[CSBindings] Prevent BindingSet::isViable from dropping viable bindings (v2)

The original attempt to do this was reverted by #77653

The problem is that the fix was too broad, I narrowed it down to
non-exact uses of stdlib collections that support upcasts.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -31,6 +31,12 @@ using namespace swift;
 using namespace constraints;
 using namespace inference;
 
+/// Check whether there exists a type that could be implicitly converted
+/// to a given type i.e. is the given type is Double or Optional<..> this
+/// function is going to return true because CGFloat could be converted
+/// to a Double and non-optional value could be injected into an optional.
+static bool hasConversions(Type);
+
 static std::optional<Type> checkTypeOfBinding(TypeVariableType *typeVar,
                                               Type type);
 
@@ -1209,7 +1215,31 @@ bool BindingSet::isViable(PotentialBinding &binding, bool isTransitive) {
     if (!existingNTD || NTD != existingNTD)
       continue;
 
-    // FIXME: What is going on here needs to be thoroughly re-evaluated.
+    // What is going on in this method needs to be thoroughly re-evaluated!
+    //
+    // This logic aims to skip dropping bindings if
+    // collection type has conversions i.e. in situations like:
+    //
+    // [$T1] conv $T2
+    // $T2 conv [(Int, String)]
+    // $T2.Element equal $T5.Element
+    //
+    // `$T1` could be bound to `(i: Int, v: String)` after
+    // `$T2` is bound to `[(Int, String)]` which is is a problem
+    // because it means that `$T2` was attempted to early
+    // before the solver had a chance to discover all viable
+    // bindings.
+    //
+    // Let's say existing binding is `[(Int, String)]` and
+    // relation is "exact", in this case there is no point
+    // tracking `[$T1]` because upcasts are only allowed for
+    // subtype and other conversions.
+    if (existing->Kind != AllowedBindingKind::Exact) {
+      if (existingType->isKnownStdlibCollectionType() &&
+          hasConversions(existingType)) {
+        continue;
+      }
+    }
 
     // If new type has a type variable it shouldn't
     // be considered  viable.