResultType, FPT->getArgTypes(), EPI));
+  }
+}
+
 /// getComplexType - Return the uniqued reference to the type for a complex
 /// number with the specified element type.
 QualType ASTContext::getComplexType(QualType T) const {
@@ -3564,18 +3574,20 @@ QualType ASTContext::getUnaryTransformType(QualType BaseType,
   return QualType(Ty, 0);
 }
 
-/// getAutoType - We only unique auto types after they've been deduced.
-QualType ASTContext::getAutoType(QualType DeducedType,
-                                 bool IsDecltypeAuto,
+/// getAutoType - Return the uniqued reference to the 'auto' type which has been
+/// deduced to the given type, or to the canonical undeduced 'auto' type, or the
+/// canonical deduced-but-dependent 'auto' type.
+QualType ASTContext::getAutoType(QualType DeducedType, bool IsDecltypeAuto,
                                  bool IsDependent) const {
+  if (DeducedType.isNull() && !IsDecltypeAuto && !IsDependent)
+    return getAutoDeductType();
+
+  // Look in the folding set for an existing type.
   void *InsertPos = 0;
-  if (!DeducedType.isNull()) {
-    // Look in the folding set for an existing type.
-    llvm::FoldingSetNodeID ID;
-    AutoType::Profile(ID, DeducedType, IsDecltypeAuto, IsDependent);
-    if (AutoType *AT = AutoTypes.FindNodeOrInsertPos(ID, InsertPos))
-      return QualType(AT, 0);
-  }
+  llvm::FoldingSetNodeID ID;
+  AutoType::Profile(ID, DeducedType, IsDecltypeAuto, IsDependent);
+  if (AutoType *AT = AutoTypes.FindNodeOrInsertPos(ID, InsertPos))
+    return QualType(AT, 0);
 
   AutoType *AT = new (*this, TypeAlignment) AutoType(DeducedType,
                                                      IsDecltypeAuto,
@@ -3617,8 +3629,10 @@ QualType ASTContext::getAtomicType(QualType T) const {
 /// getAutoDeductType - Get type pattern for deducing against 'auto'.
 QualType ASTContext::getAutoDeductType() const {
   if (AutoDeductTy.isNull())
-    AutoDeductTy = getAutoType(QualType(), false);
-  assert(!AutoDeductTy.isNull() && "can't build 'auto' pattern");
+    AutoDeductTy = QualType(
+      new (*this, TypeAlignment) AutoType(QualType(), /*decltype(auto)*/false,
+                                          /*dependent*/false),
+      0);
   return AutoDeductTy;
 }
 
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index f7826771cc..06697fafe6 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -2441,12 +2441,22 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, Decl *OldD, Scope *S) {
     //   Certain function declarations cannot be overloaded:
     //     -- Function declarations that differ only in the return type
     //        cannot be overloaded.
-    QualType OldReturnType = OldType->getResultType();
-    QualType NewReturnType = cast<FunctionType>(NewQType)->getResultType();
+
+    // Go back to the type source info to compare the declared return types,
+    // per C++1y [dcl.type.auto]p??:
+    //   Redeclarations or specializations of a function or function template
+    //   with a declared return type that uses a placeholder type shall also
+    //   use that placeholder, not a deduced type.
+    QualType OldDeclaredReturnType = (Old->getTypeSourceInfo()
+      ? Old->getTypeSourceInfo()->getType()->castAs<FunctionType>()
+      : OldType)->getResultType();
+    QualType NewDeclaredReturnType = (New->getTypeSourceInfo()
+      ? New->getTypeSourceInfo()->getType()->castAs<FunctionType>()
+      : NewType)->getResultType();
     QualType ResQT;
-    if (OldReturnType != NewReturnType) {
-      if (NewReturnType->isObjCObjectPointerType()
-          && OldReturnType->isObjCObjectPointerType())
+    if (!Context.hasSameType(OldDeclaredReturnType, NewDeclaredReturnType)) {
+      if (NewDeclaredReturnType->isObjCObjectPointerType() &&
+          OldDeclaredReturnType->isObjCObjectPointerType())
         ResQT = Context.mergeObjCGCQualifiers(NewQType, OldQType);
       if (ResQT.isNull()) {
         if (New->isCXXClassMember() && New->isOutOfLine())
@@ -2461,8 +2471,21 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, Decl *OldD, Scope *S) {
         NewQType = ResQT;
     }
 
