Improve error message for refutable quoted patterns (#16669)

As noted in
#16649 (comment)

Author: bishabosha

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

@@ -846,6 +846,10 @@ trait Checking {
               case _ => EmptyTree
           if extractor.isEmpty then
             em"pattern binding uses refutable extractor"
+          else if extractor.symbol eq defn.QuoteMatching_ExprMatch then
+            em"pattern binding uses refutable extractor `'{...}`"
+          else if extractor.symbol eq defn.QuoteMatching_TypeMatch then
+            em"pattern binding uses refutable extractor `'[...]`"
           else
             em"pattern binding uses refutable extractor `$extractor`"
 

tests/neg-custom-args/fatal-warnings/i16649-refutable.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg-custom-args/fatal-warnings/i16649-refutable.scala:4:6 ----------------------------------------------
+4 |  val '{ ($y: Int) + ($z: Int) } = x // error
+  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |      pattern binding uses refutable extractor `'{...}`
+  |
+  |      If this usage is intentional, this can be communicated by adding `: @unchecked` after the expression,
+  |      which may result in a MatchError at runtime.
+  |      This patch can be rewritten automatically under -rewrite -source 3.2-migration.

tests/neg-custom-args/fatal-warnings/i16649-refutable.scala

@@ -0,0 +1,4 @@
+import quoted.*
+
+def foo(using Quotes)(x: Expr[Int]) =
+  val '{ ($y: Int) + ($z: Int) } = x // error