[CSDiagnostics] Add a diagnostic for pack expansions without pack references

(cherry picked from commit cb2fdd9)

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -5422,7 +5422,8 @@ ERROR(cannot_convert_tuple_into_pack_expansion_parameter,none,
 NOTE(cannot_convert_tuple_into_pack_expansion_parameter_note,none,
      "value pack expansion at parameter #%0 expects %1 separate arguments",
      (unsigned, unsigned))
-
+ERROR(value_expansion_not_variadic,none,
+      "value pack expansion must contain at least one pack reference", ())
 
 ERROR(expansion_not_same_shape,none,
       "pack expansion %0 requires that %1 and %2 have the same shape",

lib/Sema/CSDiagnostics.cpp

@@ -8899,3 +8899,8 @@ bool DestructureTupleToUseWithPackExpansionParameter::diagnoseAsNote() {
       argLoc.getParamIdx(), ParamShape->getNumElements());
   return true;
 }
+
+bool ValuePackExpansionWithoutPackReferences::diagnoseAsError() {
+  emitDiagnostic(diag::value_expansion_not_variadic);
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -3000,6 +3000,23 @@ class DestructureTupleToUseWithPackExpansionParameter final
   bool diagnoseAsNote() override;
 };
 
+/// Diagnose situations when value pack expansion doesn't have any pack
+/// references i.e.:
+///
+/// ```swift
+/// func test(x: Int) {
+///   repeat x
+/// }
+/// ```
+class ValuePackExpansionWithoutPackReferences final : public FailureDiagnostic {
+public:
+  ValuePackExpansionWithoutPackReferences(const Solution &solution,
+                                          ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift