[c++1z] Disallow deduction guides with deduced types that don't syntactically match the template being deduced.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294773 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 37 insertions, 3 deletions

diff --git a/lib/Sema/SemaDeclCXX.cpp b/lib/Sema/SemaDeclCXX.cpp
index 0acf91d3d0..c12c9b27bb 100644
--- a/lib/Sema/SemaDeclCXX.cpp
+++ b/lib/Sema/SemaDeclCXX.cpp
@@ -8192,12 +8192,46 @@ void Sema::CheckDeductionGuideDeclarator(Declarator &D, QualType &R,
            diag::err_deduction_guide_no_trailing_return_type);
       break;
     }
+
+    // Check that the return type is written as a specialization of
+    // the template specified as the deduction-guide's name.
+    ParsedType TrailingReturnType = Chunk.Fun.getTrailingReturnType();
+    TemplateName GuidedTemplate = D.getName().TemplateName.get().get();
+    TypeSourceInfo *TSI = nullptr;
+    QualType RetTy = GetTypeFromParser(TrailingReturnType, &TSI);
+    assert(TSI && "deduction guide has valid type but invalid return type?");
+    bool AcceptableReturnType = false;
+    bool MightInstantiateToSpecialization = false;
+    if (auto RetTST =
+            TSI->getTypeLoc().getAs<TemplateSpecializationTypeLoc>()) {
+      TemplateName SpecifiedName = RetTST.getTypePtr()->getTemplateName();
+      bool TemplateMatches =
+          Context.hasSameTemplateName(SpecifiedName, GuidedTemplate);
+      if (SpecifiedName.getKind() == TemplateName::Template && TemplateMatches)
+        AcceptableReturnType = true;
+      else {
+        // This could still instantiate to the right type, unless we know it
+        // names the wrong class template.
+        auto *TD = SpecifiedName.getAsTemplateDecl();
+        MightInstantiateToSpecialization = !(TD && isa<ClassTemplateDecl>(TD) &&
+                                             !TemplateMatches);
+      }
+    } else if (!RetTy.hasQualifiers() && RetTy->isDependentType()) {
+      MightInstantiateToSpecialization = true;
+    }
+
+    if (!AcceptableReturnType) {
+      Diag(TSI->getTypeLoc().getLocStart(),
+           diag::err_deduction_guide_bad_trailing_return_type)
+        << GuidedTemplate << TSI->getType() << MightInstantiateToSpecialization
+        << TSI->getTypeLoc().getSourceRange();
+    }
+
+    // Keep going to check that we don't have any inner declarator pieces (we
+    // could still have a function returning a pointer to a function).
     FoundFunction = true;
   }
 
-  // FIXME: Check that the return type can instantiate to a specialization of
-  // the template specified as the deduction-guide's name.
-
   if (D.isFunctionDefinition())
     Diag(D.getIdentifierLoc(), diag::err_deduction_guide_defines_function);
 }