Report a clearer error for non-integral iterator differences

clang/lib/Sema/SemaStmt.cpp

@@ -4002,7 +4002,7 @@ StmtResult Sema::BuildCilkForRangeStmt(CXXForRangeStmt *ForRange) {
   }
   ExprResult LimitExpr = ActOnBinOp(S, ForRange->getColonLoc(), tok::minus,
                                     EndRef.get(), BeginRef.get());
-  if (LimitExpr.isInvalid()) {
+  if (LimitExpr.isInvalid() || !LimitExpr.get()->getType()->isIntegerType()) {
     // CilkForRange currently only supports random access iterators.
     Diag(ForRange->getForLoc(), diag::err_cilk_for_range_end_minus_begin);
     return StmtError();

clang/test/Cilk/cilkfor-range-error.cpp

@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 %s -std=c++20 -triple x86_64-unknown-linux-gnu -fopencilk -fsyntax-only -verify
+
+template<class Datum>
+struct Iterator {
+  void *data;
+  Iterator operator+(long);
+  bool operator!=(const Iterator &) const;
+  Iterator &operator++();
+  Datum &operator*();
+  Datum *operator->();
+};
+
+struct S {
+  int data;
+  struct I : public Iterator<S> {
+    float operator-(const I &) const;
+  };
+  I begin(), end();
+};
+
+struct T {
+  int data;
+  struct I : public Iterator<T> {
+    long operator-(const I &) const;
+  };
+  I begin(), end();
+};
+
+void sum(S &s, T &t) {
+  _Cilk_for (auto i : s) { // expected-warning{{experimental}}
+    // expected-error@-1{{cannot determine length}}
+  }
+  _Cilk_for (auto i : _Cilk_spawn t) { // expected-warning{{experimental}}
+    // expected-error@-1{{'cilk_spawn' not allowed in this scope}}
+  }
+}