[AST] Allocate PlaceholderTypes in the correct arena

We shouldn't be allocating placeholders for
type variables in the permanent arena, and we
should be caching them such that equality works.

Author: hamishknight

lib/AST/ASTContext.cpp

@@ -431,6 +431,7 @@ struct ASTContext::Implementation {
     llvm::DenseMap<Type, InOutType*> InOutTypes;
     llvm::DenseMap<std::pair<Type, void*>, DependentMemberType *>
       DependentMemberTypes;
+    llvm::DenseMap<void *, PlaceholderType *> PlaceholderTypes;
     llvm::DenseMap<Type, DynamicSelfType *> DynamicSelfTypes;
     llvm::DenseMap<std::pair<EnumDecl*, Type>, EnumType*> EnumTypes;
     llvm::DenseMap<std::pair<StructDecl*, Type>, StructType*> StructTypes;
@@ -3125,8 +3126,27 @@ Type ErrorType::get(Type originalType) {
 
 Type PlaceholderType::get(ASTContext &ctx, Originator originator) {
   assert(originator);
-  return new (ctx, AllocationArena::Permanent)
+
+  auto hasTypeVariables = [&]() -> bool {
+    if (originator.is<TypeVariableType *>())
+      return true;
+
+    if (auto *depTy = originator.dyn_cast<DependentMemberType *>())
+      return depTy->hasTypeVariable();
+
+    return false;
+  }();
+  auto arena = hasTypeVariables ? AllocationArena::ConstraintSolver
+                                : AllocationArena::Permanent;
+
+  auto &cache = ctx.getImpl().getArena(arena).PlaceholderTypes;
+  auto &entry = cache[originator.getOpaqueValue()];
+  if (entry)
+    return entry;
+
+  entry = new (ctx, arena)
       PlaceholderType(ctx, originator, RecursiveTypeProperties::HasPlaceholder);
+  return entry;
 }
 
 BuiltinIntegerType *BuiltinIntegerType::get(BuiltinIntegerWidth BitWidth,

test/Constraints/rdar44770297.swift

@@ -8,4 +8,8 @@ func foo<T: P>(_: () throws -> T) -> T.A? { // expected-note {{where 'T' = 'Neve
   fatalError()
 }
 
-let _ = foo() {fatalError()} & nil // expected-error {{global function 'foo' requires that 'Never' conform to 'P'}}
+let _ = foo() {fatalError()} & nil
+// expected-error@-1 {{global function 'foo' requires that 'Never' conform to 'P'}}
+// expected-error@-2 {{value of optional type 'Never.A?' must be unwrapped to a value of type 'Never.A'}}
+// expected-note@-3 {{coalesce using '??' to provide a default when the optional value contains 'nil'}}
+// expected-note@-4 {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}