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, 3 insertions, 3 deletions

diff --git a/clang/test/SemaCXX/constexpr-value-init.cpp b/clang/test/SemaCXX/constexpr-value-init.cpp
index 3657c18ddba..06511113355 100644
--- a/clang/test/SemaCXX/constexpr-value-init.cpp
+++ b/clang/test/SemaCXX/constexpr-value-init.cpp
@@ -14,7 +14,7 @@ void f() {
   constexpr A a; // expected-error {{constant expression}} expected-note {{in call to 'A()'}}
 }
 
-constexpr B b1; // expected-error {{without a user-provided default constructor}} expected-note {{add an explicit initializer to initialize 'b1'}}
+constexpr B b1; // expected-error {{without a user-provided default constructor}}
 constexpr B b2 = B(); // ok
 static_assert(b2.a.a == 1, "");
 static_assert(b2.a.b == 2, "");
@@ -23,9 +23,9 @@ struct C {
   int c;
 };
 struct D : C { int d; };
-constexpr C c1; // expected-error {{without a user-provided default constructor}} expected-note{{add an explicit initializer to initialize 'c1'}}
+constexpr C c1; // expected-error {{without a user-provided default constructor}}
 constexpr C c2 = C(); // ok
-constexpr D d1; // expected-error {{without a user-provided default constructor}} expected-note{{add an explicit initializer to initialize 'd1'}}
+constexpr D d1; // expected-error {{without a user-provided default constructor}}
 constexpr D d2 = D(); // ok with DR1452
 static_assert(D().c == 0, "");
 static_assert(D().d == 0, "");