introduce tyInferred for the unbound concept type params

* Why is tyInferred needed?

The bindings in TCandidate are capable of inferring types within a single
call expression. In concepts, we need to infer types in the same way, but
across the whole body of the concept.

Previously, once a concept type param was inferred, it was destructively
mutated using t.assignType, but this proved to be problematic in the presence
of overloads, because the bindings established while a non-matching overload
is tested must be reverted/forgotten. tyInferred offers a non-destructive way to
keep track of the inference progress.

While introducing new types usually requires a lot of code paths in the compiler
to updated, currently tyInferred is only a short-lived type within the concept body
pass and it's unlikely to introduce breakage elsewhere in the compiler.

compiler/ast.nim

@@ -354,44 +354,52 @@ type
     tyUnused,
     tyProxy # used as errornous type (for idetools)
 
-    tyBuiltInTypeClass #\
+    tyBuiltInTypeClass
       # Type such as the catch-all object, tuple, seq, etc
 
-    tyUserTypeClass #\
+    tyUserTypeClass
       # the body of a user-defined type class
 
-    tyUserTypeClassInst #\
+    tyUserTypeClassInst
       # Instance of a parametric user-defined type class.
       # Structured similarly to tyGenericInst.
       # tyGenericInst represents concrete types, while
       # this is still a "generic param" that will bind types
       # and resolves them during sigmatch and instantiation.
 
-    tyCompositeTypeClass #\
+    tyCompositeTypeClass
       # Type such as seq[Number]
       # The notes for tyUserTypeClassInst apply here as well
       # sons[0]: the original expression used by the user.
       # sons[1]: fully expanded and instantiated meta type
       # (potentially following aliases)
 
-    tyAnd, tyOr, tyNot #\
+    tyInferred
+      # In the initial state `base` stores a type class constraining
+      # the types that can be inferred. After a candidate type is
+      # selected, it's stored in `lastSon`. Between `base` and `lastSon`
+      # there may be 0, 2 or more types that were also considered as
+      # possible candidates in the inference process (i.e. lastSon will
+      # be updated to store a type best conforming to all candidates)
+
+    tyAnd, tyOr, tyNot
       # boolean type classes such as `string|int`,`not seq`,
       # `Sortable and Enumable`, etc
 
-    tyAnything #\
+    tyAnything
       # a type class matching any type
 
-    tyStatic #\
+    tyStatic
       # a value known at compile type (the underlying type is .base)
 
-    tyFromExpr #\
+    tyFromExpr
       # This is a type representing an expression that depends
       # on generic parameters (the expression is stored in t.n)
       # It will be converted to a real type only during generic
       # instantiation and prior to this it has the potential to
       # be any type.
 
-    tyFieldAccessor #\
+    tyFieldAccessor
       # Expressions such as Type.field (valid in contexts such
       # as the `is` operator and magics like `high` and `low`).
       # Could be lifted to a single argument proc returning the
@@ -400,7 +408,7 @@ type
       # sons[1]: field type
       # .n: nkDotExpr storing the field name
 
-    tyVoid #\
+    tyVoid
       # now different from tyEmpty, hurray!
 
 static:
@@ -482,7 +490,6 @@ type
     tfHasStatic
     tfGenericTypeParam
     tfImplicitTypeParam
-    tfInferrableTypeClassTypeParam
     tfWildcard        # consider a proc like foo[T, I](x: Type[T, I])
                       # T and I here can bind to both typedesc and static types
                       # before this is determined, we'll consider them to be a
@@ -1037,6 +1044,9 @@ proc newStrNode*(kind: TNodeKind, strVal: string): PNode =
   result = newNode(kind)
   result.strVal = strVal
 
+template previouslyInferred*(t: PType): PType =
+  if t.sons.len > 1: t.lastSon else: nil
+
 proc newSym*(symKind: TSymKind, name: PIdent, owner: PSym,
              info: TLineInfo): PSym =
   # generates a symbol and initializes the hash field too
@@ -1279,6 +1289,8 @@ proc copyType*(t: PType, owner: PSym, keepId: bool): PType =
     when debugIds: registerId(result)
   result.sym = t.sym          # backend-info should not be copied
 
