[Sema][Diagnostics] Improving diagnostics for function type coerce failure

Author: LucianoPAlmeida

lib/Sema/CSDiagnostics.cpp

@@ -2761,6 +2761,17 @@ bool ContextualFailure::diagnoseAsNote() {
  }
  }
 
+ // Note that mentions candidate type number of parameters diff.
+ auto fromFnType = getFromType()->getAs<FunctionType>();
+ auto toFnType = getToType()->getAs<FunctionType>();
+ if (fromFnType && toFnType &&
+ fromFnType->getNumParams() > toFnType->getNumParams()) {
+ emitDiagnosticAt(decl, diag::candidate_with_extraneous_args, fromFnType,
+ fromFnType->getNumParams(), toFnType,
+ toFnType->getNumParams());
+ return true;
+ }
+
  emitDiagnosticAt(decl, diag::found_candidate_type, getFromType());
  return true;
 }

lib/Sema/CSSimplify.cpp

@@ -3425,18 +3425,36 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
  if (!anchor)
  return getTypeMatchFailure(argumentLocator);
 
- // If there are missing arguments, let's add them
- // using parameter as a template.
- if (diff < 0) {
- if (fixMissingArguments(*this, anchor, func1Params, func2Params,
- abs(diff), loc))
+ // The param diff is in a function type coercion context
+ //
+ // func fn(_: Int) {}
+ // let i: Int = 0
+ // (fn as (Int, Int) -> Void)(i, i)
+ //
+ // Since we are not in a function argument application, so simply record
+ // a function type mismatch instead of an argument fix.
+ // Except for when a closure is a subexpr because missing and extraneous
+ // arg fix can properly handle closure diagnostic.
+ if (loc->isForCoercion() && !isExpr<ClosureExpr>(anchor)) {
+ auto *fix = ContextualMismatch::create(*this, func1, func2, loc);
+ if (recordFix(fix))
  return getTypeMatchFailure(argumentLocator);
  } else {
- // If there are extraneous arguments, let's remove
- // them from the list.
- if (fixExtraneousArguments(*this, func2, func1Params, diff, loc))
- return getTypeMatchFailure(argumentLocator);
+ // If there are missing arguments, let's add them
+ // using parameter as a template.
+ if (diff < 0) {
+ if (fixMissingArguments(*this, anchor, func1Params, func2Params,
+ abs(diff), loc))
+ return getTypeMatchFailure(argumentLocator);
+ } else {
+ // If there are extraneous arguments, let's remove
+ // them from the list.
+ if (fixExtraneousArguments(*this, func2, func1Params, diff, loc))
+ return getTypeMatchFailure(argumentLocator);
+ }
+ }
 
+ if (diff > 0) {
  // Drop all of the extraneous arguments.
  auto numParams = func2Params.size();
  func1Params.erase(func1Params.begin() + numParams, func1Params.end());