Detect irrefutable quoted patterns

Fixes #16649

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -862,7 +862,9 @@ class Definitions {
 
   @tu lazy val QuoteMatchingClass: ClassSymbol = requiredClass("scala.quoted.runtime.QuoteMatching")
     @tu lazy val QuoteMatching_ExprMatch: Symbol = QuoteMatchingClass.requiredMethod("ExprMatch")
+    @tu lazy val QuoteMatching_ExprMatchModule: Symbol = QuoteMatchingClass.requiredClass("ExprMatchModule")
     @tu lazy val QuoteMatching_TypeMatch: Symbol = QuoteMatchingClass.requiredMethod("TypeMatch")
+    @tu lazy val QuoteMatching_TypeMatchModule: Symbol = QuoteMatchingClass.requiredClass("TypeMatchModule")
 
   @tu lazy val ToExprModule: Symbol = requiredModule("scala.quoted.ToExpr")
     @tu lazy val ToExprModule_BooleanToExpr: Symbol = ToExprModule.requiredMethod("BooleanToExpr")

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -322,6 +322,27 @@ object SpaceEngine {
       case funRef: TermRef => isIrrefutable(funRef, argLen)
       case _: ErrorType => false
   }
+
+  /** Is this an `'{..}` or `'[..]` irrefutable quoted patterns?
+   *  @param  unapp The unapply function tree
+   *  @param  implicits The implicits of the unapply
+   *  @param  pt The scrutinee type
+   */
+  def isIrrefutableQuotedPattern(unapp: tpd.Tree, implicits: List[tpd.Tree], pt: Type)(using Context): Boolean = {
+    implicits.headOption match
+      // pattern '{ $x: T }
+      case Some(tpd.Apply(tpd.Select(tpd.Quoted(tpd.TypeApply(fn, List(tpt))), nme.apply), _))
+          if unapp.symbol.owner.eq(defn.QuoteMatching_ExprMatchModule)
+          && fn.symbol.eq(defn.QuotedRuntimePatterns_patternHole) =>
+        pt <:< defn.QuotedExprClass.typeRef.appliedTo(tpt.tpe)
+
+      // pattern '[T]
+      case Some(tpd.Apply(tpd.TypeApply(fn, List(tpt)), _))
+          if unapp.symbol.owner.eq(defn.QuoteMatching_TypeMatchModule) =>
+        pt =:= defn.QuotedTypeClass.typeRef.appliedTo(tpt.tpe)
+
+      case _ => false
+  }
 }
 
 /** Scala implementation of space logic */

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -33,7 +33,7 @@ import NameOps._
 import SymDenotations.{NoCompleter, NoDenotation}
 import Applications.unapplyArgs
 import Inferencing.isFullyDefined
-import transform.patmat.SpaceEngine.isIrrefutable
+import transform.patmat.SpaceEngine.{isIrrefutable, isIrrefutableQuotedPattern}
 import config.Feature
 import config.Feature.sourceVersion
 import config.SourceVersion._
@@ -888,9 +888,9 @@ trait Checking {
         pat match
           case Bind(_, pat1) =>
             recur(pat1, pt)
-          case UnApply(fn, _, pats) =>
+          case UnApply(fn, implicits, pats) =>
             check(pat, pt) &&
-            (isIrrefutable(fn, pats.length) || fail(pat, pt, Reason.RefutableExtractor)) && {
+            (isIrrefutable(fn, pats.length) || isIrrefutableQuotedPattern(fn, implicits, pt) || fail(pat, pt, Reason.RefutableExtractor)) && {
               val argPts = unapplyArgs(fn.tpe.widen.finalResultType, fn, pats, pat.srcPos)
               pats.corresponds(argPts)(recur)
             }

tests/pos-special/fatal-warnings/i16649-irrefutable.scala

@@ -0,0 +1,7 @@
+import quoted.*
+
+def foo(using Quotes)(x: Expr[Int]) =
+  val '{ $y } = x
+  val '{ $a: Any } = x
+  val '{ $b: Int } = x
+  val '[List[Int]] = Type.of[List[Int]]