+proc exactReplica*(t: PType): PType = copyType(t, t.owner, true)
+
 proc copySym*(s: PSym, keepId: bool = false): PSym =
   result = newSym(s.kind, s.name, s.owner, s.info)
   #result.ast = nil            # BUGFIX; was: s.ast which made problems

compiler/ccgtypes.nim

@@ -165,7 +165,7 @@ proc mapType(typ: PType): TCTypeKind =
   of tyOpenArray, tyArray, tyVarargs: result = ctArray
   of tyObject, tyTuple: result = ctStruct
   of tyGenericBody, tyGenericInst, tyGenericParam, tyDistinct, tyOrdinal,
-     tyTypeDesc, tyAlias:
+     tyTypeDesc, tyAlias, tyInferred:
     result = mapType(lastSon(typ))
   of tyEnum:
     if firstOrd(typ) < 0:

compiler/ccgutils.nim

@@ -110,7 +110,7 @@ proc getUniqueType*(key: PType): PType =
     of tyDistinct:
       if key.deepCopy != nil: result = key
       else: result = getUniqueType(lastSon(key))
-    of tyGenericInst, tyOrdinal, tyStatic, tyAlias:
+    of tyGenericInst, tyOrdinal, tyStatic, tyAlias, tyInferred:
       result = getUniqueType(lastSon(key))
       #let obj = lastSon(key)
       #if obj.sym != nil and obj.sym.name.s == "TOption":

compiler/jsgen.nim

@@ -167,6 +167,8 @@ proc mapType(typ: PType): TJSTypeKind =
      tyNone, tyFromExpr, tyForward, tyEmpty, tyFieldAccessor,
      tyExpr, tyStmt, tyTypeDesc, tyTypeClasses, tyVoid, tyAlias:
     result = etyNone
+  of tyInferred:
+    result = mapType(typ.lastSon)
   of tyStatic:
     if t.n != nil: result = mapType(lastSon t)
     else: result = etyNone

compiler/semasgn.nim

@@ -226,7 +226,7 @@ proc liftBodyAux(c: var TLiftCtx; t: PType; body, x, y: PNode) =
      tyGenericParam, tyGenericBody, tyNil, tyExpr, tyStmt,
      tyTypeDesc, tyGenericInvocation, tyForward:
     internalError(c.info, "assignment requested for type: " & typeToString(t))
-  of tyOrdinal, tyRange,
+  of tyOrdinal, tyRange, tyInferred,
      tyGenericInst, tyFieldAccessor, tyStatic, tyVar, tyAlias:
     liftBodyAux(c, lastSon(t), body, x, y)
   of tyUnused, tyUnused0, tyUnused1, tyUnused2: internalError("liftBodyAux")

compiler/semexprs.nim

@@ -323,7 +323,6 @@ proc isOpImpl(c: PContext, n: PNode): PNode =
     maybeLiftType(t2, c, n.info)
     var m: TCandidate
     initCandidate(c, m, t2)
-    discard inferTypeClassParam(m, t1, rhsOrigType)
     let match = typeRel(m, t2, t1) >= isSubtype # isNone
     result = newIntNode(nkIntLit, ord(match))
 