-    const CXXMethodDecl* OldMethod = dyn_cast<CXXMethodDecl>(Old);
-    CXXMethodDecl* NewMethod = dyn_cast<CXXMethodDecl>(New);
+    QualType OldReturnType = OldType->getResultType();
+    QualType NewReturnType = cast<FunctionType>(NewQType)->getResultType();
+    if (OldReturnType != NewReturnType) {
+      // If this function has a deduced return type and has already been
+      // defined, copy the deduced value from the old declaration.
+      AutoType *OldAT = Old->getResultType()->getContainedAutoType();
+      if (OldAT && OldAT->isDeduced()) {
+        New->setType(SubstAutoType(New->getType(), OldAT->getDeducedType()));
+        NewQType = Context.getCanonicalType(
+            SubstAutoType(NewQType, OldAT->getDeducedType()));
+      }
+    }
+
+    const CXXMethodDecl *OldMethod = dyn_cast<CXXMethodDecl>(Old);
+    CXXMethodDecl *NewMethod = dyn_cast<CXXMethodDecl>(New);
     if (OldMethod && NewMethod) {
       // Preserve triviality.
       NewMethod->setTrivial(OldMethod->isTrivial());
@@ -6047,6 +6070,10 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
         // Okay: Add virtual to the method.
         NewFD->setVirtualAsWritten(true);
       }
+
+      if (getLangOpts().CPlusPlus1y &&
+          NewFD->getResultType()->isUndeducedType())
+        Diag(D.getDeclSpec().getVirtualSpecLoc(), diag::err_auto_fn_virtual);
     }
 
     // C++ [dcl.fct.spec]p3:
@@ -8860,6 +8887,21 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
   if (FD) {
     FD->setBody(Body);
 
+    if (getLangOpts().CPlusPlus1y && !FD->isInvalidDecl() &&
+        !FD->isDependentContext()) {
+      if (FD->getResultType()->isUndeducedType()) {
+        // If the function has a deduced result type but contains no 'return'
+        // statements, the result type as written must be exactly 'auto', and
+        // the deduced result type is 'void'.
+        if (!FD->getResultType()->getAs<AutoType>()) {
+          Diag(dcl->getLocation(), diag::err_auto_fn_no_return_but_not_auto)
+            << FD->getResultType();
+          FD->setInvalidDecl();
+        }
+        Context.adjustDeducedFunctionResultType(FD, Context.VoidTy);
+      }
+    }
+
     // The only way to be included in UndefinedButUsed is if there is an
     // ODR use before the definition. Avoid the expensive map lookup if this
     // is the first declaration.
diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index 60dbdc14be..2acbe5961e 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -55,6 +55,12 @@ bool Sema::CanUseDecl(NamedDecl *D) {
   if (FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
     if (FD->isDeleted())
       return false;
+
+    // If the function has a deduced return type, and we can't deduce it,
+    // then we can't use it either.
+    if (getLangOpts().CPlusPlus1y && FD->getResultType()->isUndeducedType() &&
+        DeduceReturnType(FD, SourceLocation(), /*Diagnose*/false))
+      return false;
   }
 
   // See if this function is unavailable.
@@ -278,6 +284,12 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, SourceLocation Loc,
       NoteDeletedFunction(FD);
       return true;
     }
+
+    // If the function has a deduced return type, and we can't deduce it,
+    // then we can't use it either.
+    if (getLangOpts().CPlusPlus1y && FD->getResultType()->isUndeducedType() &&
+        DeduceReturnType(FD, Loc))
+      return true;
   }
   DiagnoseAvailabilityOfDecl(*this, D, Loc, UnknownObjCClass);
 
diff --git a/lib/Sema/SemaOverload.cpp b/lib/Sema/SemaOverload.cpp
index 480c4a0d2b..e59c811186 100644
--- a/lib/Sema/SemaOverload.cpp
+++ b/lib/Sema/SemaOverload.cpp
@@ -5726,6 +5726,14 @@ Sema::AddConversionCandidate(CXXConversionDecl *Conversion,
   if (!CandidateSet.isNewCandidate(Conversion))
     return;
 
