Index: clang/lib/AST/Interp/Interp.cpp
===================================================================
--- clang/lib/AST/Interp/Interp.cpp
+++ clang/lib/AST/Interp/Interp.cpp
@@ -553,6 +553,25 @@
     return false;
   }
 
+  if (const auto *VD = dyn_cast<VarDecl>(D)) {
+    if (!VD->getType().isConstant(S.getCtx())) {
+      if (VD->getType()->isIntegralOrEnumerationType())
+        S.FFDiag(E, diag::note_constexpr_ltor_non_const_int, 1) << VD;
+      else
+        S.FFDiag(E, diag::note_constexpr_ltor_non_constexpr, 1) << VD;
+
+      S.Note(VD->getLocation(), diag::note_declared_at) << VD->getSourceRange();
+      return false;
+    }
+
+    // const, but no initializer.
+    if (!VD->getAnyInitializer()) {
+      S.FFDiag(E, diag::note_constexpr_var_init_unknown, 1) << VD;
+      S.Note(VD->getLocation(), diag::note_declared_at) << VD->getSourceRange();
+      return false;
+    }
+  }
+
   return false;
 }
 
Index: clang/test/AST/Interp/literals.cpp
===================================================================
--- clang/test/AST/Interp/literals.cpp
+++ clang/test/AST/Interp/literals.cpp
@@ -1056,3 +1056,31 @@
   int array[(intptr_t)(char*)0]; // ref-warning {{variable length array folded to constant array}} \
                                  // expected-warning {{variable length array folded to constant array}}
 }
+
+namespace InvalidDeclRefs {
+  bool b00; // ref-note {{declared here}} \
+            // expected-note {{declared here}}
+  static_assert(b00, ""); // ref-error {{not an integral constant expression}} \
+                          // ref-note {{read of non-const variable}} \
+                          // expected-error {{not an integral constant expression}} \
+                          // expected-note {{read of non-const variable}}
+
+  float b01; // ref-note {{declared here}} \
+             // expected-note {{declared here}}
+  static_assert(b01, ""); // ref-error {{not an integral constant expression}} \
+                          // ref-note {{read of non-constexpr variable}} \
+                          // expected-error {{not an integral constant expression}} \
+                          // expected-note {{read of non-constexpr variable}}
+
+  extern const int b02; // ref-note {{declared here}} \
+                        // expected-note {{declared here}}
+  static_assert(b02, ""); // ref-error {{not an integral constant expression}} \
+                          // ref-note {{initializer of 'b02' is unknown}} \
+                          // expected-error {{not an integral constant expression}} \
+                          // expected-note {{initializer of 'b02' is unknown}}
+
+  /// FIXME: This should also be diagnosed in the new interpreter.
+  int b03 = 3; // ref-note {{declared here}}
+  static_assert(b03, ""); // ref-error {{not an integral constant expression}} \
+                          // ref-note {{read of non-const variable}}
+}