better type inference for numerical types; prerequisitive for version 1

changelog.md

@@ -235,3 +235,6 @@ styledEcho "Red on Green.", resetStyle
 - ``\n`` is now only the single line feed character like in most
   other programming languages. The new platform specific newline escape sequence is
   written as ``\p``. This change only affects the Windows platform.
+- Type inference for generic type parameters involving numeric types is now symetric. See
+  [Generic type inference for numeric types](https://nim-lang.org/docs/manual.html#generics-generic-type-inference-fornumeric-types)
+  for more information.

compiler/sigmatch.nim

@@ -913,6 +913,25 @@ proc isCovariantPtr(c: var TCandidate, f, a: PType): bool =
   else:
     return false
 
+proc maxNumericType(prev, candidate: PType): PType =
+  let c = candidate.skipTypes({tyRange})
+  template greater(s) =
+    if c.kind in s: result = c
+  case prev.kind
+  of tyInt: greater({tyInt64})
+  of tyInt8: greater({tyInt, tyInt16, tyInt32, tyInt64})
+  of tyInt16: greater({tyInt, tyInt32, tyInt64})
+  of tyInt32: greater({tyInt64})
+
+  of tyUInt: greater({tyUInt64})
+  of tyUInt8: greater({tyUInt, tyUInt16, tyUInt32, tyUInt64})
+  of tyUInt16: greater({tyUInt, tyUInt32, tyUInt64})
+  of tyUInt32: greater({tyUInt64})
+
+  of tyFloat32: greater({tyFloat64, tyFloat128})
+  of tyFloat64: greater({tyFloat128})
+  else: discard
+
 proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
                  flags: TTypeRelFlags = {}): TTypeRelation =
   # typeRel can be used to establish various relationships between types:
@@ -1602,9 +1621,16 @@ proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
     elif x.kind == tyGenericParam:
       result = isGeneric
     else:
-      result = typeRel(c, x, a) # check if it fits
-      if result > isGeneric: result = isGeneric
-
+      # Special type binding rule for numeric types.
+      # See section "Generic type inference for numeric types" of the
+      # manual for further details:
+      let rebinding = maxNumericType(x.skipTypes({tyRange}), a)
+      if rebinding != nil:
+        put(c, f, rebinding)
+        result = isGeneric
+      else:
+        result = typeRel(c, x, a) # check if it fits
+        if result > isGeneric: result = isGeneric
   of tyStatic:
     let prev = PType(idTableGet(c.bindings, f))
     if prev == nil:

doc/manual/generics.txt

@@ -63,6 +63,8 @@ The following example shows a generic binary tree can be modelled:
   for str in preorder(root):
     stdout.writeLine(str)
 
+The ``T`` is called a `generic type parameter `:idx:.
+
 
 Is operator
 -----------
@@ -710,3 +712,30 @@ definition):
 
 But a ``bind`` is rarely useful because symbol binding from the definition
 scope is the default.
+
+
+Generic type inference for numeric types
+----------------------------------------
+
+A `numeric`:idx: type is any signed, unsigned integer type, floating point
+type or a subrange thereof. Let ``maxNumericType(T1, T2)`` be the "greater"
+type of ``T1`` and ``T2``, that is the type that uses more bits. For
+example ``maxNumericType(int32, int64) == int64``. ``maxNumericType`` is only
+defined for numeric types of the same class (signed, unsigned, floating point).
+``maxNumericType`` strips away subranges,
+``maxNumericType(subrangeof(int16), int8)`` produces ``int16`` not its
+subrange. The definition ``maxNumericType`` is extended to take a variable
+number of arguments in the obvious way;
+``maxNumericType(x, y, z) == maxNumericType(maxNumericType(x, y), z)``.
+
+A generic type parameter ``T`` that is bound to multiple numeric types ``N1``,
+``N2``, ``N3``, ... during type checking is inferred to
+be ``maxNumericType(N1, N2, N3, ...)``. This special type inference rule ensures
+that the builtin arithmetic operators can be written in an intuitive way:
+
+.. code-block:: nim
+  proc `@`[T: int|int16|int32](x, y: T): T
+
+  4'i32 @ 6'i64 # inferred to be of type ``int64``
+
+  4'i64 @ 6'i32 # inferred to be of type ``int64``

tests/generics/tspecial_numeric_inference.nim

@@ -0,0 +1,12 @@
+discard """
+  output: '''int64
+int64'''
+"""
+
+import typetraits
+
+proc `@`[T: SomeInteger](x, y: T): T = x
+
+echo(type(5'i64 @ 6'i32))
+
+echo(type(5'i32 @ 6'i64))