[clang][Interp] Lazily visit unknown global declarations

[tbaeder]

Summary

In C, we don't get a evaluateAsInitializer() call for all global
declarations, yet we have to handle DeclRefExpr pointing to them.

[aaron.ballman]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

2172–2173

Do we want to assert/limit this functionality to C instead of doing it in both C and C++?

clang/test/AST/Interp/c.c

15–16

Pedantically, this is also not a constant expression. Probably worth adding -pedantic to the RUN lines to ensure we properly warn with both constexpr engines.

[tbaeder]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

2172–2173

I never know how to do something for just C. I know there's stuff in LangOpts for C++, but no LangOpts::C?

[cor3ntin]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

2172–2173

!LangOpts.CPlusPlus afaik

[tbaeder]

Ping

[tbaeder]

Ping

[tbaeder]

Ping

[tbaeder]

Ping

[cor3ntin]

LGTM

clang/lib/AST/Interp/ByteCodeExprGen.cpp

if (auto It = this->LambdaCaptures.find(D);
       It != this->LambdaCaptures.end()) {
     auto [Offset, IsReference] = It->second;
 
     if (IsReference)
       return this->emitGetThisFieldPtr(Offset, E);
     return this->emitGetPtrThisField(Offset, E);
   }
 
+  // Lazily visit global declarations we haven't seen yet.
+  // This happens in C.
+  if (const auto *VD = dyn_cast<VarDecl>(D); VD && VD->hasGlobalStorage()) {
+    if (!this->visitVarDecl(VD))
+      return false;
+    // Retry.
+    return this->VisitDeclRefExpr(E);
+  }
   return false;
 }
 
 template <class Emitter>
 void ByteCodeExprGen<Emitter>::emitCleanup() {
   for (VariableScope<Emitter> *C = VarScope; C; C = C->getParent())
     C->emitDestruction();
 }

clang/test/AST/Interp/c.c

 // RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify %s
 // RUN: %clang_cc1 -verify=ref %s
 
 /// expected-no-diagnostics
-/// ref-no-diagnostics
 
 _Static_assert(1, "");
 _Static_assert(0 != 1, "");
 _Static_assert(1.0 == 1.0, "");
 _Static_assert( (5 > 4) + (3 > 2) == 2, "");
 
+/// FIXME: Should also be rejected in the new interpreter
 int a = (1 == 1 ? 5 : 3);
+_Static_assert(a == 5, ""); // ref-error {{ not an integral constant expression}}
+
+const int b = 3;
+_Static_assert(b == 3, "");