Do not compute protoFormal if param.tpt is empty (#18288)

This was accidentally moved before of the `if (!param.tpt.isEmpty)`
guard in
0f7c3ab#diff-8c9ece1772bd78160fc1c31e988664586c9df566a1d22ff99ef99dd6d5627a90R1534.
`protoFormal` has a side effect (error reporting) that caused this
issue.

Fixes #18276

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

@@ -1606,32 +1606,31 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     if desugared.isEmpty then
       val inferredParams: List[untpd.ValDef] =
         for ((param, i) <- params.zipWithIndex) yield
-          val (formalBounds, isErased) = protoFormal(i)
-          val param0 =
-            if (!param.tpt.isEmpty) param
-            else
-              val formal = formalBounds.loBound
-              val isBottomFromWildcard = (formalBounds ne formal) && formal.isExactlyNothing
-              val knownFormal = isFullyDefined(formal, ForceDegree.failBottom)
-              // If the expected formal is a TypeBounds wildcard argument with Nothing as lower bound,
-              // try to prioritize inferring from target. See issue 16405 (tests/run/16405.scala)
-              val paramType =
-                // Strip inferred erased annotation, to avoid accidentally inferring erasedness
-                val formal0 = if !isErased then formal.stripAnnots(_.symbol != defn.ErasedParamAnnot) else formal
-                if knownFormal && !isBottomFromWildcard then
-                  formal0
-                else
-                  inferredFromTarget(param, formal, calleeType, isErased, paramIndex).orElse(
-                    if knownFormal then formal0
-                    else errorType(AnonymousFunctionMissingParamType(param, tree, inferredType = formal, expectedType = pt), param.srcPos)
-                  )
-              val paramTpt = untpd.TypedSplice(
-                  (if knownFormal then InferredTypeTree() else untpd.TypeTree())
-                    .withType(paramType.translateFromRepeated(toArray = false))
-                    .withSpan(param.span.endPos)
+          if (!param.tpt.isEmpty) param
+          else
+            val (formalBounds, isErased) = protoFormal(i)
+            val formal = formalBounds.loBound
+            val isBottomFromWildcard = (formalBounds ne formal) && formal.isExactlyNothing
+            val knownFormal = isFullyDefined(formal, ForceDegree.failBottom)
+            // If the expected formal is a TypeBounds wildcard argument with Nothing as lower bound,
+            // try to prioritize inferring from target. See issue 16405 (tests/run/16405.scala)
+            val paramType =
+              // Strip inferred erased annotation, to avoid accidentally inferring erasedness
+              val formal0 = if !isErased then formal.stripAnnots(_.symbol != defn.ErasedParamAnnot) else formal
+              if knownFormal && !isBottomFromWildcard then
+                formal0
+              else
+                inferredFromTarget(param, formal, calleeType, isErased, paramIndex).orElse(
+                  if knownFormal then formal0
+                  else errorType(AnonymousFunctionMissingParamType(param, tree, inferredType = formal, expectedType = pt), param.srcPos)
                 )
-              cpy.ValDef(param)(tpt = paramTpt)
-          if isErased then param0.withAddedFlags(Flags.Erased) else param0
+            val paramTpt = untpd.TypedSplice(
+                (if knownFormal then InferredTypeTree() else untpd.TypeTree())
+                  .withType(paramType.translateFromRepeated(toArray = false))
+                  .withSpan(param.span.endPos)
+              )
+            val param0 = cpy.ValDef(param)(tpt = paramTpt)
+            if isErased then param0.withAddedFlags(Flags.Erased) else param0
       desugared = desugar.makeClosure(Nil, inferredParams, fnBody, resultTpt, tree.span)
 
     typed(desugared, pt)

tests/pos/i18276a.scala

@@ -0,0 +1,15 @@
+import scala.language.implicitConversions
+
+case class Assign(left: String, right: String)
+class SyntaxAnalyser extends ParsersBase {
+  val x: Parser[String ~ String] = ???
+  val y: Parser[Assign] = x.map(Assign.apply)
+}
+
+class ParsersBase {
+  trait ~[+T, +U]
+  abstract class Parser[+T]:
+    def map[U](f: T => U): Parser[U] = ???
+
+  given [A, B, X]: Conversion[(A, B) => X, (A ~ B) => X] = ???
+}

tests/pos/i18276b.scala

@@ -0,0 +1,9 @@
+import scala.language.implicitConversions
+
+def foo(a: Int): Int = ???
+def bar(f: () => Int): Int = ???
+
+given f: Conversion[Int => Int, () => Int] = ???
+
+def test1: Int = bar(foo) // implicit conversion applied to foo
+def test2: Int = bar(f(foo))