Improve the error message when a function overload candidate is rejected

because it expects a reference and receives a non-l-value.

For example, given:

  int foo(int &);
  template<int x> void b() { foo(x); }

clang will now print "expects an l-value for 1st argument" instead of
"no known conversion from 'int' to 'int &' for 1st argument". The change
in wording (and associated code to detect the case) was prompted by
comment #5 in PR3104, and should be the last bit of work needed for the
bug.

llvm-svn: 158691

1 files changed, 20 insertions, 5 deletions

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index f73c5c1f35d..6e62af5bfe4 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -8153,12 +8153,27 @@ void DiagnoseBadConversion(Sema &S, OverloadCandidate *Cand, unsigned I) {
               FromIface->isSuperClassOf(ToIface))
             BaseToDerivedConversion = 2;
   } else if (const ReferenceType *ToRefTy = ToTy->getAs<ReferenceType>()) {
-      if (ToRefTy->getPointeeType().isAtLeastAsQualifiedAs(FromTy) &&
-          !FromTy->isIncompleteType() &&
-          !ToRefTy->getPointeeType()->isIncompleteType() &&
-          S.IsDerivedFrom(ToRefTy->getPointeeType(), FromTy))
-        BaseToDerivedConversion = 3;
+    if (ToRefTy->getPointeeType().isAtLeastAsQualifiedAs(FromTy) &&
+        !FromTy->isIncompleteType() &&
+        !ToRefTy->getPointeeType()->isIncompleteType() &&
+        S.IsDerivedFrom(ToRefTy->getPointeeType(), FromTy)) {
+      BaseToDerivedConversion = 3;
+    } else if (ToTy->isLValueReferenceType() && !FromExpr->isLValue() &&
+               ToTy.getNonReferenceType().getCanonicalType() ==
+               FromTy.getNonReferenceType().getCanonicalType()) {
+      QualType T1 = ToTy.getCanonicalType();
+      QualType T2 = ToTy.getNonReferenceType();
+      QualType T3 = T2.getUnqualifiedType();
+      QualType T4 = FromTy.getCanonicalType();
+      (void)T1; (void)T2; (void)T3; (void)T4;
+      S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_lvalue)
+        << (unsigned) FnKind << FnDesc
+        << (FromExpr ? FromExpr->getSourceRange() : SourceRange())
+        << (unsigned) isObjectArgument << I + 1;
+      MaybeEmitInheritedConstructorNote(S, Fn);
+      return;
     }
+  }
 
   if (BaseToDerivedConversion) {
     S.Diag(Fn->getLocation(),