[clang] perform semantic checking in constant context

Summary:
Since the addition of __builtin_is_constant_evaluated the result of an expression can change based on whether it is evaluated in constant context. a lot of semantic checking performs evaluations with out specifying context. which can lead to wrong diagnostics.
for example:
```
constexpr int i0 = (long long)__builtin_is_constant_evaluated() * (1ll << 33); //#1
constexpr int i1 = (long long)!__builtin_is_constant_evaluated() * (1ll << 33); //#2
```
before the patch, #2 was diagnosed incorrectly and #1 wasn't diagnosed.
after the patch #1 is diagnosed as it should and #2 isn't.

Changes:
 - add a flag to Sema to passe in constant context mode.
 - in SemaChecking.cpp calls to Expr::Evaluate* are now done in constant context when they should.
 - in SemaChecking.cpp diagnostics for UB are not checked for in constant context because an error will be emitted by the constant evaluator.
 - in SemaChecking.cpp diagnostics for construct that cannot appear in constant context are not checked for in constant context.
 - in SemaChecking.cpp diagnostics on constant expression are always emitted because constant expression are always evaluated.
 - semantic checking for initialization of constexpr variables is now done in constant context.
 - adapt test that were depending on warning changes.
 - add test.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62009

llvm-svn: 363488

1 files changed, 1 insertions, 0 deletions

diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 077bb0a61ac..d7a05e731ea 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -158,6 +158,7 @@ Sema::Sema(Preprocessor &pp, ASTContext &ctxt, ASTConsumer &consumer,
       ThreadSafetyDeclCache(nullptr), VarDataSharingAttributesStack(nullptr),
       CurScope(nullptr), Ident_super(nullptr), Ident___float128(nullptr) {
   TUScope = nullptr;
+  isConstantEvaluatedOverride = false;
 
   LoadedExternalKnownNamespaces = false;
   for (unsigned I = 0; I != NSAPI::NumNSNumberLiteralMethods; ++I)