1 files changed, 131 insertions, 7 deletions

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index a8923ce9e27..58614322f76 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -2806,6 +2806,67 @@ checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
   llvm_unreachable("unexpected BuiltinTemplateDecl!");
 }
 
+/// Determine whether this alias template is "enable_if_t".
+static bool isEnableIfAliasTemplate(TypeAliasTemplateDecl *AliasTemplate) {
+  return AliasTemplate->getName().equals("enable_if_t");
+}
+
+/// Collect all of the separable terms in the given condition, which
+/// might be a conjunction.
+///
+/// FIXME: The right answer is to convert the logical expression into
+/// disjunctive normal form, so we can find the first failed term
+/// within each possible clause.
+static void collectConjunctionTerms(Expr *Clause,
+                                    SmallVectorImpl<Expr *> &Terms) {
+  if (auto BinOp = dyn_cast<BinaryOperator>(Clause->IgnoreParenImpCasts())) {
+    if (BinOp->getOpcode() == BO_LAnd) {
+      collectConjunctionTerms(BinOp->getLHS(), Terms);
+      collectConjunctionTerms(BinOp->getRHS(), Terms);
+    }
+
+    return;
+  }
+
+  Terms.push_back(Clause);
+}
+
+/// Find the failed subexpression within enable_if, and describe it
+/// with a string.
+static std::pair<Expr *, std::string>
+findFailedEnableIfCondition(Sema &S, Expr *Cond) {
+  // Separate out all of the terms in a conjunction.
+  SmallVector<Expr *, 4> Terms;
+  collectConjunctionTerms(Cond, Terms);
+
+  // Determine which term failed.
+  Expr *FailedCond = nullptr;
+  for (Expr *Term : Terms) {
+    // The initialization of the parameter from the argument is
+    // a constant-evaluated context.
+    EnterExpressionEvaluationContext ConstantEvaluated(
+      S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+
+    bool Succeeded;
+    if (Term->EvaluateAsBooleanCondition(Succeeded, S.Context) &&
+        !Succeeded) {
+      FailedCond = Term->IgnoreParenImpCasts();
+      break;
+    }
+  }
+
+  if (!FailedCond)
+    FailedCond = Cond->IgnoreParenImpCasts();
+
+  std::string Description;
+  {
+    llvm::raw_string_ostream Out(Description);
+    FailedCond->printPretty(Out, nullptr,
+                            PrintingPolicy(S.Context.getLangOpts()));
+  }
+  return { FailedCond, Description };
+}
+
 QualType Sema::CheckTemplateIdType(TemplateName Name,
                                    SourceLocation TemplateLoc,
                                    TemplateArgumentListInfo &TemplateArgs) {
@@ -2852,12 +2913,12 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     if (Pattern->isInvalidDecl())
       return QualType();
 
-    TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack,
-                                      Converted);
+    TemplateArgumentList StackTemplateArgs(TemplateArgumentList::OnStack,
+                                           Converted);
 
