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, 2 insertions, 2 deletions

diff --git a/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp b/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
index 81876192ca..5cf281c185 100644
--- a/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
+++ b/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
@@ -36,7 +36,7 @@ struct S3 {
 constexpr S3 s3a = S3(0);
 constexpr S3 s3b = s3a;
 constexpr S3 s3c = S3();
-constexpr S3 s3d; // expected-error {{default initialization of an object of const type 'const S3' without a user-provided default constructor}} expected-note{{add an explicit initializer to initialize 's3d'}}
+constexpr S3 s3d; // expected-error {{default initialization of an object of const type 'const S3' without a user-provided default constructor}}
 
 struct S4 {
   S4() = default;
@@ -119,6 +119,6 @@ namespace PR13492 {
   };
 
   void f() {
-    const B b; // expected-error {{default initialization of an object of const type 'const PR13492::B' without a user-provided default constructor}} expected-note {{add an explicit initializer to initialize 'b'}}
+    const B b; // expected-error {{default initialization of an object of const type 'const PR13492::B' without a user-provided default constructor}}
   }
 }