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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@280838 91177308-0d34-0410-b5e6-96231b3b80d8

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,
+      CCE == Sema::CCEK_ConstexprIf
-                          /*SuppressUserConversions=*/false,
+          ? TryContextuallyConvertToBool(S, From)
-                          /*InOverloadResolution=*/false,
+          : TryCopyInitialization(S, From, T,
-                          /*AllowObjcWritebackConversion=*/false,
+                                  /*SuppressUserConversions=*/false,
-                          /*AllowExplicit=*/false);
+                                  /*InOverloadResolution=*/false,
+                                  /*AllowObjcWritebackConversion=*/false,
+                                  /*AllowExplicit=*/false);
   StandardConversionSequence *SCS = nullptr;
   switch (ICS.getKind()) {
   case ImplicitConversionSequence::StandardConversion:

test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp

@@ -46,6 +46,11 @@ namespace ccce {
     if constexpr (N) {} // expected-error {{cannot be narrowed}}
   }
   template void g<5>(); // expected-note {{instantiation of}}
+  void h() {
+    if constexpr (4.3) {} // expected-error{{conversion from 'double' to 'bool' is not allowed in a converted constant expression}}
+    constexpr void *p = nullptr;
+    if constexpr (p) {} // expected-error{{conversion from 'void *const' to 'bool' is not allowed in a converted constant expression}}
+  }
 }
 
 namespace generic_lambda {

test/CodeGenCXX/cxx1z-constexpr-if.cpp

@@ -2,7 +2,15 @@
 
 void should_be_used_1();
 void should_be_used_2();
+void should_be_used_3();
 void should_not_be_used();
+
+struct A {
+  constexpr explicit operator bool() const {
+    return true;
+  }
+};
+
 void f() {
   if constexpr (false)
     should_not_be_used();
@@ -15,7 +23,12 @@ void f() {
     goto foo;
 foo: should_not_be_used();
   }
+  if constexpr (A())
+    should_be_used_3();
+  else
+    should_not_be_used();
 }
 
 // CHECK: should_be_used_1
 // CHECK: should_be_used_2
+// CHECK: should_be_used_3