Add special handling when the selector of a pattern matching has Nothing type

Author: noti0na1

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

@@ -1689,7 +1689,8 @@ trait Applications extends Compatibility {
           if selType <:< unapplyArgType then
             unapp.println(i"case 1 $unapplyArgType ${ctx.typerState.constraint}")
             fullyDefinedType(unapplyArgType, "pattern selector", tree.srcPos)
-            selType.dropAnnot(defn.UncheckedAnnot) // need to drop @unchecked. Just because the selector is @unchecked, the pattern isn't.
+            if selType.isNothingType then unapplyArgType
+            else selType.dropAnnot(defn.UncheckedAnnot) // need to drop @unchecked. Just because the selector is @unchecked, the pattern isn't.
           else
             if !ctx.mode.is(Mode.InTypeTest) then
               checkMatchable(selType, tree.srcPos, pattern = true)
@@ -1711,7 +1712,7 @@ trait Applications extends Compatibility {
         val unapplyPatterns = UnapplyArgs(unapplyApp.tpe, unapplyFn, unadaptedArgs, tree.srcPos)
           .typedPatterns(qual, this)
         val result = assignType(cpy.UnApply(tree)(newUnapplyFn, unapplyImplicits(dummyArg, unapplyApp), unapplyPatterns), ownType)
-        if (ownType.stripped eq selType.stripped) || ownType.isError then result
+        if (ownType.stripped eq selType.stripped) || selType.isNothingType || ownType.isError then result
         else tryWithTypeTest(Typed(result, TypeTree(ownType)), selType)
       case tp =>
         val unapplyErr = if (tp.isError) unapplyFn else notAnExtractor(unapplyFn)

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

@@ -2816,6 +2816,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             if isStableIdentifierOrLiteral || isNamedTuplePattern then pt
             else if isWildcardStarArg(body1)
                     || pt == defn.ImplicitScrutineeTypeRef
+                    || pt.isNothingType
                     || body1.tpe <:< pt  // There is some strange interaction with gadt matching.
                                          // and implicit scopes.
                                          // run/t2755.scala fails to compile if this subtype test is omitted

tests/neg/i7294.check

@@ -1,9 +1,9 @@
--- [E007] Type Mismatch Error: tests/neg/i7294.scala:7:15 --------------------------------------------------------------
+-- [E007] Type Mismatch Error: tests/neg/i7294.scala:7:18 --------------------------------------------------------------
 7 |  case x: T => x.g(10) // error
-  |               ^
-  |               Found:    (x : Nothing)
-  |               Required: ?{ g: ? }
-  |               Note that implicit conversions were not tried because the result of an implicit conversion
-  |               must be more specific than ?{ g: [applied to (10) returning T] }
+  |               ^^^^^^^
+  |               Found:    Any
+  |               Required: T
+  |
+  |               where:    T is a type in given instance f with bounds <: foo.Foo
   |
   | longer explanation available when compiling with `-explain`