     // Only substitute for the innermost template argument list.
     MultiLevelTemplateArgumentList TemplateArgLists;
-    TemplateArgLists.addOuterTemplateArguments(&TemplateArgs);
+    TemplateArgLists.addOuterTemplateArguments(&StackTemplateArgs);
     unsigned Depth = AliasTemplate->getTemplateParameters()->getDepth();
     for (unsigned I = 0; I < Depth; ++I)
       TemplateArgLists.addOuterTemplateArguments(None);
@@ -2870,8 +2931,42 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     CanonType = SubstType(Pattern->getUnderlyingType(),
                           TemplateArgLists, AliasTemplate->getLocation(),
                           AliasTemplate->getDeclName());
-    if (CanonType.isNull())
+    if (CanonType.isNull()) {
+      // If this was enable_if and we failed to find the nested type
+      // within enable_if in a SFINAE context, dig out the specific
+      // enable_if condition that failed and present that instead.
+      if (isEnableIfAliasTemplate(AliasTemplate)) {
+        if (auto DeductionInfo = isSFINAEContext()) {
+          if (*DeductionInfo &&
+              (*DeductionInfo)->hasSFINAEDiagnostic() &&
+              (*DeductionInfo)->peekSFINAEDiagnostic().second.getDiagID() ==
+                diag::err_typename_nested_not_found_enable_if &&
+              TemplateArgs[0].getArgument().getKind()
+                == TemplateArgument::Expression) {
+            Expr *FailedCond;
+            std::string FailedDescription;
+            std::tie(FailedCond, FailedDescription) =
+              findFailedEnableIfCondition(
+                *this, TemplateArgs[0].getSourceExpression());
+
+            // Remove the old SFINAE diagnostic.
+            PartialDiagnosticAt OldDiag =
+              {SourceLocation(), PartialDiagnostic::NullDiagnostic()};
+            (*DeductionInfo)->takeSFINAEDiagnostic(OldDiag);
+
+            // Add a new SFINAE diagnostic specifying which condition
+            // failed.
+            (*DeductionInfo)->addSFINAEDiagnostic(
+              OldDiag.first,
+              PDiag(diag::err_typename_nested_not_found_requirement)
+                << FailedDescription
+                << FailedCond->getSourceRange());
+          }
+        }
+      }
+
       return QualType();
+    }
   } else if (Name.isDependent() ||
              TemplateSpecializationType::anyDependentTemplateArguments(
                TemplateArgs, InstantiationDependent)) {
@@ -9290,7 +9385,7 @@ Sema::ActOnTypenameType(Scope *S,
 /// Determine whether this failed name lookup should be treated as being
 /// disabled by a usage of std::enable_if.
 static bool isEnableIf(NestedNameSpecifierLoc NNS, const IdentifierInfo &II,
-                       SourceRange &CondRange) {
+                       SourceRange &CondRange, Expr *&Cond) {
   // We must be looking for a ::type...
   if (!II.isStr("type"))
     return false;
@@ -9320,6 +9415,19 @@ static bool isEnableIf(NestedNameSpecifierLoc NNS, const IdentifierInfo &II,
 
   // Assume the first template argument is the condition.
   CondRange = EnableIfTSTLoc.getArgLoc(0).getSourceRange();
+
+  // Dig out the condition.
+  Cond = nullptr;
+  if (EnableIfTSTLoc.getArgLoc(0).getArgument().getKind()
+        != TemplateArgument::Expression)
+    return true;
+
+  Cond = EnableIfTSTLoc.getArgLoc(0).getSourceExpression();
+
+  // Ignore Boolean literals; they add no value.
+  if (isa<CXXBoolLiteralExpr>(Cond->IgnoreParenCasts()))
+    Cond = nullptr;
+
   return true;
 }
 
@@ -9363,9 +9471,25 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
     // If we're looking up 'type' within a template named 'enable_if', produce
     // a more specific diagnostic.
     SourceRange CondRange;
-    if (isEnableIf(QualifierLoc, II, CondRange)) {
+    Expr *Cond = nullptr;
+    if (isEnableIf(QualifierLoc, II, CondRange, Cond)) {
+      // If we have a condition, narrow it down to the specific failed
+      // condition.
+      if (Cond) {
+        Expr *FailedCond;
+        std::string FailedDescription;
+        std::tie(FailedCond, FailedDescription) =
+          findFailedEnableIfCondition(*this, Cond);
+
+        Diag(FailedCond->getExprLoc(),
+             diag::err_typename_nested_not_found_requirement)
+          << FailedDescription
+          << FailedCond->getSourceRange();
+        return QualType();
+      }
+
       Diag(CondRange.getBegin(), diag::err_typename_nested_not_found_enable_if)
-        << Ctx << CondRange;
+          << Ctx << CondRange;
       return QualType();
     }