[ConstraintSystem] Fix: Try to match argument to element type if parameter is a collection

This allows better diagnostics in cases where collection element
is a generic parameter which is supposed to be inferred from argument e.g.

```swift
func foo<T>(_: [T]) {}

foo(1) // Missing brackets, so error should be about [Int] vs. Int
```

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -2659,6 +2659,31 @@ bool ConstraintSystem::repairFailures(
       break;
     }
 
+    // If parameter is a collection but argument is not, let's try
+    // to try and match collection element type to the argument to
+    // produce better diagnostics e.g.:
+    //
+    // ```
+    // func foo<T>(_: [T]) {}
+    // foo(1) // expected '[Int]', got 'Int'
+    // ```
+    if (isCollectionType(rhs)) {
+      std::function<Type(Type)> getArrayOrSetType = [&](Type type) -> Type {
+        if (auto eltTy = isArrayType(type))
+          return getArrayOrSetType(*eltTy);
+
+        if (auto eltTy = isSetType(type))
+          return getArrayOrSetType(*eltTy);
+
+        return type;
+      };
+
+      // Let's ignore any optional types associated with element e.g. `[T?]`
+      auto rhsEltTy = getArrayOrSetType(rhs)->lookThroughAllOptionalTypes();
+      (void)matchTypes(lhs, rhsEltTy, ConstraintKind::Equal, TMF_ApplyingFix,
+                       locator);
+    }
+
     conversionsOrFixes.push_back(
         AllowArgumentMismatch::create(*this, lhs, rhs, loc));
     break;