Fix name generation for anonymous given of infix types.

Fixes scala#10927

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -999,12 +999,16 @@ object desugar {
           case tree: TypeDef => tree.name.toString
           case tree: AppliedTypeTree if followArgs && tree.args.nonEmpty =>
             s"${apply(x, tree.tpt)}_${extractArgs(tree.args)}"
+          case InfixOp(left, op, right) =>
+            if followArgs then s"${op.name}_${extractArgs(List(left, right))}"
+            else op.name.toString
           case tree: LambdaTypeTree =>
             apply(x, tree.body)
           case tree: Tuple =>
             extractArgs(tree.trees)
           case tree: Function if tree.args.nonEmpty =>
-            if (followArgs) s"${extractArgs(tree.args)}_to_${apply("", tree.body)}" else "Function"
+            if followArgs then s"${extractArgs(tree.args)}_to_${apply("", tree.body)}"
+            else "Function"
           case _ => foldOver(x, tree)
         }
       else x

tests/neg/i10927.scala

@@ -0,0 +1,23 @@
+trait Transition[From, To]
+
+infix type ==>[From, To] = Transition[From, To]
+
+type A = A.type
+object A
+
+type B = B.type
+object B
+
+type C = C.type
+object C
+
+// Compiles
+given (A ==> B) = ???
+
+// was Compile error
+given (A ==> C) = ???
+
+given ==>[A, C] = ???  // error: double definition
+
+given List[A ==> B] = ???
+given List[A ==> C] = ???   // error: double definition

tests/pos/i10927.scala

@@ -0,0 +1,18 @@
+trait Transition[From, To]
+
+infix type ==>[From, To] = Transition[From, To]
+
+type A = A.type
+object A
+
+type B = B.type
+object B
+
+type C = C.type
+object C
+
+// Compiles
+given (A ==> B) = ???
+
+// was Compile error
+given (A ==> C) = ???