[spec/type] Document integer implicit conversions

Author: ntrel

spec/type.dd

@@ -460,21 +460,47 @@ void test()
 
 $(H3 $(LNAME2 disallowed-conversions, Preserving Bit Patterns))
 
+    $(P An integer of type `I` implicitly converts to another integer type `J` when
+    `J.sizeof >= I.sizeof`.)
+
+$(SPEC_RUNNABLE_EXAMPLE_FAIL
+---
+void f(byte b, ubyte ub, short s)
+{
+    b = ub; // OK, bit pattern same
+    ub = b; // OK, bit pattern same
+    s = b; // OK, widening conversion
+    b = s; // error, implicit narrowing
+}
+---
+)
+
     $(P Integer values cannot be implicitly converted to another
-    type that cannot represent the integer bit pattern after integral
-    promotion. For example:)
+    type that cannot represent the integer bit pattern after
+    $(RELATIVE_LINK2 integer-promotions, integral promotion). For example:)
 
 $(SPEC_RUNNABLE_EXAMPLE_FAIL
 ---
-ubyte  u1 = -1;       // error, -1 cannot be represented in a ubyte
-ushort u2 = -1;       // error, -1 cannot be represented in a ushort
-uint   u3 = int(-1);  // ok, -1 can be represented in an int, which can be converted to a uint
-ulong  u4 = long(-1); // ok, -1 can be represented in a long, which can be converted to a ulong
+ubyte  u1 = -1; // error, -1 cannot be represented in a ubyte
+ushort u2 = -1; // error, -1 cannot be represented in a ushort
+uint   u3 = -1; // ok, -1 can be represented in an int, which can be converted to a uint
+ulong  u4 = -1; // ok, -1 can be represented in a long, which can be converted to a ulong
 ---
 )
 
-    * Floating point types cannot be implicitly converted to
-      integral types.
+    * Integral types implicitly convert to floating point types.
+    * Floating point types cannot be implicitly converted to integral types.
+
+    $(SPEC_RUNNABLE_EXAMPLE_FAIL
+    ---
+    void f(int i, float f)
+    {
+        f = i; // OK
+        i = f; // error
+    }
+    ---
+    )
+
     * Complex or imaginary floating point types cannot be implicitly converted
       to non-complex floating point types.
     * Non-complex floating point types cannot be implicitly converted to imaginary floating