[Sema] Improve extra trailing closure diagnostic

Emit the specialized extraneous trailing closure
diagnostic for multiple trailing closures and
unlabelled unary argument lists, and add a
specialized trailing closure version for the multiple
extraneous case.

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -1267,7 +1267,8 @@ ERROR(extra_argument_named,none,
 ERROR(extra_argument_positional,none,
       "extra argument in call", ())
 ERROR(extra_arguments_in_call,none,
-      "extra arguments at positions %0 in call", (StringRef))
+      "extra %select{arguments|trailing closures}0 at positions %1 in call",
+       (bool, StringRef))
 ERROR(extra_argument_to_nullary_call,none,
       "argument passed to call that takes no arguments", ())
 ERROR(extra_trailing_closure_in_call,none,

lib/Sema/CSDiagnostics.cpp

@@ -5197,7 +5197,16 @@ bool ExtraneousArgumentsFailure::diagnoseAsError() {
       },
       [&] { OS << ", "; });
 
-  emitDiagnostic(diag::extra_arguments_in_call, OS.str());
+  bool areTrailingClosures = false;
+  if (auto *argExpr = getArgumentListExprFor(getLocator())) {
+    if (auto i = argExpr->getUnlabeledTrailingClosureIndexOfPackedArgument()) {
+      areTrailingClosures = llvm::all_of(ExtraArgs, [&](auto &pair) {
+        return pair.first >= i;
+      });
+    }
+  }
+
+  emitDiagnostic(diag::extra_arguments_in_call, areTrailingClosures, OS.str());
 
   if (auto overload = getCalleeOverloadChoiceIfAvailable(getLocator())) {
     if (auto *decl = overload->choice.getDeclOrNull()) {
@@ -5248,9 +5257,12 @@ bool ExtraneousArgumentsFailure::diagnoseSingleExtraArgument() const {
   auto argExpr = tuple ? tuple->getElement(index)
                        : cast<ParenExpr>(arguments)->getSubExpr();
 
+  auto trailingClosureIdx =
+      arguments->getUnlabeledTrailingClosureIndexOfPackedArgument();
+  auto isTrailingClosure = trailingClosureIdx && index >= *trailingClosureIdx;
+
   auto loc = argExpr->getLoc();
-  if (tuple && index == tuple->getNumElements() - 1 &&
-      tuple->hasTrailingClosure()) {
+  if (isTrailingClosure) {
     emitDiagnosticAt(loc, diag::extra_trailing_closure_in_call)
         .highlight(argExpr->getSourceRange());
   } else if (ContextualType->getNumParams() == 0) {

test/Constraints/argument_matching.swift

@@ -1762,3 +1762,23 @@ func rdar70764991() {
     bar(str, S.foo) // expected-error {{unnamed argument #1 must precede unnamed argument #2}}  {{9-12=}} {{14-14=str}}
   }
 }
+
+func testExtraTrailingClosure() {
+  func foo() {}
+  foo() {} // expected-error@:9 {{extra trailing closure passed in call}}
+  foo {} // expected-error@:7 {{extra trailing closure passed in call}}
+  foo {} x: {} // expected-error@:7 {{argument passed to call that takes no arguments}}
+
+  func bar(_ x: Int) {} // expected-note 2{{'bar' declared here}}
+  bar(5) {} // expected-error@:10 {{extra trailing closure passed in call}}
+  bar(0) {} x: {} // expected-error@:6 {{extra trailing closures at positions #2, #3 in call}}
+  bar(5, "") {} // expected-error@:6 {{extra arguments at positions #2, #3 in call}}
+
+  func baz(_ fn: () -> Void) {} // expected-note {{'baz' declared here}}
+  baz {} x: {} // expected-error@:13 {{extra trailing closure passed in call}}
+  baz({}) {} // expected-error@:11 {{extra trailing closure passed in call}}
+  baz({}) {} y: {} // expected-error@:6 {{extra trailing closures at positions #2, #3 in call}}
+
+  func qux(x: () -> Void, y: () -> Void, z: () -> Void) {} // expected-note {{'qux(x:y:z:)' declared here}}
+  qux() {} m: {} y: {} n: {} z: {} o: {} // expected-error@:6 {{extra trailing closures at positions #2, #4, #6 in call}}
+}

test/Parse/trailing_closures.swift

@@ -116,7 +116,7 @@ func f() -> Int { 42 }
 // This should be interpreted as a trailing closure, instead of being 
 // interpreted as a computed property with undesired initial value.
 struct TrickyTest {
-    var x : Int = f () { // expected-error {{argument passed to call that takes no arguments}}
+    var x : Int = f () { // expected-error {{extra trailing closure passed in call}}
         3
     }
 }