[Sema] Improve diagnostics for #selector() expressions

When the `#selector()` expression refers to a parameter or variable, we
don't want to call it a property.

Fixes the rest of SR-880.

Author: lilyball

include/swift/AST/DiagnosticsSema.def

@@ -367,8 +367,8 @@ ERROR(expr_selector_module_missing,none,
       "import the 'ObjectiveC' module to use '#selector'", ())
 ERROR(expr_selector_no_declaration,none,
       "argument of '#selector' does not refer to an initializer or method", ())
-ERROR(expr_selector_property,none,
-      "argument of '#selector' cannot refer to a property", ())
+ERROR(expr_selector_var,none,
+      "argument of '#selector' cannot refer to a %select{property|parameter|variable}0", (int))
 ERROR(expr_selector_not_method_or_init,none,
       "argument of '#selector' does not refer to a method or initializer", ())
 ERROR(expr_selector_not_objc,none,

lib/Sema/CSApply.cpp

@@ -3342,11 +3342,20 @@ namespace {
       // complain.
       auto func = dyn_cast<AbstractFunctionDecl>(foundDecl);
       if (!func) {
-        tc.diagnose(E->getLoc(),
-                    isa<VarDecl>(foundDecl)
-                      ? diag::expr_selector_property
-                      : diag::expr_selector_not_method_or_init)
-          .highlight(subExpr->getSourceRange());
+        Optional<InFlightDiagnostic> diag;
+        if (auto VD = dyn_cast<VarDecl>(foundDecl)) {
+          diag.emplace(tc.diagnose(E->getLoc(), diag::expr_selector_var,
+                                   isa<ParamDecl>(VD)
+                                   ? 1
+                                   : VD->getDeclContext()->isTypeContext()
+                                     ? 0
+                                     : 2));
+        } else {
+          diag.emplace(tc.diagnose(E->getLoc(),
+                                   diag::expr_selector_not_method_or_init));
+        }
+        diag->highlight(subExpr->getSourceRange());
+        diag.reset();
         tc.diagnose(foundDecl, diag::decl_declared_here,
                     foundDecl->getFullName());
         return E;

test/expr/unary/selector/selector.swift

@@ -26,6 +26,14 @@ class C1 {
     _ = testUnqualifiedSelector // suppress unused warning
   }
 
+  @objc func testParam(testParam: A) { // expected-note{{'testParam' declared here}}
+    _ = #selector(testParam) // expected-error{{argument of '#selector' cannot refer to a parameter}}
+  }
+
+  @objc func testVariable() {
+    let testVariable = 1 // expected-note{{'testVariable' declared here}}
+    _ = #selector(testVariable) // expected-error{{argument of '#selector' cannot refer to a variable}}
+  }
 }
 
 @objc protocol P1 {