[clang][Interp] Diagnose dummy pointers used in Inc/Dec ops

For example for unknown parameter decls.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -525,8 +525,7 @@ enum class IncDecOp {
 
 template <typename T, IncDecOp Op, PushVal DoPush>
 bool IncDecHelper(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
-  if (Ptr.isDummy())
-    return false;
+  assert(!Ptr.isDummy());
 
   if constexpr (std::is_same_v<T, Boolean>) {
     if (!S.getLangOpts().CPlusPlus14)
@@ -585,7 +584,7 @@ bool IncDecHelper(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool Inc(InterpState &S, CodePtr OpPC) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
-  if (Ptr.isDummy())
+  if (!CheckDummy(S, OpPC, Ptr))
     return false;
   if (!CheckInitialized(S, OpPC, Ptr, AK_Increment))
     return false;
@@ -599,7 +598,7 @@ bool Inc(InterpState &S, CodePtr OpPC) {
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool IncPop(InterpState &S, CodePtr OpPC) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
-  if (Ptr.isDummy())
+  if (!CheckDummy(S, OpPC, Ptr))
     return false;
   if (!CheckInitialized(S, OpPC, Ptr, AK_Increment))
     return false;
@@ -614,7 +613,7 @@ bool IncPop(InterpState &S, CodePtr OpPC) {
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool Dec(InterpState &S, CodePtr OpPC) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
-  if (Ptr.isDummy())
+  if (!CheckDummy(S, OpPC, Ptr))
     return false;
   if (!CheckInitialized(S, OpPC, Ptr, AK_Decrement))
     return false;
@@ -628,7 +627,7 @@ bool Dec(InterpState &S, CodePtr OpPC) {
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool DecPop(InterpState &S, CodePtr OpPC) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
-  if (Ptr.isDummy())
+  if (!CheckDummy(S, OpPC, Ptr))
     return false;
   if (!CheckInitialized(S, OpPC, Ptr, AK_Decrement))
     return false;

clang/test/AST/Interp/literals.cpp

@@ -839,6 +839,18 @@ namespace IncDec {
     return a[1];
   }
   static_assert(f() == 3, "");
+
+  int nonconst(int a) { // both-note 4{{declared here}}
+    static_assert(a++, ""); // both-error {{not an integral constant expression}} \
+                            // both-note {{function parameter 'a' with unknown value cannot be used in a constant expression}}
+    static_assert(a--, ""); // both-error {{not an integral constant expression}} \
+                            // both-note {{function parameter 'a' with unknown value cannot be used in a constant expression}}
+    static_assert(++a, ""); // both-error {{not an integral constant expression}} \
+                            // both-note {{function parameter 'a' with unknown value cannot be used in a constant expression}}
+    static_assert(--a, ""); // both-error {{not an integral constant expression}} \
+                            // both-note {{function parameter 'a' with unknown value cannot be used in a constant expression}}
+  }
+
 };
 #endif