Improve appearance of error messages that include polymorphic method types

Fixes #13466

Co-authored-by: Seth Tisue <seth@tisue.net>

Author: griggt

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -404,6 +404,10 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case tp: ExprType =>
         // parameterless methods require special treatment, see #11201
         (if (isParameter) ": => " else ": ") ~ toTextGlobal(tp.widenExpr)
+      case tp: PolyType =>
+        "[" ~ paramsText(tp) ~ "]"
+        ~ (Str(": ") provided !tp.resultType.isInstanceOf[MethodType])
+        ~ toTextGlobal(tp.resultType)
       case tp =>
         ": " ~ toTextGlobal(tp)
     }

tests/neg/i10666.check

@@ -1,8 +1,8 @@
 -- Error: tests/neg/i10666.scala:8:6 -----------------------------------------------------------------------------------
 8 |class Bar extends Foo { // error
   |      ^
-  |      class Bar needs to be abstract, since def foo: [T <: B](tx: T): Unit in trait Foo is not defined 
+  |      class Bar needs to be abstract, since def foo[T <: B](tx: T): Unit in trait Foo is not defined 
   |      (Note that
-  |       parameter T in def foo: [T <: B](tx: T): Unit in trait Foo does not match
-  |       parameter T in def foo: [T <: A](tx: T): Unit in class Bar
+  |       parameter T in def foo[T <: B](tx: T): Unit in trait Foo does not match
+  |       parameter T in def foo[T <: A](tx: T): Unit in class Bar
   |       class B is a subclass of class A, but method parameter types must match exactly.)

tests/neg/i13466.check

@@ -0,0 +1,11 @@
+-- Error: tests/neg/i13466.scala:9:6 -----------------------------------------------------------------------------------
+9 |given none: SomeTrait[Finally] with {} // error
+  |      ^
+  |      object creation impossible, since:
+  |      it has 3 unimplemented members.
+  |      /** As seen from module class none$, the missing signatures are as follows.
+  |       *  For convenience, these are usable as stub implementations.
+  |       */
+  |        def foo: Finally[Unit] = ???
+  |        def withTV[A]: Finally[A] = ???
+  |        def withTV2[A, B]: Finally[(A, B)] = ???

tests/neg/i13466.scala

@@ -0,0 +1,9 @@
+opaque type Finally[A] = A
+
+trait SomeTrait[F[_]] {
+  def foo: F[Unit]
+  def withTV[A]: F[A]
+  def withTV2[A, B]: F[(A, B)]
+}
+
+given none: SomeTrait[Finally] with {} // error

tests/neg/i7359-f.check

@@ -2,7 +2,7 @@
 1 |trait SAMTrait: // error
   |      ^
   |      Name clash between inherited members:
-  |      def equals: [T >: Boolean <: Boolean](obj: Any): T in trait SAMTrait at line 3 and
+  |      def equals[T >: Boolean <: Boolean](obj: Any): T in trait SAMTrait at line 3 and
   |      def equals(x$0: Any): Boolean in class Any
   |      have the same type after erasure.
   |

tests/neg/mixin-forwarder-clash1.check

@@ -3,7 +3,7 @@
    |      ^
    |      Name clash between inherited members:
    |      def concat(suffix: Int): X in trait One at line 4 and
-   |      def concat: [Dummy](suffix: Int): Y in trait Two at line 8
+   |      def concat[Dummy](suffix: Int): Y in trait Two at line 8
    |      have the same type after erasure.
    |
    |      Consider adding a @targetName annotation to one of the conflicting definitions

tests/neg/mixin-forwarder-clash2.check

@@ -4,7 +4,7 @@
   |      ^
   |      Name clash between inherited members:
   |      def concat(suffix: Int): X in trait One at line 4 and
-  |      def concat: [Dummy](suffix: Int): Y in trait Two at line 8
+  |      def concat[Dummy](suffix: Int): Y in trait Two at line 8
   |      have the same type after erasure.
   |
   |      Consider adding a @targetName annotation to one of the conflicting definitions