[Diagnostic] Add a diagnostic for invalid declaration refs

If AST node is a reference to an invalid declaration let's
diagnose it as such unless some errors have been emitted
(invalid declaration itself should have a diagnostic
describing a reason why it's invalid).

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -3726,6 +3726,9 @@ ERROR(capture_across_type_decl,none,
       "%0 declaration cannot close over value %1 defined in outer scope",
       (DescriptiveDeclKind, Identifier))
 
+ERROR(reference_to_invalid_decl,none,
+      "cannot reference invalid declaration %0", (DeclName))
+
 //------------------------------------------------------------------------------
 // MARK: Type Check Statements
 //------------------------------------------------------------------------------

lib/Sema/CSDiagnostics.cpp

@@ -7002,3 +7002,21 @@ bool MissingContextualTypeForNil::diagnoseAsError() {
   emitDiagnostic(diag::unresolved_nil_literal);
   return true;
 }
+
+bool ReferenceToInvalidDeclaration::diagnoseAsError() {
+  auto *decl = castToExpr<DeclRefExpr>(getAnchor())->getDecl();
+  assert(decl);
+
+  auto &DE = getASTContext().Diags;
+  // This problem should have been already diagnosed during
+  // validation of the declaration.
+  if (DE.hadAnyError())
+    return true;
+
+  // If no errors have been emitted yet, let's emit one
+  // about reference to an invalid declaration.
+
+  emitDiagnostic(diag::reference_to_invalid_decl, decl->getName());
+  emitDiagnosticAt(decl, diag::decl_declared_here, decl->getName());
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -2281,6 +2281,21 @@ class MissingContextualTypeForNil final : public FailureDiagnostic {
   bool diagnoseAsError() override;
 };
 
+/// Diagnostic situations where AST node references an invalid declaration.
+///
+/// \code
+/// let foo = doesntExist // or something invalid
+/// foo(42)
+/// \endcode
+class ReferenceToInvalidDeclaration final : public FailureDiagnostic {
+public:
+  ReferenceToInvalidDeclaration(const Solution &solution,
+                                ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -1618,7 +1618,8 @@ SpecifyContextualTypeForNil::create(ConstraintSystem &cs,
 
 bool AllowRefToInvalidDecl::diagnose(const Solution &solution,
                                      bool asNote) const {
-  return false;
+  ReferenceToInvalidDeclaration failure(solution, getLocator());
+  return failure.diagnose(asNote);
 }
 
 AllowRefToInvalidDecl *