Fix #8032: Make import suggestions less chatty

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

@@ -148,9 +148,17 @@ trait ImportSuggestions with
       System.currentTimeMillis < deadLine
       && {
         given Context = ctx.fresh.setExploreTyperState()
-        pt match
-          case pt: ViewProto => pt.isMatchedBy(ref)
-          case _ => normalize(ref, pt) <:< pt
+        def test(pt: Type): Boolean = pt match
+          case ViewProto(argType, OrType(rt1, rt2)) =>
+            // Union types do not constrain results, since comparison with a union
+            // type on the right might lose information. See ProtoTypes.disregardProto.
+            // To regain precision, test both sides separately.
+            test(ViewProto(argType, rt1)) || test(ViewProto(argType, rt2))
+          case pt: ViewProto =>
+            pt.isMatchedBy(ref)
+          case _ =>
+            normalize(ref, pt) <:< pt
+        test(pt)
       }
 
     /** Test whether a full given term can be synthesized that matches

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

@@ -61,6 +61,8 @@ object ProtoTypes {
 
     private def disregardProto(pt: Type)(implicit ctx: Context): Boolean = pt.dealias match {
       case _: OrType => true
+        // Don't constrain results with union types, since comparison with a union
+        // type on the right might commit too early into one side.
       case pt => pt.isRef(defn.UnitClass)
     }
 

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

@@ -283,6 +283,7 @@ trait TypeAssigner {
                  |Note that `$name` is treated as an infix operator in Scala 3.
                  |If you do not want that, insert a `;` or empty line in front
                  |or drop any spaces behind the operator."""
+            else if qualType.isBottomType then ""
             else
               var add = importSuggestionAddendum(
                 ViewProto(qualType.widen,

tests/neg/i8032.check

@@ -0,0 +1,9 @@
+-- [E007] Type Mismatch Error: tests/neg/i8032.scala:1:15 --------------------------------------------------------------
+1 |val x: 1 | 2 = 3 // error
+  |               ^
+  |               Found:    (3 : Int)
+  |               Required: (1 : Int) | (2 : Int)
+-- [E008] Member Not Found Error: tests/neg/i8032.scala:3:12 -----------------------------------------------------------
+3 |val y = ???.map // error
+  |        ^^^^^^^
+  |        value map is not a member of Nothing

tests/neg/i8032.scala

@@ -0,0 +1,3 @@
+val x: 1 | 2 = 3 // error
+
+val y = ???.map // error