Fix #2762: Make sure constrainResult does not change context when false

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

@@ -216,12 +216,11 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
       case methType: MethodType =>
         // apply the result type constraint, unless method type is dependent
         val resultApprox = resultTypeApprox(methType)
-        val savedConstraint = ctx.typerState.constraint
         if (!constrainResult(resultApprox, resultType))
           if (ctx.typerState.isCommittable)
             // defer the problem until after the application;
             // it might be healed by an implicit conversion
-            assert(ctx.typerState.constraint eq savedConstraint)
+            ()
           else
             fail(err.typeMismatchMsg(methType.resultType, resultType))
         // match all arguments with corresponding formal parameters

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

@@ -54,20 +54,23 @@ object ProtoTypes {
     /** Check that the result type of the current method
      *  fits the given expected result type.
      */
-    def constrainResult(mt: Type, pt: Type)(implicit ctx: Context): Boolean = pt match {
-      case pt: FunProto =>
-        mt match {
-          case mt: MethodType =>
-            constrainResult(resultTypeApprox(mt), pt.resultType)
-          case _ =>
-            true
-        }
-      case _: ValueTypeOrProto if !disregardProto(pt) =>
-        isCompatible(normalize(mt, pt), pt)
-      case pt: WildcardType if pt.optBounds.exists =>
-        isCompatible(normalize(mt, pt), pt)
-      case _ =>
-        true
+    def constrainResult(mt: Type, pt: Type)(implicit ctx: Context): Boolean = {
+      val savedConstraint = ctx.typerState.constraint
+      val res = pt match {
+        case pt: FunProto =>
+          mt match {
+            case mt: MethodType => constrainResult(resultTypeApprox(mt), pt.resultType)
+            case _ => true
+          }
+        case _: ValueTypeOrProto if !disregardProto(pt) =>
+          isCompatible(normalize(mt, pt), pt)
+        case pt: WildcardType if pt.optBounds.exists =>
+          isCompatible(normalize(mt, pt), pt)
+        case _ =>
+          true
+      }
+      if (!res) ctx.typerState.constraint = savedConstraint
+      res
     }
   }
 

tests/neg/i2672.scala

@@ -0,0 +1,16 @@
+class Foo[T]
+
+trait Prepend {
+  type Out
+}
+
+object Test {
+  def foo()(implicit ev: Prepend): Foo[ev.Out] = ???
+
+  def test: Unit = {
+    foo(): Foo[Any] // error: found: Prepend => Foo[_] required: Foo[Any]
+
+  }
+
+  implicit val p: Prepend = ???
+}

tests/pos/i2672.scala

@@ -0,0 +1,15 @@
+class Foo[+T]
+
+trait Prepend {
+  type Out
+}
+
+object Test {
+  def foo()(implicit ev: Prepend): Foo[ev.Out] = ???
+
+  def test: Unit = {
+    foo(): Foo[Any]
+  }
+
+  implicit val p: Prepend = ???
+}