Make this.type nullable again (unless under -Yexplicit-nulls). (#18399)

Author: sjrd

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

@@ -955,6 +955,9 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 // of null and the value denoted by p (i.e., the value v for which v eq p). [Otherwise,] the type
                 // denotes the set consisting of only the value denoted by p.
                 !ctx.explicitNulls && isNullable(tp.underlying) && tp.isStable
+              case tp: ThisType =>
+                // Same as above; this.type is also a singleton type in spec language
+                !ctx.explicitNulls && isNullable(tp.underlying)
               case tp: RefinedOrRecType => isNullable(tp.parent)
               case tp: AppliedType => isNullable(tp.tycon)
               case AndType(tp1, tp2) => isNullable(tp1) && isNullable(tp2)

tests/explicit-nulls/neg/i17467.check

@@ -29,3 +29,12 @@
    |                      must be more specific than (c1 : Null)
    |
    | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/explicit-nulls/neg/i17467.scala:18:24 ---------------------------------------------
+18 |    def me: this.type = null // error
+   |                        ^^^^
+   |                      Found:    Null
+   |                      Required: (Bar.this : Test.Bar)
+   |                      Note that implicit conversions were not tried because the result of an implicit conversion
+   |                      must be more specific than (Bar.this : Test.Bar)
+   |
+   | longer explanation available when compiling with `-explain`

tests/explicit-nulls/neg/i17467.scala

@@ -13,4 +13,7 @@ object Test:
     val c1: Null = null
     val c2: c1.type = null // error
   end test
+
+  class Bar:
+    def me: this.type = null // error
 end Test

tests/neg/i17467.check

@@ -53,12 +53,12 @@
    |                      must be more specific than (g1 : AnyRef)
    |
    | longer explanation available when compiling with `-explain`
--- [E007] Type Mismatch Error: tests/neg/i17467.scala:33:24 ------------------------------------------------------------
-33 |    def me: this.type = null // error
+-- [E007] Type Mismatch Error: tests/neg/i17467.scala:36:24 ------------------------------------------------------------
+36 |    def me: this.type = null // error
    |                        ^^^^
    |                      Found:    Null
-   |                      Required: (Bar.this : Test.Bar)
+   |                      Required: (Baz.this : Test.Baz)
    |                      Note that implicit conversions were not tried because the result of an implicit conversion
-   |                      must be more specific than (Bar.this : Test.Bar)
+   |                      must be more specific than (Baz.this : Test.Baz)
    |
    | longer explanation available when compiling with `-explain`

tests/neg/i17467.scala

@@ -30,5 +30,8 @@ object Test:
   object Mod
 
   class Bar:
+    def me: this.type = null
+
+  class Baz(val x: String) extends AnyVal:
     def me: this.type = null // error
 end Test