feat: update match parser

Support for
```
def fib (x : Nat) : Nat :=
  match x with
  | 0 | 1 => 1
  | x+2   => fib (x+1) + fib x
```

TODO: expand `matchAlts`

src/Lean/Elab/Binders.lean

@@ -388,7 +388,7 @@ def elabFunBinders {α} (binders : Array Syntax) (expectedType? : Option Expr) (
 /- Helper function for `expandEqnsIntoMatch` -/
 private def getMatchAltsNumPatterns (matchAlts : Syntax) : Nat :=
   let alt0 := matchAlts[0][0]
-  let pats := alt0[1].getSepArgs
+  let pats := alt0[1][0].getSepArgs
   pats.size
 
 def expandWhereDecls (whereDecls : Syntax) (body : Syntax) : MacroM Syntax :=

src/Lean/Elab/Do.lean

@@ -1040,7 +1040,7 @@ partial def toTerm : Code → M Syntax
     let mut termAlts := #[]
     for alt in alts do
       let rhs ← toTerm alt.rhs
-      let termAlt := mkNode `Lean.Parser.Term.matchAlt #[mkAtomFrom alt.ref "|", alt.patterns, mkAtomFrom alt.ref "=>", rhs]
+      let termAlt := mkNode `Lean.Parser.Term.matchAlt #[mkAtomFrom alt.ref "|", mkNullNode #[alt.patterns], mkAtomFrom alt.ref "=>", rhs]
       termAlts := termAlts.push termAlt
     let termMatchAlts := mkNode `Lean.Parser.Term.matchAlts #[mkNullNode termAlts]
     return mkNode `Lean.Parser.Term.«match» #[mkAtomFrom ref "match", genParam, optMotive, discrs, mkAtomFrom ref "with", termMatchAlts]
@@ -1459,8 +1459,9 @@ mutual
     let optMotive := doMatch[2]
     let discrs    := doMatch[3]
     let matchAlts := doMatch[5][0].getArgs -- Array of `doMatchAlt`
+    -- TODO: expand `matchAlt`
     let alts ←  matchAlts.mapM fun matchAlt => do
-      let patterns := matchAlt[1]
+      let patterns := matchAlt[1][0]
       let vars ← getPatternsVarsEx patterns.getSepArgs
       withRef patterns <| checkNotShadowingMutable vars
       let rhs  := matchAlt[3]

src/Lean/Elab/Match.lean

@@ -1087,6 +1087,7 @@ enforce this. -/
   | _ => elabMatchDefault stx expectedType?
 where
   elabMatchDefault (stx : Syntax) (expectedType? : Option Expr) : TermElabM Expr := do
+    -- TODO: expand matchAlts
     match (← expandNonAtomicDiscrs? stx) with
     | some stxNew => withMacroExpansion stx stxNew <| elabTerm stxNew expectedType?
     | none =>

src/Lean/Parser/Term.lean

@@ -135,7 +135,7 @@ def «forall» := leading_parser:leadPrec unicodeSymbol "∀" "forall" >> many1
 
 def matchAlt (rhsParser : Parser := termParser) : Parser :=
   nodeWithAntiquot "matchAlt" `Lean.Parser.Term.matchAlt $
-    "| " >> ppIndent (sepBy1 termParser ", " >> darrow >> checkColGe "alternative right-hand-side to start in a column greater than or equal to the corresponding '|'" >> rhsParser)
+    "| " >> ppIndent (sepBy1 (sepBy1 termParser ", ") "|" >> darrow >> checkColGe "alternative right-hand-side to start in a column greater than or equal to the corresponding '|'" >> rhsParser)
 /--
   Useful for syntax quotations. Note that generic patterns such as `` `(matchAltExpr| | ... => $rhs) `` should also
   work with other `rhsParser`s (of arity 1). -/

stage0/src/stdlib_flags.h

@@ -8,7 +8,7 @@ options get_default_options() {
     // switch to `true` for ABI-breaking changes affecting meta code
     opts = opts.update({"interpreter", "prefer_native"}, false);
     // switch to `true` for changing built-in parsers used in quotations
-    opts = opts.update({"internal", "parseQuotWithCurrentStage"}, false);
+    opts = opts.update({"internal", "parseQuotWithCurrentStage"}, true);
 #endif
     return opts;
 }