[clang][analyzer] Fix empty enum handling in EnumCastOutOfRange checker

The alpha.cplusplus.EnumCastOutOfRange checker previously gave many
false positives because a warning was given if the initializer value
did not appear in the enumerator list.
The strict handling caused std::byte to always give a warning, as it
is implemented as an enum class without any declarators.

Reviewed By: donat.nagy, steakhal

Differential Revision: https://reviews.llvm.org/D153954

clang/lib/StaticAnalyzer/Checkers/EnumCastOutOfRangeChecker.cpp

@@ -129,6 +129,14 @@ void EnumCastOutOfRangeChecker::checkPreStmt(const CastExpr *CE,
   const EnumDecl *ED = T->castAs<EnumType>()->getDecl();
 
   EnumValueVector DeclValues = getDeclValuesForEnum(ED);
+
+  // If the declarator list is empty, bail out.
+  // Every initialization an enum with a fixed underlying type but without any
+  // enumerators would produce a warning if we were to continue at this point.
+  // The most notable example is std::byte in the C++17 standard library.
+  if (DeclValues.size() == 0)
+    return;
+
   // Check if any of the enum values possibly match.
   bool PossibleValueMatch = llvm::any_of(
       DeclValues, ConstraintBasedEQEvaluator(C, *ValueToCast));

clang/test/Analysis/enum-cast-out-of-range.cpp

@@ -198,3 +198,20 @@ void enumBitFieldAssignment() {
   s.E = static_cast<unscoped_unspecified_t>(4); // OK.
   s.E = static_cast<unscoped_unspecified_t>(5); // expected-warning {{The value provided to the cast expression is not in the valid range of values for the enum}}
 }
+
+
+enum class empty_unspecified {};
+
+enum class empty_specified: char {};
+
+enum class empty_specified_unsigned: unsigned char {};
+
+void ignore_unused(...);
+
+void empty_enums_init_with_zero_should_not_warn() {
+  auto eu = static_cast<empty_unspecified>(0); //should always be OK to zero initialize any enum
+  auto ef = static_cast<empty_specified>(0);
+  auto efu = static_cast<empty_specified_unsigned>(0);
+
+  ignore_unused(eu, ef, efu);
+}