fix generics treating symchoice symbols as uninstantiated

fixes nim-lang#23853

compiler/semtypinst.nim

@@ -132,9 +132,13 @@ proc prepareNode(cl: var TReplTypeVars, n: PNode): PNode =
       else:
         replaceTypeVarsS(cl, n.sym, replaceTypeVarsT(cl, n.sym.typ))
   let isCall = result.kind in nkCallKinds
+  # don't try to instantiate symchoice symbols, they can be
+  # generic procs which the compiler will think are uninstantiated
+  # because their type will contain uninstantiated params
+  let isSymChoice = result.kind in nkSymChoices
   for i in 0..<n.safeLen:
     # XXX HACK: ``f(a, b)``, avoid to instantiate `f`
-    if isCall and i == 0: result.add(n[i])
+    if isSymChoice or (isCall and i == 0): result.add(n[i])
     else: result.add(prepareNode(cl, n[i]))
 
 proc isTypeParam(n: PNode): bool =

tests/generics/t23853.nim

@@ -0,0 +1,91 @@
+# issue #23853
+
+block simplified:
+  type QuadraticExt[F] = object
+    coords: array[2, F]
+  template Name(E: type QuadraticExt): int = 123
+  template getBigInt(Name: static int): untyped = int
+  type Foo[GT] = object
+    a: getBigInt(GT.Name)
+  var x: Foo[QuadraticExt[int]]
+
+import std/macros
+
+type
+  Algebra* = enum
+    BN254_Snarks
+    BLS12_381
+
+  Fp*[Name: static Algebra] = object
+    limbs*: array[4, uint64]
+
+  QuadraticExt*[F] = object
+    ## Quadratic Extension field
+    coords*: array[2, F]
+
+  CubicExt*[F] = object
+    ## Cubic Extension field
+    coords*: array[3, F]
+
+  ExtensionField*[F] = QuadraticExt[F] or CubicExt[F]
+
+  Fp2*[Name: static Algebra] =
+    QuadraticExt[Fp[Name]]
+
+  Fp4*[Name: static Algebra] =
+    QuadraticExt[Fp2[Name]]
+
+  Fp6*[Name: static Algebra] =
+    CubicExt[Fp2[Name]]
+
+  Fp12*[Name: static Algebra] =
+    CubicExt[Fp4[Name]]
+    # QuadraticExt[Fp6[Name]]
+
+template Name*(E: type ExtensionField): Algebra =
+  E.F.Name
+
+const BLS12_381_Order = [uint64 0x1, 0x2, 0x3, 0x4]
+const BLS12_381_Modulus = [uint64 0x5, 0x6, 0x7, 0x8]
+
+
+{.experimental: "dynamicBindSym".}
+
+macro baseFieldModulus*(Name: static Algebra): untyped =
+  result = bindSym($Name & "_Modulus")
+
+macro scalarFieldModulus*(Name: static Algebra): untyped =
+  result = bindSym($Name & "_Order")
+
+type FieldKind* = enum
+  kBaseField
+  kScalarField
+
+template getBigInt*(Name: static Algebra, kind: static FieldKind): untyped =
+  # Workaround:
+  # in `ptr UncheckedArray[BigInt[EC.getScalarField().bits()]]
+  # EC.getScalarField is not accepted by the compiler
+  #
+  # and `ptr UncheckedArray[BigInt[Fr[EC.F.Name].bits]]` gets undeclared field: 'Name'
+  #
+  # but `ptr UncheckedArray[getBigInt(EC.getName(), kScalarField)]` works fine
+  when kind == kBaseField:
+    Name.baseFieldModulus().typeof()
+  else:
+    Name.scalarFieldModulus().typeof()
+
+# ------------------------------------------------------------------------------
+
+type BenchMultiexpContext*[GT] = object
+  elems: seq[GT]
+  exponents: seq[getBigInt(GT.Name, kScalarField)]
+
+proc createBenchMultiExpContext*(GT: typedesc, inputSizes: openArray[int]): BenchMultiexpContext[GT] =
+  discard
+
+# ------------------------------------------------------------------------------
+
+proc main() =
+  let ctx = createBenchMultiExpContext(Fp12[BLS12_381], [2, 4, 8, 16])
+
+main()