🛡️Restrict number extension methods to INumber<T> (#1281)

Fixes #1269 

Quantities implement `IConvertible.ToDouble()` by converting the `Value`
property to `double`, and similar for other number types.

Number extension methods expects a number argument, but take any
argument type and call `Convert.ToDouble()` on it. This means you can
run into subtle bugs:

```cs
Length.FromCentimeters(100).Millimeters(); // Length of 100 mm
Length.FromCentimeters(100).Millimeters; // Value of 1000
```

The quantity implements conversions like this:
```cs
        double IConvertible.ToDouble(IFormatProvider? provider)
        {
            return Convert.ToDouble(_value);
        }
```

The extension method looks like this:
```cs
public static Length Millimeters<T>(this T value)
{
	return Length.FromMillimeters(Convert.ToDouble(value));
}
```

So `Convert.ToDouble()` happily converts `Length.FromCentimeters(100)`
to `100`.


### Changes
- Restrict number extension methods to `INumber<T>` for net7.0 and later

Author: angularsen

CodeGen/Generators/UnitsNetGen/NumberExtensionsGenerator.cs

@@ -25,6 +25,10 @@ public string Generate()
 $@"
 using System;
 
+#if NET7_0_OR_GREATER
+using System.Numerics;
+#endif
+
 #nullable enable
 
 namespace UnitsNet.NumberExtensions.NumberTo{_quantityName}
@@ -45,8 +49,12 @@ public static class NumberTo{_quantityName}Extensions
 
                 Writer.WLIfText(2, GetObsoleteAttributeOrNull(unit.ObsoleteText));
 
-                Writer.WL(2, $@"public static {_quantityName} {unit.PluralName}<T>(this T value) =>
-            {_quantityName}.From{unit.PluralName}(Convert.ToDouble(value));
+                Writer.WL(2, $@"public static {_quantityName} {unit.PluralName}<T>(this T value)
+            where T : notnull
+#if NET7_0_OR_GREATER
+            , INumber<T>
+#endif
+            => {_quantityName}.From{unit.PluralName}(Convert.ToDouble(value));
 ");
             }
 

UnitsNet.NumberExtensions.Tests/UnitsNet.NumberExtensions.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net7.0</TargetFramework>
     <RootNamespace>UnitsNet.NumberExtensions.Tests</RootNamespace>
     <LangVersion>latest</LangVersion>
     <Nullable>enable</Nullable>