Move fixit for const init from note to diag, weaken to warning in MS mode.

r235046 turned "extern __declspec(selectany) int a;" from a declaration into
a definition to fix PR23242 (required for compatibility with mc.exe output).
However, this broke parsing Windows headers: A  d3d11 headers contain something
like

  struct SomeStruct {};
  extern const __declspec(selectany) SomeStruct some_struct;

This is now a definition, and const objects either need an explicit default
ctor or an initializer so this errors out with 

  d3d11.h(1065,48) :
    error: default initialization of an object of const type
           'const CD3D11_DEFAULT' without a user-provided default constructor

(cl.exe just doesn't implement this rule, independent of selectany.)

To work around this, weaken this error into a warning for selectany decls
in microsoft mode, and recover with zero-initialization.

Doing this is a bit hairy since it adds a fixit on an error emitted
by InitializationSequence – this means it needs to build a correct AST, which
in turn means InitializationSequence::Failed() cannot return true when this
fixit is applied. As a workaround, the patch adds a fixit member to
InitializationSequence, and InitializationSequence::Perform() prints the
diagnostic if the fixit member is set right after its call to Diagnose.
That function is usually called when InitializationSequences are used –
InitListChecker::PerformEmptyInit() doesn't call it, but the InitListChecker
case never performs default-initialization, so this is technically OK.

This is the alternative, original fix for PR20208 that got reviewed in the
thread "[patch] Improve diagnostic on default-initializing const variables
(PR20208)".  This change basically reverts r213725, adds the original fix for
PR20208, and makes the error a warning in Microsoft mode.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@235166 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 11 insertions, 1 deletions

diff --git a/test/SemaCXX/attr-selectany.cpp b/test/SemaCXX/attr-selectany.cpp
index c27a915921..7d9cf7aea4 100644
--- a/test/SemaCXX/attr-selectany.cpp
+++ b/test/SemaCXX/attr-selectany.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fms-extensions -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fms-compatibility -fms-extensions -fsyntax-only -verify -std=c++11 %s
 // MSVC produces similar diagnostics.
 
 __declspec(selectany) void foo() { } // expected-error{{'selectany' can only be applied to data items with external linkage}}
@@ -34,3 +34,13 @@ __declspec(selectany) X x(1);
 
 namespace { class Internal {}; }
 __declspec(selectany) auto x8 = Internal(); // expected-error {{'selectany' can only be applied to data items with external linkage}}
+
+
+// The D3D11 headers do something like this.  MSVC doesn't error on this at
+// all, even without the __declspec(selectany), in violation of the standard.
+// We fall back to a warning for selectany to accept headers.
+struct SomeStruct {};
+extern const __declspec(selectany) SomeStruct some_struct; // expected-warning {{default initialization of an object of const type 'const SomeStruct' without a user-provided default constructor is a Microsoft extension}}
+
+// Without selectany, this should stay an error.
+const SomeStruct some_struct2; // expected-error {{default initialization of an object of const type 'const SomeStruct' without a user-provided default constructor}}