[clang][bytecode] Fix fallthrough to switch labels (#168484)

We need to fallthrough here in case we're not jumping to the labels.
This is only needed in expression contexts.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -6057,13 +6057,15 @@ bool Compiler<Emitter>::visitSwitchStmt(const SwitchStmt *S) {
                           DefaultLabel);
   if (!this->visitStmt(S->getBody()))
     return false;
+  this->fallthrough(EndLabel);
   this->emitLabel(EndLabel);
 
   return LS.destroyLocals();
 }
 
 template <class Emitter>
 bool Compiler<Emitter>::visitCaseStmt(const CaseStmt *S) {
+  this->fallthrough(CaseLabels[S]);
   this->emitLabel(CaseLabels[S]);
   return this->visitStmt(S->getSubStmt());
 }

clang/test/AST/ByteCode/literals.cpp

@@ -1270,6 +1270,17 @@ namespace StmtExprs {
   namespace CrossFuncLabelDiff {
     constexpr long a(bool x) { return x ? 0 : (intptr_t)&&lbl + (0 && ({lbl: 0;})); }
   }
+
+  /// GCC agrees with the bytecode interpreter here.
+  void switchInSE() {
+    static_assert(({ // ref-error {{not an integral constant expression}}
+          int i = 20;
+           switch(10) {
+             case 10: i = 300; // ref-note {{a constant expression cannot modify an object that is visible outside that expression}}
+           }
+           i;
+        }) == 300);
+  }
 }
 #endif