compiler/sigmatch.nim

@@ -49,6 +49,10 @@ type
                              # a distrinct type
     typedescMatched*: bool
     isNoCall*: bool          # misused for generic type instantiations C[T]
+    inferredTypes: seq[PType] # inferred types during the current signature
+                              # matching. they will be reset if the matching
+                              # is not successful. may replace the bindings
+                              # table in the future.
     mutabilityProblem*: uint8 # tyVar mismatch
     inheritancePenalty: int  # to prefer closest father object type
     errors*: CandidateErrors # additional clarifications to be displayed to the
@@ -600,17 +604,17 @@ proc matchUserTypeClass*(c: PContext, m: var TCandidate,
       case typ.kind
       of tyStatic:
         param = paramSym skConst
-        param.typ = typ.base
+        param.typ = typ.exactReplica
         param.ast = typ.n
       of tyUnknown:
         param = paramSym skVar
-        param.typ = typ
+        param.typ = typ.exactReplica
       else:
         param = paramSym skType
-        param.typ = makeTypeDesc(c, typ)
-
-      if typ.isMetaType:
-        param.typ.flags.incl tfInferrableTypeClassTypeParam
+        param.typ = if typ.isMetaType:
+                      c.newTypeWithSons(tyInferred, @[typ])
+                    else:
+                      makeTypeDesc(c, typ)
 
       addDecl(c, param)
       typeParams.safeAdd((param, typ))
@@ -718,8 +722,51 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
 
   assert(aOrig != nil)
 
+  var
+    useTypeLoweringRuleInTypeClass = c.c.inTypeClass > 0 and
+                                     not c.isNoCall and
+                                     f.kind != tyTypeDesc
+
+    aOrig = if useTypeLoweringRuleInTypeClass:
+          aOrig.skipTypes({tyTypeDesc, tyFieldAccessor})
+        else:
+          aOrig
+
+  if aOrig.kind == tyInferred:
+    # echo "INFER A"
+    # debug f
+    # debug aOrig
+    let prev = aOrig.previouslyInferred
+    if prev != nil:
+      return typeRel(c, f, prev)
+    else:
+      var candidate = f
+
+      case f.kind
+      of tyGenericParam:
+        var prev  = PType(idTableGet(c.bindings, f))
+        if prev != nil: candidate = prev
+      of tyFromExpr:
+        let computedType = tryResolvingStaticExpr(c, f.n).typ
+        case computedType.kind
+        of tyTypeDesc:
+          candidate = computedType.base
+        of tyStatic:
+          candidate = computedType
+        else:
+          localError(f.n.info, errTypeExpected)
+      else:
+        discard
+
+      result = typeRel(c, aOrig.base, candidate)
+      if result != isNone:
+        c.inferredTypes.safeAdd aOrig
+        aOrig.sons.add candidate
+        result = isEqual
+      return
+
   # var and static arguments match regular modifier-free types
-  let a = aOrig.skipTypes({tyStatic, tyVar}).maybeSkipDistinct(c.calleeSym)
+  var a = aOrig.skipTypes({tyStatic, tyVar}).maybeSkipDistinct(c.calleeSym)
   # XXX: Theoretically, maybeSkipDistinct could be called before we even
   # start the param matching process. This could be done in `prepareOperand`
   # for example, but unfortunately `prepareOperand` is not called in certain
@@ -1258,6 +1305,20 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
       # XXX endless recursion?
       #result = typeRel(c, prev, aOrig)
       result = isNone
+
+  of tyInferred:
+    # echo "INFER F"
+    # debug f
+    # debug a
+    let prev = f.previouslyInferred
+    if prev != nil:
+      result = typeRel(c, prev, a)
+    else:
+      result = typeRel(c, f.base, a)
+      if result != isNone:
+        c.inferredTypes.safeAdd f
+        f.sons.add a
+
   of tyTypeDesc:
     var prev = PType(idTableGet(c.bindings, f))
     if prev == nil:
@@ -1401,59 +1462,14 @@ proc incMatches(m: var TCandidate; r: TTypeRelation; convMatch = 1) =
   of isEqual: inc(m.exactMatches)
   of isNone: discard
 
