Revert "Basic Sema support for covariant collection casting."

Ubuntu 14.04 and 15.10 bots failing due to this commit.

This reverts commit 4b6a572.

Author: shahmishal

include/swift/Basic/LangOptions.h

@@ -143,9 +143,6 @@ namespace swift {
     /// \brief Enable experimental nested generic types feature.
     bool EnableExperimentalNestedGenericTypes = false;
 
-    /// \brief Enable generalized collection casting.
-    bool EnableExperimentalCollectionCasts = false;
-
     /// Should we check the target OSs of serialized modules to see that they're
     /// new enough?
     bool EnableTargetOSChecking = true;

include/swift/Option/FrontendOptions.td

@@ -230,10 +230,6 @@ def enable_experimental_nested_generic_types :
   Flag<["-"], "enable-experimental-nested-generic-types">,
   HelpText<"Enable experimental support for nested generic types">;
 
-def enable_experimental_collection_casts :
-  Flag<["-"], "enable-experimental-collection-casts">,
-  HelpText<"Enable experimental support for general collection casting">;
-
 def disable_availability_checking : Flag<["-"],
   "disable-availability-checking">,
   HelpText<"Disable checking for potentially unavailable APIs">;

lib/Frontend/CompilerInvocation.cpp

@@ -743,9 +743,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalNestedGenericTypes |=
     Args.hasArg(OPT_enable_experimental_nested_generic_types);
 
-  Opts.EnableExperimentalCollectionCasts |=
-    Args.hasArg(OPT_enable_experimental_collection_casts);
-
   Opts.DisableAvailabilityChecking |=
       Args.hasArg(OPT_disable_availability_checking);
 

lib/Sema/CSSimplify.cpp

@@ -3820,15 +3820,6 @@ ConstraintSystem::simplifyRestrictedConstraint(ConversionRestrictionKind restric
     Type baseType1 = getBaseTypeForArrayType(t1);
     Type baseType2 = getBaseTypeForArrayType(t2);
 
-    if (TC.getLangOpts().EnableExperimentalCollectionCasts) {
-      return matchTypes(baseType1,
-                        baseType2,
-                        matchKind,
-                        subFlags,
-                        locator.withPathElement(
-                      ConstraintLocator::PathElement::getGenericArgument(0)));
-    }
-
     // Look through type variables in the first element type; we need to know
     // it's structure before we can decide whether this can be an array upcast.
     TypeVariableType *baseTypeVar1;
@@ -3882,29 +3873,6 @@ ConstraintSystem::simplifyRestrictedConstraint(ConversionRestrictionKind restric
     Type key2, value2;
     std::tie(key2, value2) = *isDictionaryType(t2);
 
-    if (TC.getLangOpts().EnableExperimentalCollectionCasts) {
-      // The source key and value types must be subtypes of the destination
-      // key and value types, respectively.
-      auto result = matchTypes(key1, key2, matchKind, subFlags, 
-                               locator.withPathElement(
-                      ConstraintLocator::PathElement::getGenericArgument(0)));
-      if (result == SolutionKind::Error)
-        return result;
-
-      switch (matchTypes(value1, value2, matchKind, subFlags, 
-                         locator.withPathElement(
-                    ConstraintLocator::PathElement::getGenericArgument(1)))) {
-      case SolutionKind::Solved:
-        return result;
-
-      case SolutionKind::Unsolved:
-        return SolutionKind::Unsolved;
-
-      case SolutionKind::Error:
-        return SolutionKind::Error;
-      }
-    }
-
     // Look through type variables in the first key and value type; we
     // need to know their structure before we can decide whether this
     // can be an upcast.
@@ -4011,15 +3979,6 @@ ConstraintSystem::simplifyRestrictedConstraint(ConversionRestrictionKind restric
     auto t2 = type2->getDesugaredType();
     Type baseType2 = getBaseTypeForSetType(t2);
 
-    if (TC.getLangOpts().EnableExperimentalCollectionCasts) {
-      return matchTypes(baseType1,
-                        baseType2,
-                        matchKind,
-                        subFlags,
-                        locator.withPathElement(
-                      ConstraintLocator::PathElement::getGenericArgument(0)));
-    }
-
     // Look through type variables in the first base type; we need to know
     // their structure before we can decide whether this can be an upcast.
     TypeVariableType *baseTypeVar1 = nullptr;

test/SILGen/collection_subtype_downcast.swift

@@ -48,7 +48,6 @@ var aa: [[A]] = []
 var bb: [[B]] = []
 
 aa = bb // expected-error {{cannot assign value of type '[[B]]' to type '[[A]]'}}
-bb = aa // expected-error {{cannot assign value of type '[[A]]' to type '[[B]]'}}
 
 class C {
 }