[clang] [clang][bytecode] Fix 'if consteval' in non-constant contexts (PR #104707)

[Timm Baeder via cfe-commits]

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@@ -4368,8 +4363,19 @@ template <class Emitter> bool Compiler<Emitter>::visitIfStmt(const IfStmt *IS) {
     if (!visitDeclStmt(CondDecl))
       return false;
 
-  if (!this->visitBool(IS->getCond()))
-    return false;
+  // Compile condition.
+  if (IS->isNonNegatedConsteval()) {
+    if (!this->emitIsConstantContext(IS))
+      return false;
+  } else if (IS->isNegatedConsteval()) {
+    if (!this->emitIsConstantContext(IS))
+      return false;
+    if (!this->emitInv(IS))
+      return false;
+  } else {
+    if (!this->visitBool(IS->getCond()))
+      return false;
+  }
----------------
tbaederr wrote:

Stuff like that _needs_ to be "implemented" in the interpreter. It must be, because it must be rejected as well. It's the same example as the `goto` statement above. It needs to be compiled to bytecode, somehow. If it isn't then using it in a constexpr function (without the `if consteval` will also fail).

Again, this is no different than any other if statement. I could imagine compiling two different versions of functions (one for consteval = true, one for =false), but we have the same problem with other if statements. This is just a difference between the interpreting and compiling approach.

https://github.com/llvm/llvm-project/pull/104707