Fix #18649: Use loBound of param types when materializing a context function.

Since the param types come from type arguments to
`ContextFunctionN[...]`, nothing prevents them a priori from being
wildcard type params, i.e., `TypeBounds`. However, that is not a
valid type to give to a concrete term param.

We can arbitrarily choose any type that conforms to the bounds,
which realistically means one of the two bounds. Since type
inference already chooses the lower bound when explicitly writing
the context function, we align and choose the lower bound when
materializing it.

[Cherry-picked 242e68f]

Author: sjrd

Diff for: compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -571,6 +571,14 @@ object TreeChecker {
       super.typedClassDef(cdef, cls)
     }
 
+    override def typedValDef(vdef: untpd.ValDef, sym: Symbol)(using Context): Tree =
+      val tpdTree = super.typedValDef(vdef, sym)
+      vdef.tpt.tpe match
+        case _: ValueType => () // ok
+        case _: ExprType if sym.isOneOf(TermParamOrAccessor) => () // ok
+        case _ => assert(false, i"wrong type, expected a value type for ${sym.fullName}, but found: ${sym.info}")
+      tpdTree
+
     override def typedDefDef(ddef: untpd.DefDef, sym: Symbol)(using Context): Tree =
       def defParamss = ddef.paramss.filter(!_.isEmpty).nestedMap(_.symbol)
       def layout(symss: List[List[Symbol]]): String =

Diff for: compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3160,7 +3160,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val paramTypes = {
       val hasWildcard = formals.exists(_.existsPart(_.isInstanceOf[WildcardType], StopAt.Static))
       if hasWildcard then formals.map(_ => untpd.TypeTree())
-      else formals.map(untpd.TypeTree)
+      else formals.map(formal => untpd.TypeTree(formal.loBound)) // about loBound, see tests/pos/i18649.scala
     }
 
     val erasedParams = pt match {

Diff for: tests/pos/i18649.scala

@@ -0,0 +1,7 @@
+object Test:
+  // always inferred Nothing for `x`
+  def contextFunctionWildcardExplicit: ? ?=> String = x ?=> "foo"
+
+  // used to infer TYPEBOUNDS for the type of the argument
+  def contextFunctionWildcardInserted: ? ?=> String = "foo"
+end Test