[clang][Interp] Emit Error op for contains-error expressions

Instead of aborting interpretation right away. This way we can still
successfully evaluate such functions provided we don't reach the
Error op at all.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -2244,7 +2244,7 @@ template <class Emitter> bool ByteCodeExprGen<Emitter>::discard(const Expr *E) {
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::delegate(const Expr *E) {
   if (E->containsErrors())
-    return false;
+    return this->emitError(E);
 
   // We're basically doing:
   // OptionScope<Emitter> Scope(this, DicardResult, Initializing);
@@ -2254,7 +2254,7 @@ bool ByteCodeExprGen<Emitter>::delegate(const Expr *E) {
 
 template <class Emitter> bool ByteCodeExprGen<Emitter>::visit(const Expr *E) {
   if (E->containsErrors())
-    return false;
+    return this->emitError(E);
 
   if (E->getType()->isVoidType())
     return this->discard(E);
@@ -2283,7 +2283,7 @@ bool ByteCodeExprGen<Emitter>::visitInitializer(const Expr *E) {
   assert(!classify(E->getType()));
 
   if (E->containsErrors())
-    return false;
+    return this->emitError(E);
 
   OptionScope<Emitter> Scope(this, /*NewDiscardResult=*/false,
                              /*NewInitializing=*/true);

clang/lib/AST/Interp/Interp.h

@@ -2227,6 +2227,9 @@ inline bool Invalid(InterpState &S, CodePtr OpPC) {
   return false;
 }
 
+/// Do nothing and just abort execution.
+inline bool Error(InterpState &S, CodePtr OpPC) { return false; }
+
 /// Same here, but only for casts.
 inline bool InvalidCast(InterpState &S, CodePtr OpPC, CastKind Kind) {
   const SourceLocation &Loc = S.Current->getLocation(OpPC);

clang/lib/AST/Interp/Opcodes.td

@@ -706,6 +706,7 @@ def Dup : Opcode {
 
 // [] -> []
 def Invalid : Opcode {}
+def Error : Opcode {}
 def InvalidCast : Opcode {
   let Args = [ArgCastKind];
 }

clang/test/AST/Interp/if.cpp

@@ -1,8 +1,5 @@
-// RUN: %clang_cc1 -std=c++23 -fsyntax-only -fexperimental-new-constant-interpreter %s -verify
-// RUN: %clang_cc1 -std=c++23 -fsyntax-only %s -verify=ref
-
-// expected-no-diagnostics
-// ref-no-diagnostics
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -fexperimental-new-constant-interpreter %s -verify=expected,both
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only %s -verify=ref,both
 
 namespace ConstEval {
   constexpr int f() {
@@ -51,3 +48,13 @@ namespace InitDecl {
   }
   static_assert(attrs() == 1, "");
 };
+
+/// The faulty if statement creates a RecoveryExpr with contains-errors,
+/// but the execution will never reach that.
+constexpr char g(char const (&x)[2]) {
+    return 'x';
+  if (auto [a, b] = x) // both-error {{an array type is not allowed here}} \
+                       // both-warning {{ISO C++17 does not permit structured binding declaration in a condition}}
+    ;
+}
+static_assert(g("x") == 'x');