-proc skipToInferrableParam(tt: PType): PType =
-  var t = tt
-  while t != nil:
-    if tfInferrableTypeClassTypeParam in t.flags:
-      return t
-    if t.sonsLen > 0 and t.kind == tyTypeDesc:
-      t = t.base
-    else:
-      return nil
-
-  return nil
-
-proc inferTypeClassParam*(m: var TCandidate, f, a: PType): bool =
-  var c = m.c
-  if c.inTypeClass == 0: return false
-
-  var inferrableType = a.skipToInferrableParam
-  if inferrableType == nil: return false
-
-  var inferAs = f
-
-  case f.kind
-  of tyGenericParam:
-    var prev  = PType(idTableGet(m.bindings, f))
-    if prev != nil: inferAs = prev
-
-  of tyFromExpr:
-    let computedType = tryResolvingStaticExpr(m, f.n).typ
-    case computedType.kind
-    of tyTypeDesc:
-      inferAs = computedType.base
-    of tyStatic:
-      inferAs = computedType
-    else:
-      localError(f.n.info, errTypeExpected)
-
-  else:
-    discard
-
-  inferrableType.assignType inferAs
-  return true
-
-proc paramTypesMatchAux(m: var TCandidate, f, argType: PType,
+proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
                         argSemantized, argOrig: PNode): PNode =
   var
     fMaybeStatic = f.skipTypes({tyDistinct})
     arg = argSemantized
-    argType = argType
+    a = a
     c = m.c
 
-  if inferTypeClassParam(m, f, argType):
-    return argSemantized
-
   if tfHasStatic in fMaybeStatic.flags:
     # XXX: When implicit statics are the default
     # this will be done earlier - we just have to
@@ -1461,12 +1477,12 @@ proc paramTypesMatchAux(m: var TCandidate, f, argType: PType,
 
     # XXX: weaken tyGenericParam and call it tyGenericPlaceholder
     # and finally start using tyTypedesc for generic types properly.
-    if argType.kind == tyGenericParam and tfWildcard in argType.flags:
-      argType.assignType(f)
-      # put(m.bindings, f, argType)
+    if a.kind == tyGenericParam and tfWildcard in a.flags:
+      a.assignType(f)
+      # put(m.bindings, f, a)
       return argSemantized
 
-    if argType.kind == tyStatic:
+    if a.kind == tyStatic:
       if m.callee.kind == tyGenericBody and
          argType.n == nil and
          tfGenericTypeParam notin argType.flags:
@@ -1477,20 +1493,9 @@ proc paramTypesMatchAux(m: var TCandidate, f, argType: PType,
         arg.typ = newTypeS(tyStatic, c)
         arg.typ.sons = @[evaluated.typ]
         arg.typ.n = evaluated
-        argType = arg.typ
+        a = arg.typ
 
-  var
-    useTypeLoweringRuleInTypeClass = argType != nil and
-                                     c.inTypeClass > 0 and
-                                     not m.isNoCall and
-                                     f.kind != tyTypeDesc
-
-    a = if useTypeLoweringRuleInTypeClass:
-          argType.skipTypes({tyTypeDesc, tyFieldAccessor})
-        else:
-          argType
-
-    r = typeRel(m, f, a)
+  var r = typeRel(m, f, a)
 
   if r != isNone and m.calleeSym != nil and
      m.calleeSym.kind in {skMacro, skTemplate}:
@@ -1931,6 +1936,10 @@ proc matches*(c: PContext, n, nOrig: PNode, m: var TCandidate) =
           def = implicitConv(nkHiddenStdConv, formal.typ, def, m, c)
         setSon(m.call, formal.position + 1, def)
     inc(f)
+  # forget all inferred types if the overload matching failed
+  if m.state == csNoMatch:
+    for t in m.inferredTypes:
+      if t.sonsLen > 1: t.sons.setLen 1
 
