Support comparable aggregations against base types

Looks like we were limited, previously, by the specific expression
types.

Fixes stephencelis#277.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>

Author: stephencelis

Source/Typed/AggregateFunctions.swift

@@ -84,7 +84,7 @@ extension ExpressionType where UnderlyingType : _OptionalType, UnderlyingType.Wr
 
 }
 
-extension ExpressionType where UnderlyingType : protocol<Value, Comparable> {
+extension ExpressionType where UnderlyingType : Value, UnderlyingType.Datatype : Comparable {
 
     /// Builds a copy of the expression wrapped with the `max` aggregate
     /// function.
@@ -114,7 +114,7 @@ extension ExpressionType where UnderlyingType : protocol<Value, Comparable> {
 
 }
 
-extension ExpressionType where UnderlyingType : _OptionalType, UnderlyingType.WrappedType : protocol<Value, Comparable> {
+extension ExpressionType where UnderlyingType : _OptionalType, UnderlyingType.WrappedType : Value, UnderlyingType.WrappedType.Datatype : Comparable {
 
     /// Builds a copy of the expression wrapped with the `max` aggregate
     /// function.

Tests/AggregateFunctionsTests.swift

@@ -25,6 +25,8 @@ class AggregateFunctionsTests : XCTestCase {
         AssertSQL("max(\"doubleOptional\")", doubleOptional.max)
         AssertSQL("max(\"string\")", string.max)
         AssertSQL("max(\"stringOptional\")", stringOptional.max)
+        AssertSQL("max(\"date\")", date.max)
+        AssertSQL("max(\"dateOptional\")", dateOptional.max)
     }
 
     func test_min_wrapsComparableExpressionsWithMinFunction() {
@@ -34,6 +36,8 @@ class AggregateFunctionsTests : XCTestCase {
         AssertSQL("min(\"doubleOptional\")", doubleOptional.min)
         AssertSQL("min(\"string\")", string.min)
         AssertSQL("min(\"stringOptional\")", stringOptional.min)
+        AssertSQL("min(\"date\")", date.min)
+        AssertSQL("min(\"dateOptional\")", dateOptional.min)
     }
 
     func test_average_wrapsNumericExpressionsWithAvgFunction() {