[C++11] Fix warning when dropping cv-qualifiers when assigning to a reference with a braced initializer list

[Rakete1111]

Summary

This fixes PR34912 :)

[Rakete1111]

Rebased + friendly 2018 ping :)

[rsmith]

lib/Sema/SemaInit.cpp

7691–7695

Same problem exists here, and probably in a lot of these diagnostics. For example:

*int a; int &&b = {a}; says "cannot bind to lvalue of type 'void'")

• void f(int); void f() { int &&b = {f}; } asserts due to not unwrapping the InitListExpr before trying to diagnose

Can you try to address this more generally? Perhaps: add an Expr *OnlyArg, which is null if Args.size() != 1, is the list element if Args.size() == 1 and is an InitListExpr with a single element, and is otherwise Args[0], and change all the diagnostics that are talking about a one-and-only argument to use OnlyArg?

[Rakete1111]

Addressed review comments :)
I added a test case for the crashing code.

Note: I didn't change Args[0] to OnlyArg in FK_AddressOfUnaddressableFunction, because I'm pretty sure that C++ doesn't have unaddressable functions and thus there is no need to decompose an initializer list. Is this correct?

[Rakete1111]

Rebased + friendly ping :)

[Rakete1111]

Rebased + friendly ping :)

[rsmith]

In D39679#1037591, @Rakete1111 wrote:

Note: I didn't change Args[0] to OnlyArg in FK_AddressOfUnaddressableFunction, because I'm pretty sure that C++ doesn't have unaddressable functions and thus there is no need to decompose an initializer list. Is this correct?

C++ with Clang extensions has unaddressable functions. This example asserts at the moment:

void f() __attribute__((enable_if(false, "")));
int f();
void (&&p)() = {f};

... due, I think, to failing to unwrap the initializer list when diagnosing the initialization.

[rsmith]

In any case, this looks good to me. It'll look even better with the FK_AddressOfUnaddressableFunction case fixed :)

[Rakete1111]

done :)
Could you commit it please, @rsmith ?

[Rakete1111]

Rebased + friendly ping :)

[Rakete1111]

Committed in r336922 :)

lib/Sema/SemaInit.cpp

 
   case FK_ReferenceInitDropsQualifiers: {
     QualType SourceType = Args[0]->getType();
+
+    // For braced initializer lists, we want to get the type
+    // of its (only) element, and not the "type" of the list itself.
+    if (const auto *List = dyn_cast<InitListExpr>(Args[0]))
+      SourceType = List->getInit(0)->getType();
+
     QualType NonRefType = DestType.getNonReferenceType();
     Qualifiers DroppedQualifiers =
         SourceType.getQualifiers() - NonRefType.getQualifiers();

test/SemaCXX/references.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s 
 int g(int);
 
 void f() {

   //  const double& rcd2 = 2; // rcd2 refers to temporary with value 2.0
   const volatile int cvi = 1;
   const int& r = cvi; // expected-error{{binding value of type 'const volatile int' to reference to type 'const int' drops 'volatile' qualifier}}
+
+#if __cplusplus >= 201103L
+  const int& r2{cvi}; // expected-error{{binding value of type 'const volatile int' to reference to type 'const int' drops 'volatile' qualifier}}
+
+  const int a = 2;
+  int& r3 = a; // expected-error{{binding value of type 'const int' to reference to type 'int' drops 'const'}}
+#endif
 }
 
 // C++ [dcl.init.ref]p3