[clang][Interp] Handle enums

Handle DeclRefExprs of enum types. They are otherwise handled like
integers.

Differential Revision: https://reviews.llvm.org/D134020

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -861,6 +861,11 @@ bool ByteCodeExprGen<Emitter>::VisitDeclRefExpr(const DeclRefExpr *E) {
 
       FoundDecl = true;
     }
+  } else if (const auto *ECD = dyn_cast<EnumConstantDecl>(Decl)) {
+    PrimType T = *classify(ECD->getType());
+
+    return this->emitConst(T, getIntWidth(ECD->getType()), ECD->getInitVal(),
+                           E);
   }
 
   // References are implemented using pointers, so when we get here,

clang/test/AST/Interp/enums-targets.cpp

@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -triple i686-pc-linux -fexperimental-new-constant-interpreter -verify %s
+// RUN: %clang_cc1 -triple i686-pc-linux -verify %s
+// RUN: %clang_cc1 -triple x86_64-pc-linux -fexperimental-new-constant-interpreter -verify=warn %s
+// RUN: %clang_cc1 -triple x86_64-pc-linux -verify=warn %s
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -fexperimental-new-constant-interpreter -verify %s
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -verify %s
+// RUN: %clang_cc1 -triple hexagon -fexperimental-new-constant-interpreter -verify %s
+// RUN: %clang_cc1 -triple hexagon -verify %s
+
+// expected-no-diagnostics
+
+/// This test is split out from the rest since the output is target dependent.
+
+enum E { // warn-warning {{enumeration values exceed range of largest integer}}
+  E1 = -__LONG_MAX__ -1L,
+  E2 = __LONG_MAX__ *2UL+1UL
+};
+

clang/test/AST/Interp/enums.cpp

@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify %s
+// RUN: %clang_cc1 -verify=ref %s
+
+enum class EC : short {
+  A, B, C
+};
+static_assert(static_cast<int>(EC::A) == 0, "");
+static_assert(static_cast<int>(EC::B) == 1, "");
+static_assert(static_cast<int>(EC::C) == 2, "");
+static_assert(sizeof(EC) == sizeof(short), "");
+
+constexpr EC ec = EC::C;
+static_assert(static_cast<int>(ec) == 2, "");
+
+constexpr int N = 12;
+constexpr int M = 2;
+
+enum CE {
+  ONE = -1,
+  TWO = 2,
+  THREE,
+  FOUR = 4,
+  FIVE = N + M,
+  SIX = FIVE + 2,
+  MAX = __INT_MAX__ * 2U + 1U
+};
+static_assert(ONE == -1, "");
+static_assert(THREE == 3, "");
+static_assert(FIVE == 14, "");
+static_assert(SIX == 16, "");
+
+constexpr EC testEnums() {
+  EC e = EC::C;
+
+  e = EC::B;
+
+  EC::B = e; // expected-error{{expression is not assignable}} \
+             // ref-error{{expression is not assignable}}
+
+  return e;
+}
+
+constexpr EC getB() {
+  EC e = EC::C;
+  e = EC::B;
+  return e;
+}
+
+
+static_assert(getB() == EC::B, "");