+  // If the conversion function has an undeduced return type, trigger its
+  // deduction now.
+  if (getLangOpts().CPlusPlus1y && ConvType->isUndeducedType()) {
+    if (DeduceReturnType(Conversion, From->getExprLoc()))
+      return;
+    ConvType = Conversion->getConversionType().getNonReferenceType();
+  }
+
   // Overload resolution is always an unevaluated context.
   EnterExpressionEvaluationContext Unevaluated(*this, Sema::Unevaluated);
 
@@ -9173,6 +9181,13 @@ private:
           if (S.CheckCUDATarget(Caller, FunDecl))
             return false;
 
+      // If any candidate has a placeholder return type, trigger its deduction
+      // now.
+      if (S.getLangOpts().CPlusPlus1y &&
+          FunDecl->getResultType()->isUndeducedType() &&
+          S.DeduceReturnType(FunDecl, SourceExpr->getLocStart(), Complain))
+        return false;
+
       QualType ResultTy;
       if (Context.hasSameUnqualifiedType(TargetFunctionType, 
                                          FunDecl->getType()) ||
@@ -9439,6 +9454,11 @@ Sema::ResolveSingleFunctionTemplateSpecialization(OverloadExpr *ovl,
     if (FoundResult) *FoundResult = I.getPair();    
   }
 
+  if (Matched && getLangOpts().CPlusPlus1y &&
+      Matched->getResultType()->isUndeducedType() &&
+      DeduceReturnType(Matched, ovl->getExprLoc(), Complain))
+    return 0;
+
   return Matched;
 }
 
diff --git a/lib/Sema/SemaStmt.cpp b/lib/Sema/SemaStmt.cpp
index 16cc43a27f..248665ac86 100644
--- a/lib/Sema/SemaStmt.cpp
+++ b/lib/Sema/SemaStmt.cpp
@@ -2482,12 +2482,80 @@ Sema::ActOnCapScopeReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
   return Owned(Result);
 }
 
+/// Deduce the return type for a function from a returned expression, per
+/// C++1y [dcl.spec.auto]p6.
+bool Sema::DeduceFunctionTypeFromReturnExpr(FunctionDecl *FD,
+                                            SourceLocation ReturnLoc,
+                                            Expr *&RetExpr,
+                                            AutoType *AT) {
+  TypeLoc OrigResultType = FD->getTypeSourceInfo()->getTypeLoc().
+    IgnoreParens().castAs<FunctionProtoTypeLoc>().getResultLoc();
+  QualType Deduced;
+
+  if (RetExpr) {
+    //  If the deduction is for a return statement and the initializer is
+    //  a braced-init-list, the program is ill-formed.
+    if (isa<InitListExpr>(RetExpr)) {
+      Diag(RetExpr->getExprLoc(), diag::err_auto_fn_return_init_list);
+      return true;
+    }
+
+    //  Otherwise, [...] deduce a value for U using the rules of template
+    //  argument deduction.
+    DeduceAutoResult DAR = DeduceAutoType(OrigResultType, RetExpr, Deduced);
+
+    if (DAR == DAR_Failed && !FD->isInvalidDecl())
+      Diag(RetExpr->getExprLoc(), diag::err_auto_fn_deduction_failure)
+        << OrigResultType.getType() << RetExpr->getType();
+
+    if (DAR != DAR_Succeeded)
+      return true;
+  } else {
+    //  In the case of a return with no operand, the initializer is considered
+    //  to be void().
+    //
+    // Deduction here can only succeed if the return type is exactly 'cv auto'
+    // or 'decltype(auto)', so just check for that case directly.
+    if (!OrigResultType.getType()->getAs<AutoType>()) {
+      Diag(ReturnLoc, diag::err_auto_fn_return_void_but_not_auto)
+        << OrigResultType.getType();
+      return true;
+    }
+    // We always deduce U = void in this case.
+    Deduced = SubstAutoType(OrigResultType.getType(), Context.VoidTy);
+    if (Deduced.isNull())
+      return true;
+  }
+
+  //  If a function with a declared return type that contains a placeholder type
+  //  has multiple return statements, the return type is deduced for each return
+  //  statement. [...] if the type deduced is not the same in each deduction,
+  //  the program is ill-formed.
+  if (AT->isDeduced() && !FD->isInvalidDecl()) {
+    AutoType *NewAT = Deduced->getContainedAutoType();
+    if (!Context.hasSameType(AT->getDeducedType(), NewAT->getDeducedType())) {
+      Diag(ReturnLoc, diag::err_auto_fn_different_deductions)
+        << (AT->isDecltypeAuto() ? 1 : 0)
+        << NewAT->getDeducedType() << AT->getDeducedType();
+      return true;
+    }
+  } else if (!FD->isInvalidDecl()) {
+    // Update all declarations of the function to have the deduced return type.
+    Context.adjustDeducedFunctionResultType(FD, Deduced);
+  }
+
+  return false;
+}
+
 StmtResult
 Sema::ActOnReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
   // Check for unexpanded parameter packs.
   if (RetValExp && DiagnoseUnexpandedParameterPack(RetValExp))
     return StmtError();
 
