[clang][bytecode] Check reads for null block pointers (#157695)

All pointer types can be null, so check that independently from the
pointer type.

Fixes #157650

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -792,18 +792,18 @@ bool CheckLocalLoad(InterpState &S, CodePtr OpPC, const Block *B) {
 
 bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
                AccessKinds AK) {
-  // Block pointers are the only ones we can actually read from.
-  if (!Ptr.isBlockPointer()) {
-    if (Ptr.isZero()) {
-      const auto &Src = S.Current->getSource(OpPC);
+  if (Ptr.isZero()) {
+    const auto &Src = S.Current->getSource(OpPC);
 
-      if (Ptr.isField())
-        S.FFDiag(Src, diag::note_constexpr_null_subobject) << CSK_Field;
-      else
-        S.FFDiag(Src, diag::note_constexpr_access_null) << AK;
-    }
+    if (Ptr.isField())
+      S.FFDiag(Src, diag::note_constexpr_null_subobject) << CSK_Field;
+    else
+      S.FFDiag(Src, diag::note_constexpr_access_null) << AK;
     return false;
   }
+  // Block pointers are the only ones we can actually read from.
+  if (!Ptr.isBlockPointer())
+    return false;
 
   if (!Ptr.block()->isAccessible()) {
     if (!CheckLive(S, OpPC, Ptr, AK))

clang/test/AST/ByteCode/references.cpp

@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify=expected,both %s
-// RUN: %clang_cc1 -verify=ref,both %s
+// RUN: %clang_cc1                                         -verify=ref,both      %s
 
 
 constexpr int a = 10;
@@ -178,3 +178,19 @@ namespace Params {
 
   static_assert(foo());
 }
+
+namespace ReadFromNullBlockPtr {
+  struct S {
+    int *const &t;
+  };
+
+  void foo(int x) {
+    constexpr S s = {&x}; // both-error {{must be initialized by a constant expression}} \
+                          // both-note {{reference to temporary}} \
+                          // both-note {{created here}} \
+                          // ref-note {{declared here}}
+    static_assert(s.t == &x, ""); // both-error {{not an integral constant expression}} \
+                                  // expected-note {{read of dereferenced null pointer}} \
+                                  // ref-note {{initializer of 's' is not a constant expression}}
+  }
+}