[Diagnostics] Add a diagnostic for incorrect use of trailing closures

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -5491,3 +5491,20 @@ bool ExtraneousCallFailure::diagnoseAsError() {
   removeParensFixIt(diagnostic);
   return true;
 }
+
+bool InvalidUseOfTrailingClosure::diagnoseAsError() {
+  auto *anchor = getAnchor();
+  auto &cs = getConstraintSystem();
+
+  emitDiagnostic(anchor->getLoc(), diag::trailing_closure_bad_param,
+                 getToType())
+      .highlight(anchor->getSourceRange());
+
+  if (auto overload = getChoiceFor(cs.getCalleeLocator(getLocator()))) {
+    if (auto *decl = overload->choice.getDeclOrNull()) {
+      emitDiagnostic(decl, diag::decl_declared_here, decl->getFullName());
+    }
+  }
+
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -1921,6 +1921,15 @@ class ExtraneousCallFailure final : public FailureDiagnostic {
   bool diagnoseAsError() override;
 };
 
+class InvalidUseOfTrailingClosure final : public ArgumentMismatchFailure {
+public:
+  InvalidUseOfTrailingClosure(Expr *root, ConstraintSystem &cs, Type argType,
+                              Type paramType, ConstraintLocator *locator)
+      : ArgumentMismatchFailure(root, cs, argType, paramType, locator) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -996,7 +996,10 @@ RemoveInvalidCall *RemoveInvalidCall::create(ConstraintSystem &cs,
 }
 
 bool AllowInvalidUseOfTrailingClosure::diagnose(Expr *expr, bool asNote) const {
-  return false;
+  auto &cs = getConstraintSystem();
+  InvalidUseOfTrailingClosure failure(expr, cs, getFromType(), getToType(),
+                                      getLocator());
+  return failure.diagnose(asNote);
 }
 
 AllowInvalidUseOfTrailingClosure *