Refine estimation of default arguments in overloading resolution

When doing a "size-fits" check, we previously only worked with the fact
whether the given alternative had default parameters or not. We know count
the number of default parameters in the applied parameter section, which
gives us a better estimate.

Fixes #15898

Author: odersky

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

@@ -1935,11 +1935,19 @@ trait Applications extends Compatibility {
             val ptypes = tp.paramInfos
             val numParams = ptypes.length
             def isVarArgs = ptypes.nonEmpty && ptypes.last.isRepeatedParam
-            def hasDefault = alt.symbol.hasDefaultParams
-            if (numParams == numArgs) true
-            else if (numParams < numArgs) isVarArgs
-            else if (numParams > numArgs + 1) hasDefault
-            else isVarArgs || hasDefault
+            def numDefaultParams =
+              if alt.symbol.hasDefaultParams then
+                trimParamss(tp, alt.symbol.rawParamss) match
+                  case params :: _ => params.count(_.is(HasDefault))
+                  case _ => 0
+              else 0
+            if numParams < numArgs then isVarArgs
+            else if numParams == numArgs then true
+            else
+              val numNecessaryArgs = numParams - numDefaultParams
+              if numNecessaryArgs <= numArgs then true
+              else if numNecessaryArgs == numArgs + 1 then isVarArgs
+              else false
           case _ =>
             numArgs == 0
         }
@@ -2080,6 +2088,14 @@ trait Applications extends Compatibility {
     }
   end resolveOverloaded1
 
+  /** The largest suffix of `paramss` that has the same first parameter name as `t` */
+  def trimParamss(t: Type, paramss: List[List[Symbol]])(using Context): List[List[Symbol]] = t match
+    case MethodType(Nil) => trimParamss(t.resultType, paramss)
+    case t: MethodOrPoly =>
+      val firstParamName = t.paramNames.head
+      paramss.dropWhile(_.head.name != firstParamName)
+    case _ => Nil
+
   /** Resolve overloading by mapping to a different problem where each alternative's
    *  type is mapped with `f`, alternatives with non-existing types are dropped, and the
    *  expected type is `pt`. Map the results back to the original alternatives.
@@ -2089,13 +2105,7 @@ trait Applications extends Compatibility {
       val t = f(alt)
       if t.exists then
         val mappedSym = alt.symbol.asTerm.copy(info = t)
-        mappedSym.rawParamss = alt.symbol.rawParamss
-          // we need rawParamss to find parameters with default arguments,
-          // but we do not need to be precise right now, since this is just a pre-test before
-          // we look up default getters. If at some point we extract default arguments from the
-          // parameter symbols themselves, we have to find the right parameter by name, not position.
-          // That means it's OK to copy parameters wholesale rather than tailoring them to always
-          // correspond to the type transformation.
+        mappedSym.rawParamss = trimParamss(t, alt.symbol.rawParamss)
         Some((TermRef(NoPrefix, mappedSym), alt))
       else
         None

tests/neg/i15898.scala

@@ -0,0 +1,22 @@
+object O {
+  class AC(code: => Unit) {
+    def apply() = code
+
+    def this(code: => Unit, key: Int = 1, modifiers: Int = 0) = {
+      this(code)
+    }
+  }
+
+  class Doc {
+    def method: Boolean = true
+  }
+
+  val doc = new Doc
+
+  val ac = new AC(doc.method) // error
+
+  def foo[T](code: => Unit): Unit = ()
+  def foo[T](code: => Unit, key: Int = 1, modifiers: Int = 0): Unit = foo(code)
+  foo(doc.method) // error
+  foo[Int](doc.method) // error
+}

tests/pos/i15898.scala

@@ -0,0 +1,22 @@
+object O {
+  class AC(code: => Unit) {
+    def apply() = code
+
+    def this(code: => Unit, key: Int, modifiers: Int = 0) = {
+      this(code)
+    }
+  }
+
+  class Doc {
+    def method: Boolean = true
+  }
+
+  val doc = new Doc
+
+  val ac = new AC(doc.method)
+
+  def foo[T](code: => Unit): Unit = ()
+  def foo[T](code: => Unit, key: Int, modifiers: Int = 0): Unit = foo(code)
+  foo(doc.method)
+  foo[Int](doc.method)
+}