Try contextually converting condition of constexpr if to Boolean value

Summary:
C++1z 6.4.1/p2:
 If the if statement is of the form if constexpr, the value of the
 condition shall be a contextually converted constant expression of type
 bool [...]
C++1z 5.20/p4:
 [...] A contextually converted constant expression of type bool is an
 expression, contextually converted to bool (Clause4), where the
 converted expression is a constant expression and the conversion
 sequence contains only the conversions above. [...]

Contextually converting result of an expression `e` to a Boolean value
requires `bool t(e)` to be well-formed.

An explicit conversion function is only considered as a user-defined
conversion for direct-initialization, which is essentially what
//contextually converted to bool// requires.

Also, fixes PR28470.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D24158

llvm-svn: 280838

1 files changed, 11 insertions, 5 deletions

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 37552b57d66..183fdf4272e 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -5164,12 +5164,18 @@ static ExprResult CheckConvertedConstantExpression(Sema &S, Expr *From,
   //  implicitly converted to type T, where the converted
   //  expression is a constant expression and the implicit conversion
   //  sequence contains only [... list of conversions ...].
+  // C++1z [stmt.if]p2:
+  //  If the if statement is of the form if constexpr, the value of the
+  //  condition shall be a contextually converted constant expression of type
+  //  bool.
   ImplicitConversionSequence ICS =
-    TryCopyInitialization(S, From, T,
-                          /*SuppressUserConversions=*/false,
-                          /*InOverloadResolution=*/false,
-                          /*AllowObjcWritebackConversion=*/false,
-                          /*AllowExplicit=*/false);
+      CCE == Sema::CCEK_ConstexprIf
+          ? TryContextuallyConvertToBool(S, From)
+          : TryCopyInitialization(S, From, T,
+                                  /*SuppressUserConversions=*/false,
+                                  /*InOverloadResolution=*/false,
+                                  /*AllowObjcWritebackConversion=*/false,
+                                  /*AllowExplicit=*/false);
   StandardConversionSequence *SCS = nullptr;
   switch (ICS.getKind()) {
   case ImplicitConversionSequence::StandardConversion: