[clang][Interp] Allow visiting extern variables

If the variable is additionally const(expr), visit them like normal
but omit the initializer.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -2665,18 +2665,12 @@ bool ByteCodeExprGen<Emitter>::visitVarDecl(const VarDecl *VD) {
     if (P.getGlobal(VD))
       return true;
 
-    // Ignore external declarations. We will instead emit a dummy
-    // pointer when we see a DeclRefExpr for them.
-    if (VD->hasExternalStorage())
-      return true;
-
     std::optional<unsigned> GlobalIndex = P.createGlobal(VD, Init);
 
     if (!GlobalIndex)
       return false;
 
-    assert(Init);
-    {
+    if (Init) {
       DeclScope<Emitter> LocalScope(this, VD);
 
       if (VarT) {
@@ -2686,6 +2680,7 @@ bool ByteCodeExprGen<Emitter>::visitVarDecl(const VarDecl *VD) {
       }
       return this->visitGlobalInitializer(Init, *GlobalIndex);
     }
+    return true;
   } else {
     VariableScope<Emitter> LocalScope(this);
     if (VarT) {

clang/test/AST/Interp/literals.cpp

@@ -1190,6 +1190,11 @@ namespace incdecbool {
 constexpr int externvar1() { // both-error {{never produces a constant expression}}
   extern char arr[]; // ref-note {{declared here}}
    return arr[0]; // ref-note {{read of non-constexpr variable 'arr'}} \
-                  // expected-note {{indexing of array without known bound}}
+                  // expected-note {{array-to-pointer decay of array member without known bound is not supported}}
 }
 #endif
+
+namespace Extern {
+  constexpr extern char Oops = 1;
+  static_assert(Oops == 1, "");
+}