Improve implicit parameter error message with aliases

Fixes scala#17122
dotty-staging · Mar 17, 2023 · b223c20 · b223c20
1 parent 2dec682
commit b223c20
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Showing 3 changed files with 13 additions and 1 deletion.
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/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"

diff --git a/tests/neg/i17122.check b/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
diff --git a/tests/neg/i17122.scala 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