[AST] Canonicalize original type for ErrorType

Make sure we canonicalize the original type for an ErrorType to ensure
that diagnostic logic can coalesce ErrorTypes that have the same
canonical type.

Author: hamishknight

include/swift/AST/Types.h

@@ -1668,9 +1668,10 @@ class ErrorType final : public TypeBase {
     return props;
   }
 
-  // The Error type is always canonical.
   ErrorType(ASTContext &C, Type originalType)
-      : TypeBase(TypeKind::Error, &C, getProperties(originalType)) {
+      : TypeBase(TypeKind::Error,
+                 (!originalType || originalType->isCanonical()) ? &C : nullptr,
+                 getProperties(originalType)) {
     if (originalType) {
       Bits.ErrorType.HasOriginalType = true;
       *reinterpret_cast<Type *>(this + 1) = originalType;

lib/AST/Type.cpp

@@ -1765,7 +1765,6 @@ CanType TypeBase::computeCanonicalType() {
 #define ALWAYS_CANONICAL_TYPE(id, parent) case TypeKind::id:
 #define TYPE(id, parent)
 #include "swift/AST/TypeNodes.def"
-  case TypeKind::Error:
   case TypeKind::TypeVariable:
   case TypeKind::Placeholder:
   case TypeKind::BuiltinTuple:
@@ -1779,6 +1778,14 @@ CanType TypeBase::computeCanonicalType() {
 #define TYPE(id, parent)
 #include "swift/AST/TypeNodes.def"
 
+  case TypeKind::Error: {
+    auto errTy = cast<ErrorType>(this);
+    auto originalTy = errTy->getOriginalType();
+    ASSERT(originalTy && "The bare ErrorType singleton is already canonical");
+    Result = ErrorType::get(originalTy->getCanonicalType()).getPointer();
+    break;
+  }
+
   case TypeKind::Enum:
   case TypeKind::Struct:
   case TypeKind::Class:

test/Constraints/tuple_arguments.swift

@@ -1728,7 +1728,7 @@ do {
 do {
   func f(_: Int...) {}
   let _ = [(1, 2, 3)].map(f) // expected-error {{no exact matches in call to instance method 'map'}}
-  // expected-note@-1 2{{found candidate with type '(((Int, Int, Int)) -> T) -> [T]'}}
+  // expected-note@-1 {{found candidate with type '(((Int, Int, Int)) -> T) -> [T]'}}
 }
 
 // rdar://problem/48443263 - cannot convert value of type '() -> Void' to expected argument type '(_) -> Void'