Fix regressions in asSeenFrom introduced in 3.7 (#23438)

The body of isLegalPrefix used to read:

    pre.isStable || !ctx.phase.isTyper

but was changed to drop the second condition in #21954 (originally
included in 3.6.4-RC1, reverted from 3.6.4 final but back in 3.7). This
has led to a number of regressions, the last ones discussed in #23423.

To make the testcases added in #21594 pass, this PR proposes a less
drastic change: relax isLegalPrefix as before, unless we're doing an
implicit search. This should dramatically reduce the possibility for
regressions.

Fixes #23423.
Fixes #22676.

Author: smarter

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

@@ -125,7 +125,9 @@ object TypeOps:
   }
 
   def isLegalPrefix(pre: Type)(using Context): Boolean =
-    pre.isStable
+    // isLegalPrefix is relaxed after typer unless we're doing an implicit
+    // search (this matters when doing summonInline in an inline def like in tests/pos/i17222.8.scala).
+    pre.isStable || !ctx.phase.isTyper && ctx.mode.is(Mode.ImplicitsEnabled)
 
   /** Implementation of Types#simplified */
   def simplify(tp: Type, theMap: SimplifyMap | Null)(using Context): Type = {

tests/neg-custom-args/captures/lazylist.check

@@ -29,7 +29,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylist.scala:41:42 -------------------------------------
 41 |  val ref4c: LazyList[Int]^{cap1, ref3} = ref4 // error
    |                                          ^^^^
-   |                                          Found:    (ref4 : lazylists.LazyList[Int]^{cap3, ref1, ref2})
+   |                                          Found:    (ref4 : lazylists.LazyList[Int]^{cap3, cap1, cap2})
    |                                          Required: lazylists.LazyList[Int]^{cap1, ref3}
    |
    | longer explanation available when compiling with `-explain`

tests/neg/6314-6.check

@@ -4,13 +4,13 @@
    |object creation impossible, since def apply(fa: String): Int in trait XX in object Test3 is not defined 
    |(Note that
    | parameter String in def apply(fa: String): Int in trait XX in object Test3 does not match
-   | parameter Test3.Bar[X & (X & Y)] in def apply(fa: Test3.Bar[X & YY.this.Foo]): Test3.Bar[Y & YY.this.Foo] in trait YY in object Test3
+   | parameter Test3.Bar[X & Object with Test3.YY {...}#Foo] in def apply(fa: Test3.Bar[X & YY.this.Foo]): Test3.Bar[Y & YY.this.Foo] in trait YY in object Test3
    | )
 -- Error: tests/neg/6314-6.scala:52:3 ----------------------------------------------------------------------------------
 52 |  (new YY {}).boom // error: object creation impossible
    |   ^
    |object creation impossible, since def apply(fa: String): Int in trait XX in object Test4 is not defined 
    |(Note that
    | parameter String in def apply(fa: String): Int in trait XX in object Test4 does not match
-   | parameter Test4.Bar[X & (X & Y)] in def apply(fa: Test4.Bar[X & YY.this.FooAlias]): Test4.Bar[Y & YY.this.FooAlias] in trait YY in object Test4
+   | parameter Test4.Bar[X & Object with Test4.YY {...}#FooAlias] in def apply(fa: Test4.Bar[X & YY.this.FooAlias]): Test4.Bar[Y & YY.this.FooAlias] in trait YY in object Test4
    | )

tests/pos-macros/i23423/A_1.scala

@@ -0,0 +1,16 @@
+package pkg
+
+import scala.quoted.*
+
+trait HasElem {
+  type Elem
+  type Alias = Elem
+}
+
+object Macro:
+  inline def foo: Unit = ${fooImpl}
+  def fooImpl(using Quotes): Expr[Unit] =
+    '{
+      val lll: (he: HasElem) => he.Alias =
+        (hx: HasElem) => ???
+    }

tests/pos-macros/i23423/B_2.scala

@@ -0,0 +1,6 @@
+object Test:
+  def test: Unit = pkg.Macro.foo
+  // used to be error:
+  // Found:    (hx: pkg.HasElem) => hx.Elem
+  // Required: (he: pkg.HasElem) => he.Elem
+

tests/pos/i22676.scala

@@ -0,0 +1,11 @@
+package example
+
+trait Example {
+  class Input
+
+  type Output[A] = A match {
+    case Input => Int
+  }
+}
+
+class Ref(ref: Example#Input)