Type mismatch on init of static[T] object with T being a static[U]

[mratsim]

The following works:

type
  MyObjParam = object
    x: int

  MyObj[P: static MyObjParam] = object
    y: int

const P = MyObjParam(x: 2)
let Q = MyObj[P](y: 2)
echo Q

But the following fails

type
  MyObjParam[N: static int] = object
    x: int

  MyObj[P: static MyObjParam] = object
    y: int

const P = MyObjParam[256](x: 2)
let Q = MyObj[P](y: 2)
echo Q

The error is:

Error: cannot instantiate MyObj
got: <static[MyObjParam[256]]((x: 2))>
but expected: <P>

I expect the issue rootcause is closely related to #9679.

Use case: I'm trying to implement a bigint backend for an elliptic curve crypto library. Everything is done modulo a bigint that is known at compile-time. The types are like this:

type
  # https://github.com/status-im/nim-constantine/blob/master/constantine/bigints.nim
  BigInt*[bits: static int] = object
    limbs*: array[bits.wordsRequired, Word]
  
  # https://github.com/status-im/nim-constantine/blob/master/constantine/field_fp.nim
  Fp*[P: static BigInt] = object
    ## P is a prime number
    ## All operations on a field are modulo P
    value: type(P)

I expect that @andreaferretti would also like to use this in Emmy

[mratsim]

Investigating a bit where the semcheck is failing:

Full stacktrace

Error: cannot instantiate MyObj
got: <static[MyObjParam[256]]((x: 2))>
but expected: <P>
Traceback (most recent call last)
nim.nim(98)              nim
nim.nim(75)              handleCmdLine
cmdlinehelper.nim(92)    loadConfigsAndRunMainCommand
main.nim(194)            mainCommand
main.nim(90)             commandCompileToC
modules.nim(147)         compileProject
modules.nim(88)          compileModule
passes.nim(197)          processModule
passes.nim(86)           processTopLevelStmt
sem.nim(603)             myProcess
sem.nim(571)             semStmtAndGenerateGenerics
semstmts.nim(2159)       semStmt
semexprs.nim(939)        semExprNoType
semexprs.nim(2624)       semExpr
semstmts.nim(2099)       semStmtList
semexprs.nim(2627)       semExpr
semstmts.nim(447)        semVarOrLet
semexprs.nim(64)         semExprWithType
semexprs.nim(2590)       semExpr
semobjconstr.nim(258)    semObjConstr
semtypes.nim(1665)       semTypeNode
semtypes.nim(1329)       semGeneric
msgs.nim(530)            localError
msgs.nim(515)            liMessage
msgs.nim(346)            handleError
msgs.nim(331)            quit
FAILURE

Investigation

Diving a bit into sigmatch, the failure seems to be triggered on this code path:

Nim/compiler/sigmatch.nim

Lines 2399 to 2408 in bb8e2ee

	else:
	m.baseTypeMatch = false
	m.typedescMatched = false
	n.sons[a] = prepareOperand(c, formal.typ, n.sons[a])
	arg = paramTypesMatch(m, formal.typ, n.sons[a].typ,
	n.sons[a], nOrig.sons[a])
	if arg == nil:
	m.state = csNoMatch
	m.firstMismatch = f
	return

Small stacktrace before returning from the branch

semtypes.nim(1665)       semTypeNode
semtypes.nim(1323)       semGeneric
sigmatch.nim(2481)       matches
sigmatch.nim(2424)       matchesAux

Diving deeper into sigmatch, it's typeRel called by paramTypesMatchAux that doesn't match.

Nim/compiler/sigmatch.nim

Line 1968 in bb8e2ee

var r = typeRel(m, f, a)

Tracing typeRel

Turning on tracing produces this interesting output for the inner static param, notice that we have tyStatic on one side and tyObject on the other.

----- TYPE REL 4
{
  "kind": "tyStatic",
  "id": 171017,
  "sym": {
    "kind": "skGenericParam",
    "name": "P",
    "id": 171019,
    "typ": <defined 7 lines upwards>
  },
  "flags": ["tfHasMeta", "tfHasStatic", "tfGenericTypeParam"],
  "sons": [{
    "kind": "tyGenericBody",
    "id": 171004,
    "sym": {
      "kind": "skType",
      "name": "MyObjParam",
      "id": 171003,
      "flags": ["sfUsed", "sfGlobal"],
      "typ": <defined 8 lines upwards>
    },
    "flags": ["tfHasMeta"],
    "sons": [{
      "kind": "tyStatic",
      "id": 171009,
      "sym": {
        "kind": "skGenericParam",
        "name": "N",
        "id": 171011,
        "typ": <defined 7 lines upwards>
      },
      "flags": ["tfHasStatic", "tfGenericTypeParam"],
      "sons": [{
        "kind": "tyInt",
        "id": 42006,
        "sym": {
          "kind": "skType",
          "name": "int",
          "id": 42005,
          "flags": ["sfUsed", "sfExported", "sfGlobal"],
          "typ": <defined 8 lines upwards>
        },
        "flags": ["tfCheckedForDestructor"]
      }]
    }, {
      "kind": "tyObject",
      "id": 171013,
      "sym": <defined 33 lines upwards>,
      "flags": ["tfFinal"],
      "sons": [null],
      "n": {
        "kind": "nkRecList",
        "sons": [{
          "kind": "nkSym",
          "sym": {
            "kind": "skField",
            "name": "x",
            "id": 171014,
            "position": 0,
            "typ": <defined 27 lines upwards>
          }
        }]
      }
    }],
    "n": {
      "kind": "nkGenericParams",
      "sons": [{
        "kind": "nkSym",
        "sym": <defined 43 lines upwards>
      }]
    }
  }]
}
{
  "kind": "tyObject",
  "id": 171013,
  "sym": {
    "kind": "skType",
    "name": "MyObjParam",
    "id": 171003,
    "flags": ["sfUsed", "sfGlobal"],
    "typ": {
      "kind": "tyGenericBody",
      "id": 171004,
      "sym": <defined 8 lines upwards>,
      "flags": ["tfHasMeta"],
      "sons": [{
        "kind": "tyStatic",
        "id": 171009,
        "sym": {
          "kind": "skGenericParam",
          "name": "N",
          "id": 171011,
          "typ": <defined 7 lines upwards>
        },
        "flags": ["tfHasStatic", "tfGenericTypeParam"],
        "sons": [{
          "kind": "tyInt",
          "id": 42006,
          "sym": {
            "kind": "skType",
            "name": "int",
            "id": 42005,
            "flags": ["sfUsed", "sfExported", "sfGlobal"],
            "typ": <defined 8 lines upwards>
          },
          "flags": ["tfCheckedForDestructor"]
        }]
      }, <defined 35 lines upwards>],
      "n": {
        "kind": "nkGenericParams",
        "sons": [{
          "kind": "nkSym",
          "sym": <defined 24 lines upwards>
        }]
      }
    }
  },
  "flags": ["tfFinal"],
  "sons": [null],
  "n": {
    "kind": "nkRecList",
    "sons": [{
      "kind": "nkSym",
      "sym": {
        "kind": "skField",
        "name": "x",
        "id": 171014,
        "position": 0,
        "typ": <defined 33 lines upwards>
      }
    }]
  }
}
----- TYPE REL 4 RESULT: isNone

[cooldome]

Works as expected now. My latest change, fixed it.