+  // FIXME: Unify this and C++1y auto function handling. In particular, we
+  // should allow 'return { 1, 2, 3 };' in a lambda to deduce
+  // 'std::initializer_list<int>'.
   if (isa<CapturingScopeInfo>(getCurFunction()))
     return ActOnCapScopeReturnStmt(ReturnLoc, RetValExp);
 
@@ -2510,6 +2578,23 @@ Sema::ActOnReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
   } else // If we don't have a function/method context, bail.
     return StmtError();
 
+  // FIXME: Add a flag to the ScopeInfo to indicate whether we're performing
+  // deduction.
+  bool HasDependentReturnType = FnRetType->isDependentType();
+  if (getLangOpts().CPlusPlus1y) {
+    if (AutoType *AT = FnRetType->getContainedAutoType()) {
+      FunctionDecl *FD = cast<FunctionDecl>(CurContext);
+      if (CurContext->isDependentContext())
+        HasDependentReturnType = true;
+      else if (DeduceFunctionTypeFromReturnExpr(FD, ReturnLoc, RetValExp, AT)) {
+        FD->setInvalidDecl();
+        return StmtError();
+      } else {
+        FnRetType = FD->getResultType();
+      }
+    }
+  }
+
   ReturnStmt *Result = 0;
   if (FnRetType->isVoidType()) {
     if (RetValExp) {
@@ -2575,7 +2660,7 @@ Sema::ActOnReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
     }
 
     Result = new (Context) ReturnStmt(ReturnLoc, RetValExp, 0);
-  } else if (!RetValExp && !FnRetType->isDependentType()) {
+  } else if (!RetValExp && !HasDependentReturnType) {
     unsigned DiagID = diag::warn_return_missing_expr;  // C90 6.6.6.4p4
     // C99 6.8.6.4p1 (ext_ since GCC warns)
     if (getLangOpts().C99) DiagID = diag::ext_return_missing_expr;
@@ -2586,9 +2671,9 @@ Sema::ActOnReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
       Diag(ReturnLoc, DiagID) << getCurMethodDecl()->getDeclName() << 1/*meth*/;
     Result = new (Context) ReturnStmt(ReturnLoc);
   } else {
-    assert(RetValExp || FnRetType->isDependentType());
+    assert(RetValExp || HasDependentReturnType);
     const VarDecl *NRVOCandidate = 0;
-    if (!FnRetType->isDependentType() && !RetValExp->isTypeDependent()) {
+    if (!HasDependentReturnType && !RetValExp->isTypeDependent()) {
       // we have a non-void function with an expression, continue checking
 
       QualType RetType = (RelatedRetType.isNull() ? FnRetType : RelatedRetType);
diff --git a/lib/Sema/SemaTemplateDeduction.cpp b/lib/Sema/SemaTemplateDeduction.cpp
index bbe34d865b..8efc7a0263 100644
--- a/lib/Sema/SemaTemplateDeduction.cpp
+++ b/lib/Sema/SemaTemplateDeduction.cpp
@@ -2804,21 +2804,25 @@ Sema::FinishTemplateArgumentDeduction(FunctionTemplateDecl *FunctionTemplate,
 
