SI-7668 Better return type inheritance for dep. method types

Return type inheritance already handles substitution of
type parameters of the overriding method for those of the
overriding.

This commit extends this to do the same for parameter symbols.

2 files changed, 15 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Namers.scala b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
index 1282cfb416..08837d9a54 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Namers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
@@ -1084,6 +1084,9 @@ trait Namers extends MethodSynthesis {
             overriddenTp = overriddenTp.resultType
           }
 
+          // SI-7668 Substitute parameters from the parent method with those of the overriding method.
+          overriddenTp = overriddenTp.substSym(overridden.paramss.flatten, vparamss.flatten.map(_.symbol))
+
           overriddenTp match {
             case NullaryMethodType(rtpe) => overriddenTp = rtpe
             case MethodType(List(), rtpe) => overriddenTp = rtpe
diff --git a/test/files/pos/t7668.scala b/test/files/pos/t7668.scala
new file mode 100644
index 0000000000..222a13d039
--- /dev/null
+++ b/test/files/pos/t7668.scala
@@ -0,0 +1,12 @@
+trait Space {
+  type T
+  val x: T
+}
+
+trait Extractor {
+  def extract(s: Space): s.T
+}
+
+class Sub extends Extractor {
+  def extract(s: Space) = s.x
+}