Fixes SI-5801, error messages regression. Review by @adriaanm

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -109,6 +109,8 @@ trait Typers extends Modes with Adaptations with Taggings {
       case MethodType(params, _) =>
         val argResultsBuff = new ListBuffer[SearchResult]()
         val argBuff = new ListBuffer[Tree]()
+        // paramFailed cannot be initialized with params.exists(_.tpe.isError) because that would
+        // hide some valid errors for params preceding the erroneous one.
         var paramFailed = false
 
         def mkPositionalArg(argTree: Tree, paramName: Name) = argTree
@@ -124,7 +126,7 @@ trait Typers extends Modes with Adaptations with Taggings {
           for(ar <- argResultsBuff)
             paramTp = paramTp.subst(ar.subst.from, ar.subst.to)
 
-          val res = if (paramFailed) SearchFailure else inferImplicit(fun, paramTp, context.reportErrors, false, context)
+          val res = if (paramFailed || (paramTp.isError && {paramFailed = true; true})) SearchFailure else inferImplicit(fun, paramTp, context.reportErrors, false, context)
           argResultsBuff += res
 
           if (res != SearchFailure) {

test/files/neg/t5801.check

@@ -0,0 +1,22 @@
+t5801.scala:1: error: object sth is not a member of package scala
+import scala.sth
+       ^
+t5801.scala:4: error: not found: value sth
+  def foo(a: Int)(implicit b: sth.Sth): Unit = {}
+                              ^
+t5801.scala:7: error: not found: value sth
+  def bar(x: Int)(implicit y: Int): sth.Sth = null
+                                    ^
+t5801.scala:8: error: could not find implicit value for parameter y: Int
+  bar(1)
+     ^
+t5801.scala:10: error: not found: value sth
+  def meh(x: Int)(implicit a: sth.Sth, b: Int): Unit = {}
+                              ^
+t5801.scala:13: error: not found: value sth
+  def meh2(x: Int)(implicit b: Int, a: sth.Sth): Unit = {}
+                                       ^
+t5801.scala:14: error: could not find implicit value for parameter b: Int
+  meh2(1)
+      ^
+7 errors found

test/files/neg/t5801.scala

@@ -0,0 +1,16 @@
+import scala.sth
+
+object Test extends App {
+  def foo(a: Int)(implicit b: sth.Sth): Unit = {}
+  foo(1)
+
+  def bar(x: Int)(implicit y: Int): sth.Sth = null
+  bar(1)
+
+  def meh(x: Int)(implicit a: sth.Sth, b: Int): Unit = {}
+  meh(1)
+
+  def meh2(x: Int)(implicit b: Int, a: sth.Sth): Unit = {}
+  meh2(1)
+}
+