[clang][Interp] Emit diagnostics if final ltor conversion fails

Author: tbaederr

clang/lib/AST/Interp/EvalEmitter.cpp

@@ -169,7 +169,9 @@ template <> bool EvalEmitter::emitRet<PT_Ptr>(const SourceInfo &Info) {
       return false;
     // Never allow reading from a non-const pointer, unless the memory
     // has been created in this evaluation.
-    if (!Ptr.isConst() && Ptr.block()->getEvalID() != Ctx.getEvalID())
+    if (!Ptr.isZero() && Ptr.isBlockPointer() &&
+        Ptr.block()->getEvalID() != Ctx.getEvalID() &&
+        (!CheckLoad(S, OpPC, Ptr, AK_Read) || !Ptr.isConst()))
       return false;
 
     if (std::optional<APValue> V =

clang/test/AST/Interp/cxx11.cpp

@@ -152,3 +152,11 @@ void A::f(SortOrder order) {
     return;
 }
 }
+
+namespace FinalLtorDiags {
+  template<int*> struct A {}; // both-note {{template parameter is declared here}}
+  int k;
+  int *q = &k; // both-note {{declared here}}
+  A<q> c; // both-error {{non-type template argument of type 'int *' is not a constant expression}} \
+          // both-note {{read of non-constexpr variable 'q' is not allowed in a constant expression}}
+}