Fix PR15634, better error message for template deduction failure.

When two template decls with the same name are used in this diagnostic,
force them to print their qualified names.  This changes the bad message of:

candidate template ignored: could not match 'array' against 'array'

to the better message of:

candidate template ignored: could not match 'NS2::array' against 'NS1::array'

llvm-svn: 179056

1 files changed, 26 insertions, 4 deletions

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 89d495ddc19..805fdd8b578 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -8508,13 +8508,35 @@ void DiagnoseBadDeduction(Sema &S, OverloadCandidate *Cand,
     return;
   }
 
-  case Sema::TDK_NonDeducedMismatch:
+  case Sema::TDK_NonDeducedMismatch: {
     // FIXME: Provide a source location to indicate what we couldn't match.
+    TemplateArgument FirstTA = *Cand->DeductionFailure.getFirstArg();
+    TemplateArgument SecondTA = *Cand->DeductionFailure.getSecondArg();
+    if (FirstTA.getKind() == TemplateArgument::Template &&
+        SecondTA.getKind() == TemplateArgument::Template) {
+      TemplateName FirstTN = FirstTA.getAsTemplate();
+      TemplateName SecondTN = SecondTA.getAsTemplate();
+      if (FirstTN.getKind() == TemplateName::Template &&
+          SecondTN.getKind() == TemplateName::Template) {
+        if (FirstTN.getAsTemplateDecl()->getName() ==
+            SecondTN.getAsTemplateDecl()->getName()) {
+          // FIXME: This fixes a bad diagnostic where both templates are named
+          // the same.  This particular case is a bit difficult since:
+          // 1) It is passed as a string to the diagnostic printer.
+          // 2) The diagnostic printer only attempts to find a better
+          //    name for types, not decls.
+          // Ideally, this should folded into the diagnostic printer.
+          S.Diag(Fn->getLocation(),
+                 diag::note_ovl_candidate_non_deduced_mismatch_qualified)
+              << FirstTN.getAsTemplateDecl() << SecondTN.getAsTemplateDecl();
+          return;
+        }
+      }
+    }
     S.Diag(Fn->getLocation(), diag::note_ovl_candidate_non_deduced_mismatch)
-      << *Cand->DeductionFailure.getFirstArg()
-      << *Cand->DeductionFailure.getSecondArg();
+      << FirstTA << SecondTA;
     return;
-
+  }
   // TODO: diagnose these individually, then kill off
   // note_ovl_candidate_bad_deduction, which is uselessly vague.
   case Sema::TDK_MiscellaneousDeductionFailure: