[Diagnostics] Add a tailored note when passing a closure to a non-closure accepting parameter

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -388,6 +388,9 @@ ERROR(cannot_convert_argument_value_for_swift_func,none,
 NOTE(candidate_has_invalid_argument_at_position,none,
      "candidate expects %select{|in-out }2value of type %0 for parameter #%1 (got %3)",
      (Type, unsigned, bool, Type))
+NOTE(candidate_has_invalid_closure_at_position,none,
+     "closure passed to parameter of type %0 that does not "
+     "accept a closure", (Type))
 
 ERROR(cannot_convert_array_to_variadic,none,
       "cannot pass array of type %0 as variadic arguments of type %1",

lib/Sema/CSDiagnostics.cpp

@@ -7346,10 +7346,15 @@ bool ArgumentMismatchFailure::diagnoseAsError() {
 bool ArgumentMismatchFailure::diagnoseAsNote() {
   auto *locator = getLocator();
   if (auto *callee = getCallee()) {
-    emitDiagnosticAt(callee, diag::candidate_has_invalid_argument_at_position,
-                     getToType(), getParamPosition(),
-                     locator->isLastElement<LocatorPathElt::LValueConversion>(),
-                     getFromType());
+    if (isExpr<ClosureExpr>(getAnchor())) {
+      emitDiagnosticAt(callee, diag::candidate_has_invalid_closure_at_position,
+                       getToType());
+    } else {
+      emitDiagnosticAt(callee, diag::candidate_has_invalid_argument_at_position,
+                       getToType(), getParamPosition(),
+                       locator->isLastElement<LocatorPathElt::LValueConversion>(),
+                       getFromType());
+    }
     return true;
   }
 

test/Constraints/closures.swift

@@ -1247,3 +1247,14 @@ do {
 
 
 }
+
+do {
+  func test(_: Int, _: Int) {}
+  // expected-note@-1 {{closure passed to parameter of type 'Int' that does not accept a closure}}
+  func test(_: Int, _: String) {}
+  // expected-note@-1 {{closure passed to parameter of type 'String' that does not accept a closure}}
+
+  test(42) { // expected-error {{no exact matches in call to local function 'test'}}
+    print($0)
+  }
+}