Customize the SFINAE diagnostics for enable_if to provide the failed condition.

When enable_if disables a particular overload resolution candidate,
rummage through the enable_if condition to find the specific condition
that caused the failure. For example, if we have something like:

    template<
      typename Iter,
      typename = std::enable_if_t<Random_access_iterator<Iter> &&
                                  Comparable<Iterator_value_type<Iter>>>>
    void mysort(Iter first, Iter last) {}

and we call "mysort" with "std::list<int>" iterators, we'll get a
diagnostic saying that the "Random_access_iterator<Iter>" requirement
failed. If we call "mysort" with
"std::vector<something_not_comparable>", we'll get a diagnostic saying
that the "Comparable<...>" requirement failed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307196 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/DiagnosticSemaKinds.td

@@ -3518,6 +3518,8 @@ def note_ovl_candidate_substitution_failure : Note<
     "candidate template ignored: substitution failure%0%1">;
 def note_ovl_candidate_disabled_by_enable_if : Note<
     "candidate template ignored: disabled by %0%1">;
+def note_ovl_candidate_disabled_by_requirement : Note<
+    "candidate template ignored: requirement '%0' was not satisfied%1">;
 def note_ovl_candidate_has_pass_object_size_params: Note<
     "candidate address cannot be taken because parameter %0 has "
     "pass_object_size attribute">;
@@ -4431,6 +4433,9 @@ def err_typename_nested_not_found : Error<"no type named %0 in %1">;
 def err_typename_nested_not_found_enable_if : Error<
   "no type named 'type' in %0; 'enable_if' cannot be used to disable "
   "this declaration">;
+def err_typename_nested_not_found_requirement : Error<
+  "failed requirement '%0'; 'enable_if' cannot be used to disable this "
+  "declaration">;
 def err_typename_nested_not_type : Error<
     "typename specifier refers to non-type member %0 in %1">;
 def note_typename_refers_here : Note<

include/clang/Basic/PartialDiagnostic.h

@@ -329,6 +329,15 @@ public:
 
   bool hasStorage() const { return DiagStorage != nullptr; }
 
+  /// Retrieve the string argument at the given index.
+  StringRef getStringArg(unsigned I) {
+    assert(DiagStorage && "No diagnostic storage?");
+    assert(I < DiagStorage->NumDiagArgs && "Not enough diagnostic args");
+    assert(DiagStorage->DiagArgumentsKind[I]
+             == DiagnosticsEngine::ak_std_string && "Not a string arg");
+    return DiagStorage->DiagArgumentsStr[I];
+  }
+
   friend const PartialDiagnostic &operator<<(const PartialDiagnostic &PD,
                                              unsigned I) {
     PD.AddTaggedVal(I, DiagnosticsEngine::ak_uint);

include/clang/Sema/TemplateDeduction.h

@@ -88,6 +88,12 @@ public:
     HasSFINAEDiagnostic = false;
   }
 
+  /// Peek at the SFINAE diagnostic.
+  const PartialDiagnosticAt &peekSFINAEDiagnostic() const {
+    assert(HasSFINAEDiagnostic);
+    return SuppressedDiagnostics.front();
+  }
+
   /// \brief Provide a new template argument list that contains the
   /// results of template argument deduction.
   void reset(TemplateArgumentList *NewDeduced) {

lib/Sema/SemaOverload.cpp

@@ -9830,6 +9830,15 @@ static void DiagnoseBadDeduction(Sema &S, NamedDecl *Found, Decl *Templated,
       return;
     }
 
+    // We found a specific requirement that disabled the enable_if.
+    if (PDiag && PDiag->second.getDiagID() ==
+        diag::err_typename_nested_not_found_requirement) {
+      S.Diag(Templated->getLocation(),
+             diag::note_ovl_candidate_disabled_by_requirement)
+        << PDiag->second.getStringArg(0) << TemplateArgString;
+      return;
+    }
+
     // Format the SFINAE diagnostic into the argument string.
     // FIXME: Add a general mechanism to include a PartialDiagnostic *'s
     //        formatted message in another diagnostic.

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,
+    TemplateArgumentList StackTemplateArgs(TemplateArgumentList::OnStack,
-                                      Converted);
+                                           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();
     }
 

test/SemaTemplate/constexpr-instantiate.cpp

@@ -191,7 +191,7 @@ namespace Unevaluated {
       static constexpr bool f() { return sizeof(T) < U::size; }
 
       template<typename U>
-      static typename enable_if<f<U>(), void>::type g() {} // expected-note {{disabled by 'enable_if'}}
+      static typename enable_if<f<U>(), void>::type g() {} // expected-note {{requirement 'f<Unevaluated::PR13423::U>()' was not satisfied}}
     };
 
     struct U { static constexpr int size = 2; };

test/SemaTemplate/overload-candidates.cpp

@@ -47,7 +47,7 @@ namespace boost {
   template<bool, typename = void> struct enable_if {};
   template<typename T> struct enable_if<true, T> { typedef T type; };
 }
-template<typename T> typename boost::enable_if<sizeof(T) == 4, int>::type if_size_4(); // expected-note{{candidate template ignored: disabled by 'enable_if' [with T = char]}}
+template<typename T> typename boost::enable_if<sizeof(T) == 4, int>::type if_size_4(); // expected-note{{candidate template ignored: requirement 'sizeof(char) == 4' was not satisfied [with T = char]}}
 int k = if_size_4<char>(); // expected-error{{no matching function}}
 
 namespace llvm {
@@ -61,7 +61,7 @@ void test_if_int() {
 }
 
 template<typename T> struct NonTemplateFunction {
-  typename boost::enable_if<sizeof(T) == 4, int>::type f(); // expected-error{{no type named 'type' in 'boost::enable_if<false, int>'; 'enable_if' cannot be used to disable this declaration}}
+  typename boost::enable_if<sizeof(T) == 4, int>::type f(); // expected-error{{failed requirement 'sizeof(char) == 4'; 'enable_if' cannot be used to disable this declaration}}
 };
 NonTemplateFunction<char> NTFC; // expected-note{{here}}
 
@@ -100,7 +100,7 @@ namespace PR15673 {
 #if __cplusplus <= 199711L
   // expected-warning@-2 {{default template arguments for a function template are a C++11 extension}}
 #endif
-  // expected-note@-4 {{candidate template ignored: disabled by 'enable_if' [with T = int]}}
+  // expected-note@+1 {{candidate template ignored: requirement 'a_trait<int>::value' was not satisfied [with T = int]}}
   void foo() {}
   void bar() { foo<int>(); } // expected-error {{no matching function for call to 'foo'}}
 
@@ -128,7 +128,7 @@ namespace PR15673 {
 #if __cplusplus <= 199711L
   // expected-warning@-2 {{alias declarations are a C++11 extension}}
 #endif
-  // expected-note@-4 {{candidate template ignored: disabled by 'enable_if' [with T = int]}}
+  // expected-note@+7 {{candidate template ignored: requirement 'some_trait<int>::value' was not satisfied [with T = int]}}
 
   template<typename T,
            typename Requires = unicorns<T> >