Fix #5311: Survive mistyped arguments of parameter dependent method types

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

@@ -465,7 +465,8 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
            */
           def addTyped(arg: Arg, formal: Type): Type => Type = {
             addArg(typedArg(arg, formal), formal)
-            if (methodType.isParamDependent)
+            if (methodType.isParamDependent && typeOfArg(arg).exists)
+              // `typeOfArg(arg)` could be missing because the evaluation of `arg` produced type errors
               safeSubstParam(_, methodType.paramRefs(n), typeOfArg(arg))
             else identity
           }

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

@@ -302,11 +302,13 @@ object ProtoTypes {
       typer.adapt(targ, formal, locked)
     }
 
-    /** The type of the argument `arg`.
-     *  @pre `arg` has been typed before
+    /** The type of the argument `arg`, or `NoType` if `arg` has not been typed before
+     *  or if `arg`'s typing produced a type error.
      */
-    def typeOfArg(arg: untpd.Tree)(implicit ctx: Context): Type =
-      state.typedArg(arg).tpe
+    def typeOfArg(arg: untpd.Tree)(implicit ctx: Context): Type = {
+      val t = state.typedArg(arg)
+      if (t == null) NoType else t.tpe
+    }
 
     /** The same proto-type but with all arguments combined in a single tuple */
     def tupled: FunProto = state.tupled match {

tests/neg/i5311.scala

@@ -0,0 +1,13 @@
+object m {
+  trait Foo {
+    type T[A]
+    def bar : (T[Int] => T[Int]) => T[Int => Int]
+  }
+
+  def baz (implicit S: Foo, f : S.T[Int] => S.T[Int]) : S.T[Int => Int] =
+    S.bar.apply(f)
+
+  def example(implicit s:Foo) : s.T[Int => Int] = {
+    baz((x : s.T[Int]) => x) // error
+  }
+}