 proc argtypeMatches*(c: PContext, f, a: PType): bool =
   var m: TCandidate

compiler/types.nim

@@ -55,16 +55,17 @@ const
   # TODO: Remove tyTypeDesc from each abstractX and (where necessary)
   # replace with typedescX
   abstractPtrs* = {tyVar, tyPtr, tyRef, tyGenericInst, tyDistinct, tyOrdinal,
-                   tyTypeDesc, tyAlias}
+                   tyTypeDesc, tyAlias, tyInferred}
   abstractVar* = {tyVar, tyGenericInst, tyDistinct, tyOrdinal, tyTypeDesc,
-                  tyAlias}
+                  tyAlias, tyInferred}
   abstractRange* = {tyGenericInst, tyRange, tyDistinct, tyOrdinal, tyTypeDesc,
-                    tyAlias}
+                    tyAlias, tyInferred}
   abstractVarRange* = {tyGenericInst, tyRange, tyVar, tyDistinct, tyOrdinal,
-                       tyTypeDesc, tyAlias}
-  abstractInst* = {tyGenericInst, tyDistinct, tyOrdinal, tyTypeDesc, tyAlias}
-
-  skipPtrs* = {tyVar, tyPtr, tyRef, tyGenericInst, tyTypeDesc, tyAlias}
+                       tyTypeDesc, tyAlias, tyInferred}
+  abstractInst* = {tyGenericInst, tyDistinct, tyOrdinal, tyTypeDesc, tyAlias,
+                   tyInferred}
+  skipPtrs* = {tyVar, tyPtr, tyRef, tyGenericInst, tyTypeDesc, tyAlias,
+               tyInferred}
   # typedescX is used if we're sure tyTypeDesc should be included (or skipped)
   typedescPtrs* = abstractPtrs + {tyTypeDesc}
   typedescInst* = abstractInst + {tyTypeDesc}
@@ -410,7 +411,7 @@ const
     "unused0", "unused1",
     "unused2", "varargs[$1]", "unused", "Error Type",
     "BuiltInTypeClass", "UserTypeClass",
-    "UserTypeClassInst", "CompositeTypeClass",
+    "UserTypeClassInst", "CompositeTypeClass", "inferred",
     "and", "or", "not", "any", "static", "TypeFromExpr", "FieldAccessor",
     "void"]
 
@@ -476,6 +477,10 @@ proc typeToString(typ: PType, prefer: TPreferedDesc = preferName): string =
       of tyTuple: "tuple"
       of tyOpenArray: "openarray"
       else: typeToStr[t.base.kind]
+  of tyInferred:
+    let concrete = t.previouslyInferred
+    if concrete != nil: result = typeToString(concrete)
+    else: result = "inferred[" & typeToString(t.base) & "]"
   of tyUserTypeClassInst:
     let body = t.base
     result = body.sym.name.s & "["
@@ -971,7 +976,9 @@ proc sameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
     result = sameTypeOrNilAux(a.sons[0], b.sons[0], c) and
         sameValue(a.n.sons[0], b.n.sons[0]) and
         sameValue(a.n.sons[1], b.n.sons[1])
-  of tyGenericInst, tyAlias: discard
+  of tyGenericInst, tyAlias, tyInferred:
+    cycleCheck()
+    result = sameTypeAux(a.lastSon, b.lastSon, c)
   of tyNone: result = false
   of tyUnused, tyUnused0, tyUnused1, tyUnused2: internalError("sameFlags")
 
@@ -1118,7 +1125,7 @@ proc typeAllowedAux(marker: var IntSet, typ: PType, kind: TSymKind,
     result = nil
   of tyOrdinal:
     if kind != skParam: result = t
-  of tyGenericInst, tyDistinct, tyAlias:
+  of tyGenericInst, tyDistinct, tyAlias, tyInferred:
     result = typeAllowedAux(marker, lastSon(t), kind, flags)
   of tyRange:
     if skipTypes(t.sons[0], abstractInst-{tyTypeDesc}).kind notin

compiler/vmdeps.nim

@@ -294,6 +294,7 @@ proc mapTypeToAstX(t: PType; info: TLineInfo;
   of tyOr: result = mapTypeToBracket("or", mOr, t, info)
   of tyNot: result = mapTypeToBracket("not", mNot, t, info)
   of tyAnything: result = atomicType("anything", mNone)
+  of tyInferred: internalAssert false
   of tyStatic, tyFromExpr, tyFieldAccessor:
     if inst:
       if t.n != nil: result = t.n.copyTree