Disallow normal method names starting with extension_

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

@@ -1953,7 +1953,11 @@ class Typer extends Namer
     }
 
     val ddef2 = assignType(cpy.DefDef(ddef)(name, tparams1, vparamss1, tpt1, rhs1), sym)
+
     checkSignatureRepeatedParam(sym)
+    if name.isExtensionName then
+      ctx.error(em"illegal method name: $name may not start with `extension_`",
+        ddef.source.atSpan(ddef.nameSpan))
     ddef2.setDefTree
       //todo: make sure dependent method types do not depend on implicits or by-name params
   }

docs/docs/reference/contextual/extension-methods.md

@@ -229,7 +229,22 @@ def extension_position(s: String)(ch: Char, n: Int): Int =
   if n < s.length && s(n) != ch then extension_position(s)(ch, n + 1)
   else n
 ```
+### More Details
 
+1. To avoid confusion, names of normal methods are not allowed to start with `extension_`.
+
+2. A named import such as `import a.m` of an extension method in `a` will make `m`
+only available as an extension method. To access it under
+`extension_m` that name as to be imported separately. Example:
+```scala
+object DoubleOps:
+  extension (x: Double) def ** (exponent: Int): Double =
+    require(exponent > 0)
+    if exponent == 0 then 1 else x * (x ** (exponent - 1))
+
+import DoubleOps.{**, extension_**}
+assert(2.0 ** 3 == extension_**(2.0)(3))
+```
 
 ### Syntax
 

tests/neg/override-extension_normal-methods.scala

@@ -8,6 +8,7 @@ trait AAA extends A {
 
 trait B {
   def m[T](x: T): String  = "normal method"
+  def extension_n: String = "illegal method" // error: illegal method name: extension_n may not start with `extension_`
 }
 
 trait BBB extends B {

tests/pos/reference/extension-methods.scala

@@ -92,4 +92,11 @@ object ExtMethods:
       xs.sorted.takeRight(n)
   }
 
+  object DoubleOps:
+    extension (x: Double) def ** (exponent: Int): Double =
+      require(exponent > 0)
+      if exponent == 0 then 1 else x * (x ** (exponent - 1))
+
+  import DoubleOps.{**, extension_**}
+  assert(2.0 ** 3 == extension_**(2.0)(3))
 end ExtMethods