[Sema] Mark implicitly-inserted ICE's as being part of explicit cast (PR38166)

Summary:
As discussed in [[ https://bugs.llvm.org/show_bug.cgi?id=38166 | PR38166 ]], we need to be able to distinqush whether the cast
we are visiting is actually a cast, or part of an `ExplicitCast`.
There are at least four ways to get there:
1. Introduce a new `CastKind`, and use it instead of `IntegralCast` if we are in `ExplicitCast`.

   Would work, but does not scale - what if we will need more of these cast kinds?
2. Introduce a flag in `CastExprBits`, whether this cast is part of `ExplicitCast` or not.

   Would work, but it isn't immediately clear where it needs to be set.
2. Fix `ScalarExprEmitter::VisitCastExpr()` to visit these `NoOp` casts.

   As pointed out by @rsmith, CodeGenFunction::EmitMaterializeTemporaryExpr calls

   skipRValueSubobjectAdjustments, which steps over the CK_NoOp cast`,

   which explains why we currently don't visit those.

   This is probably impossible, as @efriedma points out, that is intentional as per `[class.temporary]` in the standard
3. And the simplest one, just record which NoOp casts we skip.

   It just kinda works as-is afterwards.

But, the approach with a flag is the least intrusive one, and is probably the best one overall.

Reviewers: rsmith, rjmccall, majnemer, efriedma

Reviewed By: rsmith

Subscribers: cfe-commits, aaron.ballman, vsk, llvm-commits, rsmith

Differential Revision: https://reviews.llvm.org/D49508

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@337815 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 9 insertions, 0 deletions

diff --git a/lib/Sema/SemaCast.cpp b/lib/Sema/SemaCast.cpp
index b3f6be4aec..a669956422 100644
--- a/lib/Sema/SemaCast.cpp
+++ b/lib/Sema/SemaCast.cpp
@@ -89,6 +89,14 @@ namespace {
     void CheckCXXCStyleCast(bool FunctionalCast, bool ListInitialization);
     void CheckCStyleCast();
 
+    void updatePartOfExplicitCastFlags(CastExpr *CE) {
+      // Walk down from the CE to the OrigSrcExpr, and mark all immediate
+      // ImplicitCastExpr's as being part of ExplicitCastExpr. The original CE
+      // (which is a ExplicitCastExpr), and the OrigSrcExpr are not touched.
+      while ((CE = dyn_cast<ImplicitCastExpr>(CE->getSubExpr())))
+        CE->setIsPartOfExplicitCast(true);
+    }
+
     /// Complete an apparently-successful cast operation that yields
     /// the given expression.
     ExprResult complete(CastExpr *castExpr) {
@@ -100,6 +108,7 @@ namespace {
                                             CK_Dependent, castExpr, nullptr,
                                             castExpr->getValueKind());
       }
+      updatePartOfExplicitCastFlags(castExpr);
       return castExpr;
     }