 /// Gets the type of a function for template-argument-deducton
 /// purposes when it's considered as part of an overload set.
-static QualType GetTypeOfFunction(ASTContext &Context,
-                                  const OverloadExpr::FindResult &R,
+static QualType GetTypeOfFunction(Sema &S, const OverloadExpr::FindResult &R,
                                   FunctionDecl *Fn) {
+  // We may need to deduce the return type of the function now.
+  if (S.getLangOpts().CPlusPlus1y && Fn->getResultType()->isUndeducedType() &&
+      S.DeduceReturnType(Fn, R.Expression->getExprLoc(), /*Diagnose*/false))
+    return QualType();
+
   if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn))
     if (Method->isInstance()) {
       // An instance method that's referenced in a form that doesn't
       // look like a member pointer is just invalid.
       if (!R.HasFormOfMemberPointer) return QualType();
 
-      return Context.getMemberPointerType(Fn->getType(),
-               Context.getTypeDeclType(Method->getParent()).getTypePtr());
+      return S.Context.getMemberPointerType(Fn->getType(),
+               S.Context.getTypeDeclType(Method->getParent()).getTypePtr());
     }
 
   if (!R.IsAddressOfOperand) return Fn->getType();
-  return Context.getPointerType(Fn->getType());
+  return S.Context.getPointerType(Fn->getType());
 }
 
 /// Apply the deduction rules for overload sets.
@@ -2852,7 +2856,7 @@ ResolveOverloadForDeduction(Sema &S, TemplateParameterList *TemplateParams,
       // But we can still look for an explicit specialization.
       if (FunctionDecl *ExplicitSpec
             = S.ResolveSingleFunctionTemplateSpecialization(Ovl))
-        return GetTypeOfFunction(S.Context, R, ExplicitSpec);
+        return GetTypeOfFunction(S, R, ExplicitSpec);
     }
 
     return QualType();
@@ -2885,7 +2889,7 @@ ResolveOverloadForDeduction(Sema &S, TemplateParameterList *TemplateParams,
     }
 
     FunctionDecl *Fn = cast<FunctionDecl>(D);
-    QualType ArgType = GetTypeOfFunction(S.Context, R, Fn);
+    QualType ArgType = GetTypeOfFunction(S, R, Fn);
     if (ArgType.isNull()) continue;
 
     // Function-to-pointer conversion.
@@ -3391,6 +3395,15 @@ Sema::DeduceTemplateArguments(FunctionTemplateDecl *FunctionTemplate,
 
   Deduced.resize(TemplateParams->size());
 
+  // If the function has a deduced return type, substitute it for a dependent
+  // type so that we treat it as a non-deduced context in what follows.
+  bool HasUndeducedReturnType = false;
+  if (getLangOpts().CPlusPlus1y && InOverloadResolution &&
+      Function->getResultType()->isUndeducedType()) {
+    FunctionType = SubstAutoType(FunctionType, Context.DependentTy);
+    HasUndeducedReturnType = true;
+  }
+
   if (!ArgFunctionType.isNull()) {
     unsigned TDF = TDF_TopLevelParameterTypeList;
     if (InOverloadResolution) TDF |= TDF_InOverloadResolution;
@@ -3408,6 +3421,13 @@ Sema::DeduceTemplateArguments(FunctionTemplateDecl *FunctionTemplate,
                                           Specialization, Info))
     return Result;
 
+  // If the function has a deduced return type, deduce it now, so we can check
+  // that the deduced function type matches the requested type.
+  if (HasUndeducedReturnType &&
+      Specialization->getResultType()->isUndeducedType() &&
+      DeduceReturnType(Specialization, Info.getLocation(), false))
+    return TDK_MiscellaneousDeductionFailure;
+
   // If the requested function type does not match the actual type of the
   // specialization with respect to arguments of compatible pointer to function
   // types, template argument deduction fails.
@@ -3577,7 +3597,7 @@ namespace {
       //   auto &&lref = lvalue;
       // must transform into "rvalue reference to T" not "rvalue reference to
       // auto type deduced as T" in order for [temp.deduct.call]p3 to apply.
-      if (isa<TemplateTypeParmType>(Replacement)) {
+      if (!Replacement.isNull() && isa<TemplateTypeParmType>(Replacement)) {
         QualType Result = Replacement;
         TemplateTypeParmTypeLoc NewTL =
           TLB.push<TemplateTypeParmTypeLoc>(Result);
@@ -3601,14 +3621,21 @@ namespace {
       return E;
     }
 
