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

3 files changed, 20 insertions, 0 deletions

diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index 1572127200..e63f45c722 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/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<
diff --git a/include/clang/Basic/PartialDiagnostic.h b/include/clang/Basic/PartialDiagnostic.h
index 53ce95cab1..b2f14afe56 100644
--- a/include/clang/Basic/PartialDiagnostic.h
+++ b/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);
diff --git a/include/clang/Sema/TemplateDeduction.h b/include/clang/Sema/TemplateDeduction.h
index d92cbab4fb..cd9ed6abfa 100644
--- a/include/clang/Sema/TemplateDeduction.h
+++ b/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) {