Merge pull request #14219 from dotty-staging/fix-14214

Re-type implicit candidate if expected type is context function

Author: odersky

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

@@ -240,7 +240,6 @@ object ErrorReporting {
   def err(using Context): Errors = new Errors
 }
 
-
 class ImplicitSearchError(
   arg: tpd.Tree,
   pt: Type,
@@ -249,6 +248,7 @@ class ImplicitSearchError(
   ignoredInstanceNormalImport: => Option[SearchSuccess],
   importSuggestionAddendum: => String
 )(using ctx: Context) {
+
   def missingArgMsg = arg.tpe match {
     case ambi: AmbiguousImplicits =>
       (ambi.alt1, ambi.alt2) match {

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

@@ -1050,8 +1050,13 @@ trait Implicits:
       val generated: Tree = tpd.ref(ref).withSpan(span.startPos)
       val locked = ctx.typerState.ownedVars
       val adapted =
-        if (argument.isEmpty)
-          adapt(generated, pt.widenExpr, locked)
+        if argument.isEmpty then
+          if defn.isContextFunctionType(pt) then
+            // need to go through typed, to build the context closure
+            typed(untpd.TypedSplice(generated), pt, locked)
+          else
+            // otherwise we can skip typing and go directly to adapt
+            adapt(generated, pt.widenExpr, locked)
         else {
           def untpdGenerated = untpd.TypedSplice(generated)
           def producesConversion(info: Type): Boolean = info match

tests/pos/i14214.scala

@@ -0,0 +1,16 @@
+class Dummy
+given Dummy = ???
+trait Foo
+given foo: Foo = ???
+trait Bar
+given bar(using Dummy): Bar = ???
+
+object Test:
+  summon[Dummy ?=> Foo] // was error
+  summon[Dummy ?=> Foo](using foo)  // works
+  summon[Dummy ?=> Foo](using (_: Dummy) ?=> foo)  // works
+  summon[Dummy ?=> Bar]
+  summon[Dummy ?=> Bar](using bar)  // works
+  summon[Dummy ?=> Bar](using (_: Dummy) ?=> bar)  // works
+
+