[clang][bytecode] Allow This pointers in CPCE mode (llvm#137761)

The outermost function doesn't have a This pointers, but inner calls
can. This still doesn't match the diagnostic output of the current
interpreter properly, but it's closer I think.

Author: tbaederr

clang/lib/AST/ByteCode/Interp.h

@@ -1517,7 +1517,7 @@ inline bool InitGlobalTempComp(InterpState &S, CodePtr OpPC,
 
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool InitThisField(InterpState &S, CodePtr OpPC, uint32_t I) {
-  if (S.checkingPotentialConstantExpression())
+  if (S.checkingPotentialConstantExpression() && S.Current->getDepth() == 0)
     return false;
   const Pointer &This = S.Current->getThis();
   if (!CheckThis(S, OpPC, This))
@@ -1535,7 +1535,7 @@ template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool InitThisBitField(InterpState &S, CodePtr OpPC, const Record::Field *F,
                       uint32_t FieldOffset) {
   assert(F->isBitField());
-  if (S.checkingPotentialConstantExpression())
+  if (S.checkingPotentialConstantExpression() && S.Current->getDepth() == 0)
     return false;
   const Pointer &This = S.Current->getThis();
   if (!CheckThis(S, OpPC, This))
@@ -1602,7 +1602,7 @@ bool GetPtrField(InterpState &S, CodePtr OpPC, uint32_t Off);
 bool GetPtrFieldPop(InterpState &S, CodePtr OpPC, uint32_t Off);
 
 inline bool GetPtrThisField(InterpState &S, CodePtr OpPC, uint32_t Off) {
-  if (S.checkingPotentialConstantExpression())
+  if (S.checkingPotentialConstantExpression() && S.Current->getDepth() == 0)
     return false;
   const Pointer &This = S.Current->getThis();
   if (!CheckThis(S, OpPC, This))

clang/test/AST/ByteCode/builtin-functions.cpp

@@ -48,10 +48,10 @@ static_assert(test_address_of_incomplete_array_type() == 1234, ""); // both-erro
     constexpr NonTrivial(const NonTrivial &) : n(1) {}
     int n;
   };
-  constexpr bool test_nontrivial_memcpy() { // ref-error {{never produces a constant}}
+  constexpr bool test_nontrivial_memcpy() { // both-error {{never produces a constant}}
     NonTrivial arr[3] = {};
     __builtin_memcpy(arr, arr + 1, sizeof(NonTrivial)); // both-note {{non-trivially-copyable}} \
-                                                        // ref-note {{non-trivially-copyable}}
+                                                        // both-note {{non-trivially-copyable}}
     return true;
   }
   static_assert(test_nontrivial_memcpy()); // both-error {{constant}} \

clang/test/AST/ByteCode/unions.cpp

@@ -76,9 +76,10 @@ namespace DefaultInit {
 
   constexpr U1 u1; /// OK.
 
-  constexpr int foo() {
+  constexpr int foo() { // expected-error {{never produces a constant expression}}
     U1 u;
-    return u.a; // both-note {{read of member 'a' of union with active member 'b'}}
+    return u.a; // both-note {{read of member 'a' of union with active member 'b'}} \
+                // expected-note {{read of member 'a' of union with active member 'b'}}
   }
   static_assert(foo() == 42); // both-error {{not an integral constant expression}} \
                               // both-note {{in call to}}