Raise on Big*#to_u with negative values

spec/std/big/big_decimal_spec.cr

@@ -348,11 +348,11 @@ describe BigDecimal do
     bd6.to_i.should eq -1
 
     bd1.to_u.should eq 0
-    bd2.to_u.should eq 0
+    expect_raises(OverflowError) { bd2.to_u }
     bd3.to_u.should eq 123
-    bd4.to_u.should eq 123
-    bd5.to_u.should eq 123
-    bd6.to_u.should eq 1
+    expect_raises(OverflowError) { bd4.to_u }
+    expect_raises(OverflowError) { bd5.to_u }
+    expect_raises(OverflowError) { bd6.to_u }
 
     bd1.to_f.should eq 0.00123
     bd2.to_f.should eq -0.00123

spec/std/big/big_float_spec.cr

@@ -188,6 +188,9 @@ describe "BigFloat" do
     it { 1.34.to_big_f.to_u.should eq(1) }
     it { 123.to_big_f.to_u.should eq(123) }
     it { 4321.to_big_f.to_u.should eq(4321) }
+    it do
+      expect_raises(OverflowError) { -123.34.to_big_f.to_u }
+    end
   end
 
   describe "to_u!" do

spec/std/big/big_int_spec.cr

@@ -363,6 +363,8 @@ describe "BigInt" do
     big.to_u16!.should eq(722)
     big.to_u32.should eq(1234567890)
 
+    expect_raises(OverflowError) { BigInt.new(-1234567890).to_u }
+
     u64 = big.to_u64
     u64.should eq(1234567890)
     u64.should be_a(UInt64)

src/big/big_decimal.cr

@@ -410,39 +410,39 @@ struct BigDecimal < Number
   end
 
   private def to_big_u
+    raise OverflowError.new if self < 0
+    to_big_u!
+  end
+
+  private def to_big_u!
     (@value.abs // TEN ** @scale)
   end
 
-  # Converts to `UInt64`. Truncates anything on the right side of the decimal point,
-  # converting negative to positive.
+  # Converts to `UInt64`. Truncates anything on the right side of the decimal point.
   # Raises `OverflowError` in case of overflow.
   def to_u64
     to_big_u.to_u64
   end
 
-  # Converts to `UInt32`. Truncates anything on the right side of the decimal point,
-  # converting negative to positive.
+  # Converts to `UInt32`. Truncates anything on the right side of the decimal point.
   # Raises `OverflowError` in case of overflow.
   def to_u32
     to_big_u.to_u32
   end
 
-  # Converts to `UInt16`. Truncates anything on the right side of the decimal point,
-  # converting negative to positive.
+  # Converts to `UInt16`. Truncates anything on the right side of the decimal point.
   # Raises `OverflowError` in case of overflow.
   def to_u16
     to_big_u.to_u16
   end
 
-  # Converts to `UInt8`. Truncates anything on the right side of the decimal point,
-  # converting negative to positive.
+  # Converts to `UInt8`. Truncates anything on the right side of the decimal point.
   # Raises `OverflowError` in case of overflow.
   def to_u8
     to_big_u.to_u8
   end
 
-  # Converts to `UInt32`. Truncates anything on the right side of the decimal point,
-  # converting negative to positive.
+  # Converts to `UInt32`. Truncates anything on the right side of the decimal point.
   # Raises `OverflowError` in case of overflow.
   def to_u
     to_u32
@@ -452,28 +452,28 @@ struct BigDecimal < Number
   # converting negative to positive.
   # In case of overflow a wrapping is performed.
   def to_u8!
-    to_big_u.to_u8!
+    to_big_u!.to_u8!
   end
 
   # Converts to `UInt16`. Truncates anything on the right side of the decimal point,
   # converting negative to positive.
   # In case of overflow a wrapping is performed.
   def to_u16!
-    to_big_u.to_u16!
+    to_big_u!.to_u16!
   end
 
   # Converts to `UInt32`. Truncates anything on the right side of the decimal point,
   # converting negative to positive.
   # In case of overflow a wrapping is performed.
   def to_u32!
-    to_big_u.to_u32!
+    to_big_u!.to_u32!
   end
 
   # Converts to `UInt64`. Truncates anything on the right side of the decimal point,
   # converting negative to positive.
   # In case of overflow a wrapping is performed.
   def to_u64!
-    to_big_u.to_u64!
+    to_big_u!.to_u64!
   end
 
   # Converts to `UInt32`. Truncates anything on the right side of the decimal point,

src/big/big_float.cr

@@ -234,6 +234,7 @@ struct BigFloat < Float
   end
 
   def to_u64
+    raise OverflowError.new if self < 0
     LibGMP.mpf_get_ui(self)
   end
 

src/big/big_int.cr

@@ -500,10 +500,11 @@ struct BigInt < Int
   end
 
   def to_u32
-    LibGMP.get_ui(self).to_u32
+    to_u64.to_u32
   end
 
   def to_u64
+    raise OverflowError.new if self < 0
     if LibGMP::ULong == UInt64 || (UInt32::MIN <= self <= UInt32::MAX)
       LibGMP.get_ui(self).to_u64
     else