Improve implicit parameter error message with aliases (#17125)

Fixes #17122

Author: mbovel

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

@@ -969,7 +969,7 @@ trait Implicits:
       case Select(qual, nme.apply) if defn.isFunctionType(qual.tpe.widen) =>
         val qt = qual.tpe.widen
         val qt1 = qt.dealiasKeepAnnots
-        def addendum = if (qt1 eq qt) "" else (i"\nThe required type is an alias of: $qt1")
+        def addendum = if (qt1 eq qt) "" else (i"\nWhere $qt is an alias of: $qt1")
         i"parameter of ${qual.tpe.widen}$addendum"
       case _ =>
         i"${ if paramName.is(EvidenceParamName) then "an implicit parameter"

tests/neg/i17122.check

@@ -0,0 +1,5 @@
+-- [E172] Type Error: tests/neg/i17122.scala:7:14 ----------------------------------------------------------------------
+7 |def test = m() // error
+  |              ^
+  |              No given instance of type A was found for parameter of C
+  |              Where C is an alias of: (A) ?=> B

tests/neg/i17122.scala

@@ -0,0 +1,7 @@
+case class A()
+case class B()
+
+type C = A ?=> B
+def m(): C = ???
+
+def test = m() // error