[FE] Report ambiguity in all possible cases

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -1199,6 +1199,7 @@ public interface Errors {
     // Context receivers
     DiagnosticFactory1<KtElement, String> NO_CONTEXT_RECEIVER = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory1<KtElement, String> MULTIPLE_ARGUMENTS_APPLICABLE_FOR_CONTEXT_RECEIVER = DiagnosticFactory1.create(ERROR);
+    DiagnosticFactory0<KtElement> AMBIBIGUOS_CALL_WITH_IMPLICIT_CONTEXT_RECEIVER = DiagnosticFactory0.create(ERROR);
 
     // Error sets
     ImmutableSet<? extends DiagnosticFactory<?>> UNRESOLVED_REFERENCE_DIAGNOSTICS = ImmutableSet.of(

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -1090,6 +1090,7 @@ public class DefaultErrorMessages {
 
         MAP.put(NO_CONTEXT_RECEIVER, "No required context receiver found: {0}", TO_STRING);
         MAP.put(MULTIPLE_ARGUMENTS_APPLICABLE_FOR_CONTEXT_RECEIVER, "Multiple arguments applicable for context receiver: {0}", TO_STRING);
+        MAP.put(AMBIBIGUOS_CALL_WITH_IMPLICIT_CONTEXT_RECEIVER, "With implicit context receiver, call is ambiguous. Specify the receiver explicitly");
 
         MAP.setImmutable();
 

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/DiagnosticReporterByTrackingStrategy.kt

@@ -111,6 +111,10 @@ class DiagnosticReporterByTrackingStrategy(
                     )
                 )
             }
+            ContextReceiverAmbiguity::class.java -> {
+                val callElement = psiKotlinCall.psiCall.callElement
+                trace.report(AMBIBIGUOS_CALL_WITH_IMPLICIT_CONTEXT_RECEIVER.on(callElement))
+            }
         }
     }