checker: restrict numeric promotions to cases where no data is lost

doc/docs.md

@@ -269,6 +269,8 @@ rune // represents a Unicode code point
 
 f32 f64
 
+any_int, any_float // internal intermediate types of number literals
+
 byteptr // these two are mostly used for C interop
 voidptr
 
@@ -277,6 +279,27 @@ any // similar to C's void* and Go's interface{}
 
 Please note that unlike C and Go, `int` is always a 32 bit integer.
 
+There is an exceptions to the rule that all operators
+in V must have values of the same type on both sides. A small primitive type
+on one side can be automatically promoted if it fits
+completely into the data range of the type on the other side, i.e. when
+the promotion does not result in any data loss.
+These are the allowed possibilities:
+
+```
+   i8 → i16 → int → i64
+            ↘     ↘
+              f32 → f64
+            ↗     ↗
+ byte → u16 → u32 → u64 ⬎
+      ↘     ↘     ↘      ptr
+   i8 → i16 → int → i64 ⬏
+```
+An `int` value for example can be automatically promoted to `f64`
+or `i64` but not to `f32` or `u32`. (`f32` would mean precission
+loss for large values and `u32` would mean loss of the sign for
+negative values).
+
 ## Strings
 
 ```v