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.

llvm-svn: 235166

1 files changed, 8 insertions, 0 deletions

diff --git a/clang/test/FixIt/fixit.cpp b/clang/test/FixIt/fixit.cpp
index 6c2fb7faf14..512713aa529 100644
--- a/clang/test/FixIt/fixit.cpp
+++ b/clang/test/FixIt/fixit.cpp
@@ -387,3 +387,11 @@ struct conversion_operator {
   // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:3-[[@LINE-1]]:32}:""
   // CHECK: fix-it:"{{.*}}":{[[@LINE-2]]:44-[[@LINE-2]]:44}:" conversion_operator::* const"
 };
+
+struct const_zero_init {
+  int a;
+};
+const const_zero_init czi; // expected-error {{default initialization of an object of const type 'const const_zero_init'}}
+// CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:26-[[@LINE-1]]:26}:"{}"
+int use_czi = czi.a;
+