-    QualType Apply(TypeSourceInfo *TSI) {
-      if (TypeSourceInfo *Result = TransformType(TSI))
-        return Result->getType();
-      return QualType();
+    QualType Apply(TypeLoc TL) {
+      // Create some scratch storage for the transformed type locations.
+      // FIXME: We're just going to throw this information away. Don't build it.
+      TypeLocBuilder TLB;
+      TLB.reserve(TL.getFullDataSize());
+      return TransformType(TLB, TL);
     }
   };
 }
 
+Sema::DeduceAutoResult
+Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result) {
+  return DeduceAutoType(Type->getTypeLoc(), Init, Result);
+}
+
 /// \brief Deduce the type for an auto type-specifier (C++11 [dcl.spec.auto]p6)
 ///
 /// \param Type the type pattern using the auto type-specifier.
@@ -3616,7 +3643,7 @@ namespace {
 /// \param Result if type deduction was successful, this will be set to the
 ///        deduced type.
 Sema::DeduceAutoResult
-Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result) {
+Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result) {
   if (Init->getType()->isNonOverloadPlaceholderType()) {
     ExprResult NonPlaceholder = CheckPlaceholderExpr(Init);
     if (NonPlaceholder.isInvalid())
@@ -3624,15 +3651,16 @@ Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result) {
     Init = NonPlaceholder.take();
   }
 
-  if (Init->isTypeDependent() || Type->getType()->isDependentType()) {
+  if (Init->isTypeDependent() || Type.getType()->isDependentType()) {
     Result = SubstituteAutoTransform(*this, Context.DependentTy).Apply(Type);
+    assert(!Result.isNull() && "substituting DependentTy can't fail");
     return DAR_Succeeded;
   }
 
   // If this is a 'decltype(auto)' specifier, do the decltype dance.
   // Since 'decltype(auto)' can only occur at the top of the type, we
   // don't need to go digging for it.
-  if (const AutoType *AT = Type->getType()->getAs<AutoType>()) {
+  if (const AutoType *AT = Type.getType()->getAs<AutoType>()) {
     if (AT->isDecltypeAuto()) {
       if (isa<InitListExpr>(Init)) {
         Diag(Init->getLocStart(), diag::err_decltype_auto_initializer_list);
@@ -3643,6 +3671,8 @@ Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result) {
       // FIXME: Support a non-canonical deduced type for 'auto'.
       Deduced = Context.getCanonicalType(Deduced);
       Result = SubstituteAutoTransform(*this, Deduced).Apply(Type);
+      if (Result.isNull())
+        return DAR_FailedAlreadyDiagnosed;
       return DAR_Succeeded;
     }
   }
@@ -3704,6 +3734,8 @@ Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result) {
   }
 
   Result = SubstituteAutoTransform(*this, DeducedType).Apply(Type);
+  if (Result.isNull())
+   return DAR_FailedAlreadyDiagnosed;
 
   // Check that the deduced argument type is compatible with the original
   // argument type per C++ [temp.deduct.call]p4.
@@ -3733,6 +3765,22 @@ void Sema::DiagnoseAutoDeductionFailure(VarDecl *VDecl, Expr *Init) {
       << Init->getSourceRange();
 }
 
+bool Sema::DeduceReturnType(FunctionDecl *FD, SourceLocation Loc,
+                            bool Diagnose) {
+  assert(FD->getResultType()->isUndeducedType());
+
+  if (FD->getTemplateInstantiationPattern())
+    InstantiateFunctionDefinition(Loc, FD);
+
+  bool StillUndeduced = FD->getResultType()->isUndeducedType();
+  if (StillUndeduced && Diagnose && !FD->isInvalidDecl()) {
+    Diag(Loc, diag::err_auto_fn_used_before_defined) << FD;
+    Diag(FD->getLocation(), diag::note_callee_decl) << FD;
+  }
+
+  return StillUndeduced;
+}
+
 static void
 MarkUsedTemplateParameters(ASTContext &Ctx, QualType T,
                            bool OnlyDeduced,
diff --git a/lib/Sema/SemaTemplateInstantiateDecl.cpp b/lib/Sema/SemaTemplateInstantiateDecl.cpp
index ee471696bd..688a7360ad 100644
--- a/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2868,13 +2868,15 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
     return;
   }
 
