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

1 files changed, 9 insertions, 0 deletions

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);