fixes #26000; Cannot add members to enum-indexed array of seqs at com… (#26013)

…pile time

fixes #26000

vm: preserve lvalues for mutations of broadcast array elements

Load in-place mutation targets through their address so mutations don't
operate on detached copies of broadcast defaults. This also preserves
nested lvalues and evaluates indexed destinations only once.

Cover sequence, string, and set mutations through direct, nested, field,
enum-indexed, and range-indexed array elements.

(cherry picked from commit 2915691515)

compiler/vmgen.nim

@@ -851,14 +851,26 @@ proc genBinaryStmt(c: PCtx; n: PNode; opc: TOpcode) =
   c.freeTemp(tmp)
   c.freeTemp(dest)
 
+proc genMutatingValue(c: PCtx; n: PNode): TRegister =
+  ## Loads the value of an in-place mutation target while keeping it attached to
+  ## its original storage. Compound lvalues must be resolved through their
+  ## address: a normal value load can return a detached copy (for example, when
+  ## indexing a broadcast default array).
+  if needsAsgnPatch(n):
+    let address = c.genx(n, {gfNodeAddr})
+    result = c.getTemp(n.typ)
+    c.gABC(n, opcLdDeref, result, address)
+    c.freeTemp(address)
+  else:
+    result = c.genx(n)
+
 proc genBinaryStmtVar(c: PCtx; n: PNode; opc: TOpcode) =
   var x = n[1]
   if x.kind in {nkAddr, nkHiddenAddr}: x = x[0]
   let
-    dest = c.genx(x)
+    dest = c.genMutatingValue(x)
     tmp = c.genx(n[2])
   c.gABC(n, opc, dest, tmp, 0)
-  #c.genAsgnPatch(n[1], dest)
   c.freeTemp(tmp)
   c.freeTemp(dest)
 
@@ -1162,7 +1174,7 @@ proc genMagic(c: PCtx; n: PNode; dest: var TDest; flags: TGenFlags = {}, m: TMag
 
   of mIncl, mExcl:
     unused(c, n, dest)
-    var d = c.genx(n[1])
+    var d = c.genMutatingValue(n[1])
     var tmp = c.genx(n[2])
     c.genSetType(n[1], d)
     c.gABC(n, if m == mIncl: opcIncl else: opcExcl, d, tmp)

tests/vm/tbroadcastarraymutation.nim

@@ -0,0 +1,101 @@
+type
+  Container = object
+    numbers: seq[int]
+    text: string
+    chars: set[char]
+
+  Variant = object
+    case enabled: bool
+    of false:
+      numbers: seq[int]
+    else:
+      discard
+
+  Index = enum
+    index0, index1, index2, index3, index4, index5, index6, index7,
+    index8, index9, index10, index11, index12, index13, index14, index15,
+    index16, index17, index18, index19, index20, index21, index22, index23,
+    index24, index25, index26, index27, index28, index29, index30, index31,
+    index32
+
+  Outer = object
+    values: array[33, seq[int]]
+
+proc directSeq(): array[33, seq[int]] =
+  result[32].add 1
+
+proc directStringChar(): array[33, string] =
+  result[32].add 'a'
+
+proc directStringString(): array[33, string] =
+  result[32].add "ab"
+
+proc directSet(): array[33, set[char]] =
+  result[32].incl 'a'
+
+proc fieldSeq(): array[33, Container] =
+  result[32].numbers.add 1
+
+proc fieldStringChar(): array[33, Container] =
+  result[32].text.add 'a'
+
+proc fieldStringString(): array[33, Container] =
+  result[32].text.add "ab"
+
+proc fieldSet(): array[33, Container] =
+  result[32].chars.incl 'a'
+
+proc nestedSeq(): array[33, array[33, seq[int]]] =
+  result[32][32].add 1
+
+proc checkedFieldSeq(): array[33, Variant] =
+  result[32].numbers.add 1
+
+proc enumIndexSeq(): array[Index, seq[int]] =
+  result[index32].add 1
+
+proc rangeIndexSeq(): array[10..42, seq[int]] =
+  result[42].add 1
+
+proc firstIndexSeq(): array[33, seq[int]] =
+  result[0].add 1
+
+proc middleIndexSeq(): array[33, seq[int]] =
+  result[16].add 1
+
+proc objectArraySeq(): Outer =
+  result.values[32].add 1
+
+proc singleEvaluation(): tuple[values: array[33, seq[int]], evaluations: int] =
+  var evaluations = 0
+  proc index(): int =
+    inc evaluations
+    32
+  result.values[index()].add 1
+  result.evaluations = evaluations
+
+proc test =
+  let direct = directSeq()
+  doAssert direct[0].len == 0
+  doAssert direct[31].len == 0
+  doAssert direct[32] == @[1]
+  doAssert directStringChar()[32] == "a"
+  doAssert directStringString()[32] == "ab"
+  doAssert 'a' in directSet()[32]
+  doAssert fieldSeq()[32].numbers == @[1]
+  doAssert fieldStringChar()[32].text == "a"
+  doAssert fieldStringString()[32].text == "ab"
+  doAssert 'a' in fieldSet()[32].chars
+  doAssert nestedSeq()[32][32] == @[1]
+  doAssert checkedFieldSeq()[32].numbers == @[1]
+  doAssert enumIndexSeq()[index32] == @[1]
+  doAssert rangeIndexSeq()[42] == @[1]
+  doAssert firstIndexSeq()[0] == @[1]
+  doAssert middleIndexSeq()[16] == @[1]
+  doAssert objectArraySeq().values[32] == @[1]
+  let evaluated = singleEvaluation()
+  doAssert evaluated.values[32] == @[1]
+  doAssert evaluated.evaluations == 1
+
+static: test()
+test()