-  // C++0x [temp.explicit]p9:
-  //   Except for inline functions, other explicit instantiation declarations
-  //   have the effect of suppressing the implicit instantiation of the entity
-  //   to which they refer.
+  // C++1y [temp.explicit]p10:
+  //   Except for inline functions, declarations with types deduced from their
+  //   initializer or return value, and class template specializations, other
+  //   explicit instantiation declarations have the effect of suppressing the
+  //   implicit instantiation of the entity to which they refer.
   if (Function->getTemplateSpecializationKind()
         == TSK_ExplicitInstantiationDeclaration &&
-      !PatternDecl->isInlined())
+      !PatternDecl->isInlined() &&
+      !PatternDecl->getResultType()->isUndeducedType())
     return;
 
   if (PatternDecl->isInlined())
diff --git a/lib/Sema/SemaType.cpp b/lib/Sema/SemaType.cpp
index 4bf15f0360..0959f7d66a 100644
--- a/lib/Sema/SemaType.cpp
+++ b/lib/Sema/SemaType.cpp
@@ -2018,6 +2018,8 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
   // The TagDecl owned by the DeclSpec.
   TagDecl *OwnedTagDecl = 0;
 
+  bool ContainsPlaceholderType = false;
+
   switch (D.getName().getKind()) {
   case UnqualifiedId::IK_ImplicitSelfParam:
   case UnqualifiedId::IK_OperatorFunctionId:
@@ -2025,6 +2027,7 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
   case UnqualifiedId::IK_LiteralOperatorId:
   case UnqualifiedId::IK_TemplateId:
     T = ConvertDeclSpecToType(state);
+    ContainsPlaceholderType = D.getDeclSpec().containsPlaceholderType();
 
     if (!D.isInvalidType() && D.getDeclSpec().isTypeSpecOwned()) {
       OwnedTagDecl = cast<TagDecl>(D.getDeclSpec().getRepAsDecl());
@@ -2048,6 +2051,7 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
     // converts to.
     T = SemaRef.GetTypeFromParser(D.getName().ConversionFunctionId,
                                   &ReturnTypeInfo);
+    ContainsPlaceholderType = T->getContainedAutoType();
     break;
   }
 
@@ -2058,7 +2062,7 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
   // In C++11, a function declarator using 'auto' must have a trailing return
   // type (this is checked later) and we can skip this. In other languages
   // using auto, we need to check regardless.
-  if (D.getDeclSpec().containsPlaceholderType() &&
+  if (ContainsPlaceholderType &&
       (!SemaRef.getLangOpts().CPlusPlus11 || !D.isFunctionDeclarator())) {
     int Error = -1;
 
@@ -2101,10 +2105,12 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
       Error = 10; // Type alias
       break;
     case Declarator::TrailingReturnContext:
-      Error = 11; // Function return type
+      if (!SemaRef.getLangOpts().CPlusPlus1y)
+        Error = 11; // Function return type
       break;
     case Declarator::ConversionIdContext:
-      Error = 12; // conversion-type-id
+      if (!SemaRef.getLangOpts().CPlusPlus1y)
+        Error = 12; // conversion-type-id
       break;
     case Declarator::TypeNameContext:
       Error = 13; // Generic
@@ -2599,7 +2605,8 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
         // trailing-return-type is only required if we're declaring a function,
         // and not, for instance, a pointer to a function.
         if (D.getDeclSpec().getTypeSpecType() == DeclSpec::TST_auto &&
-            !FTI.hasTrailingReturnType() && chunkIndex == 0) {
+            !FTI.hasTrailingReturnType() && chunkIndex == 0 &&
+            !S.getLangOpts().CPlusPlus1y) {
           S.Diag(D.getDeclSpec().getTypeSpecTypeLoc(),
                diag::err_auto_missing_trailing_return);
           T = Context.IntTy;