[QoI] Use information known about call parameters via failed constraint

This makes it a lot easier to diagnose contextual mismatch related
to arguments used by the call without relying on the type of the
function expression which is not always available.

Author: xedin

lib/Sema/CSDiag.cpp

@@ -6367,6 +6367,23 @@ bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
     }
   }
 
+  // If there is a failing constraint associated with current constraint
+  // system which points to the argument/parameter mismatch, let's use
+  // that information while re-typechecking argument expression, this
+  // makes it a lot easier to determine contextual mismatch.
+  if (CS->failedConstraint && !hasTrailingClosure) {
+    auto *constraint = CS->failedConstraint;
+    if (constraint->getKind() == ConstraintKind::ArgumentTupleConversion) {
+      if (auto *locator = constraint->getLocator()) {
+        if (locator->getAnchor() == callExpr) {
+          argType = constraint->getSecondType();
+          if (auto *typeVar = argType->getAs<TypeVariableType>())
+            argType = CS->getFixedType(typeVar);
+        }
+      }
+    }
+  }
+
   // Get the expression result of type checking the arguments to the call
   // independently, so we have some idea of what we're working with.
   //

test/Compatibility/tuple_arguments.swift

@@ -1460,7 +1460,7 @@ enum DataSourcePage<T> {
 }
 
 let pages1: MutableProperty<(data: DataSourcePage<Int>, totalCount: Int)> = MutableProperty((
-    // expected-error@-1 {{cannot convert value of type 'MutableProperty<(data: _, totalCount: Int)>' to specified type 'MutableProperty<(data: DataSourcePage<Int>, totalCount: Int)>'}}
+    // expected-error@-1 {{expression type 'MutableProperty<(data: DataSourcePage<Int>, totalCount: Int)>' is ambiguous without more context}}
     data: .notLoaded,
     totalCount: 0
 ))

test/Constraints/function.swift

@@ -90,12 +90,12 @@ private class SR2657BlockClass<T> {
   init(f: T) { self.f = f }
 }
 
-func foo(block: () -> ()) {
+func foo(block: () -> ()) { // expected-note 2 {{parameter 'block' is implicitly non-escaping}}
   let a = SR2657BlockClass(f: block) // No error
   let b = SR2657BlockClass<()->()>(f: block)
-  // expected-error@-1 {{invalid conversion from non-escaping function of type '() -> ()' to potentially escaping function type '() -> ()'}}
+  // expected-error@-1 {{passing non-escaping parameter 'block' to function expecting an @escaping closure}}
   let c: SR2657BlockClass<()->()> = SR2657BlockClass(f: block)
   // expected-error@-1 {{cannot convert value of type 'SR2657BlockClass<() -> ()>' to specified type 'SR2657BlockClass<() -> ()>'}}
   let d: SR2657BlockClass<()->()> = SR2657BlockClass<()->()>(f: block)
-  // expected-error@-1 {{invalid conversion from non-escaping function of type '() -> ()' to potentially escaping function type '() -> ()'}}
+  // expected-error@-1 {{passing non-escaping parameter 'block' to function expecting an @escaping closure}}
 }

test/Generics/invalid.swift

@@ -54,10 +54,8 @@ func eatDinnerConcrete(d: Pizzas<Pepper>.DeepDish,
 func badDiagnostic1() {
 
   _ = Lunch<Pizzas<Pepper>.NewYork>.Dinner<HotDog>(
-      leftovers: Pizzas<ChiliFlakes>.NewYork(),
-      // FIXME: Quality of this diagnostic is related to the fact that failure analyzer
-      //        doesn't take advantage of informantion in constraint system such as failed constraint.
-      transformation: { _ in HotDog() }) // expected-error {{cannot convert value of type '(_) -> HotDog' to expected argument type '(_) -> _'}}
+      leftovers: Pizzas<ChiliFlakes>.NewYork(),  // expected-error {{cannot convert value of type 'Pizzas<ChiliFlakes>.NewYork' to expected argument type 'Pizzas<Pepper>.NewYork'}}
+      transformation: { _ in HotDog() })
 }
 
 func badDiagnostic2() {