Also suggest partial matches for missing implicit arguments

This used to work only for ViewProtos before.

Author: odersky

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

@@ -146,7 +146,12 @@ trait ImportSuggestions with
      */
     def shallowTest(ref: TermRef): Boolean =
       System.currentTimeMillis < deadLine
-      && (ref <:< pt)(given ctx.fresh.setExploreTyperState())
+      && {
+        given Context = ctx.fresh.setExploreTyperState()
+        pt match
+          case pt: ViewProto => pt.isMatchedBy(ref)
+          case _ => normalize(ref, pt) <:< pt
+      }
 
     /** Test whether a full given term can be synthesized that matches
      *  the expected type `pt`.

tests/neg/missing-implicit2.check

@@ -0,0 +1,18 @@
+-- Error: tests/neg/missing-implicit2.scala:10:18 ----------------------------------------------------------------------
+10 |    f(given xFromY) // error
+   |                  ^
+   |                  no implicit argument of type Y was found for parameter y of method xFromY
+   |
+   |                  The following import might fix the problem:
+   |
+   |                    import test.instances.y
+   |                           
+-- Error: tests/neg/missing-implicit2.scala:16:5 -----------------------------------------------------------------------
+16 |    f // error
+   |     ^
+   |     no implicit argument of type X was found for parameter x of method f in object test
+   |
+   |     The following import might make progress towards fixing the problem:
+   |
+   |       import instances2.xFromY
+   |              

tests/neg/missing-implicit2.scala

@@ -0,0 +1,17 @@
+trait X
+trait Y
+object test with
+  def f(given x: X) = ???
+  object instances {
+    given y: Y = ???
+  }
+  locally {
+    given xFromY(given y: Y): X = ???
+    f(given xFromY) // error
+  }
+  locally {
+    object instances2 {
+      given xFromY: Y => X = ???
+    }
+    f // error
+  }