Compilation issue on devel

[mratsim]

Reported by @Vindaar

<...>/Arraymancer/src/tensor/fallback/blas_l3_gemm_micro_kernel.nim(30, 16) Error: type mismatch: got <NimNodeKind, NimNode, NimNode, tuple of (proc (kind: NimNodeKind, children: varargs[NimNode]): NimNode{.noSideEffect, gcsafe, locks: 0.}, NimNodeKind, NimNode, NimNode, NimNode)>
but expected one of: 
proc newTree(kind: NimNodeKind; children: varargs[NimNode]): NimNode
  first type mismatch at position: 2
  required type: varargs[NimNode]
  but expression 'ident("+=")' is of type: NimNode

expression: newTree(nnkInfix, ident("+="), newTree(nnkBracketExpr, [AB, newLit(i + j * MR)]), (
    newTree, nnkInfix, ident("*"), newTree(nnkBracketExpr, [a,
    newTree(nnkInfix, [ident("+"), newLit(i), offA])]), newTree(nnkBracketExpr,
    [b, newTree(nnkInfix, [ident("+"), newLit(j), offB])])))
     Error: Execution failed with exit code 1

Nim Compiler Version 0.18.1 [Linux: amd64]
Copyright (c) 2006-2018 by Andreas Rumpf

git hash: c6f15c53f458ec0138d006edcaca349281fda753
active boot switches: -d:release

[mratsim]

Impacted code is a loop unrolling macro:

Arraymancer/src/tensor/fallback/blas_l3_gemm_micro_kernel.nim

Lines 18 to 50 in 50f7c31

	macro unroll_ukernel[MRNR, T](AB: array[MRNR, T],
	a: ptr UncheckedArray[T], offA: int,
	b: ptr UncheckedArray[T], offB: int,
	MR, NR: static[int]): untyped =
	## Unroll the following at compile time
	# for j in 0 ..< NR:
	# for i in 0 ..< MR:
	# AB[i + j*MR] += A[i + voffA] * B[j + voffB]
	result = newNimNode(nnkStmtList)
	for j in 0..NR-1:
	for i in 0..MR-1:
	result.add(
	newTree(
	nnkInfix,
	ident("+="),
	newTree(
	nnkBracketExpr,
	AB,newLit(i + j*MR)
	),
	newTree(
	nnkInfix,
	ident("*"),
	newTree(
	nnkBracketExpr,
	a, newTree(nnkInfix, ident("+"), newLit(i), offA)
	),
	newTree(
	nnkBracketExpr,
	b, newTree(nnkInfix, ident("+"), newLit(j), offB)
	),
	)
	)
	)

[Vindaar]

Just did some git bisecting through the Nim repo and this is the breaking commit:
nim-lang/Nim@47335aa

[mratsim]

Further investigation shows that it was the trailing comma that was causing the issue:

               newTree( 
                 nnkBracketExpr, 
                 b, newTree(nnkInfix, ident("+"), newLit(j), offB) 
               ),