[Sema] Fix PR30664

[arphaman]

Summary

This patch fixes an assertion failure crash that happens when a constant record reference member is initialized using an empty initializer list.

[rsmith]

lib/Sema/SemaExprCXX.cpp

3429–3430

I think you mean "if the constructor has no parameters", right?

3433

It looks like this will result in us building an expression like T(void({})). That would be invalid, and we shouldn't have it in the AST. Instead, we should probably build T{} for this case.

test/SemaCXX/cxx11-crashes.cpp

109

This case is ill-formed under http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1696

Is there any way to tickle this problem without binding a class member to a temporary? If not, I'd prefer we instead implement DR1696 to eliminate the problem.

lib/Sema/SemaExprCXX.cpp

case ImplicitConversionSequence::UserDefinedConversion: {
         BeforeToType = Context.getTagDeclType(Conv->getParent());
       } else {
         const CXXConstructorDecl *Ctor = cast<CXXConstructorDecl>(FD);
         CastKind = CK_ConstructorConversion;
         // Do no conversion if dealing with ... for the first conversion.
         if (!ICS.UserDefined.EllipsisConversion) {
           // If the user-defined conversion is specified by a constructor, the
           // initial standard conversion sequence converts the source type to
-          // the type required by the argument of the constructor
-          BeforeToType = Ctor->getParamDecl(0)->getType().getNonReferenceType();
+          // the type required by the argument of the constructor or the 'void'
+          // type if the argument has no constructors.
+          BeforeToType =
+              Ctor->param_empty()
+                  ? Context.VoidTy
+                  : Ctor->getParamDecl(0)->getType().getNonReferenceType();
         }
       }
       // Watch out for ellipsis conversion.
       if (!ICS.UserDefined.EllipsisConversion) {
         ExprResult Res =
           PerformImplicitConversion(From, BeforeToType,
                                     ICS.UserDefined.Before, AA_Converting,
                                     CCK);

test/SemaCXX/cxx11-crashes.cpp

 void Foo(S);
 
 void test(int some_number) {  // expected-note {{'some_number' declared here}}
   auto x = sum_number;  // expected-error {{use of undeclared identifier 'sum_number'; did you mean 'some_number'?}}
   auto lambda = [x] {};
   Foo(lambda);
 }
 }
+
+namespace pr30664 {
+struct foo {
+  int x;
+  foo() : x(42) { }
+};
+
+struct bar {
+  const foo &o;  // expected-note {{reference member declared here}}
+  bar() : o{} {} // expected-warning {{binding reference member 'o' to a temporary value}}
+};
+
+struct outer {
+  struct inner {
+    const outer &o;    // expected-note {{reference member declared here}}
+    inner() : o({}) {} // expected-warning {{binding reference member 'o' to a temporary value}}
+  };
+};
+
+outer::inner i;
+
+struct foobar {
+  foobar(void) { }
+  struct inner {
+    int y;
+    const foobar &o; // expected-note {{reference member declared here}}
+    inner() : y(21), o({}) {} // expected-warning {{binding reference member 'o' to a temporary value}}
+  };
+};
+}