Fix #6750: Use semi-erased by name parameter signature

Before `Erasure` the signatures will contain `=>X` instead of `Function0`. This makes signature
clashes of by-name parameters only show up after `Erasure`. This allows for `inline` methods to
have by-name parameter overloads.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -608,7 +608,19 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       case tp: TermRef =>
         sigName(tp.widen)
       case ExprType(rt) =>
-        sigName(defn.FunctionOf(Nil, rt))
+        // Before erasure the signature will be `=>X` and after erasure it will be `Function0`.
+        // This is to support overriding inline methods with by-name parameters we slghtly modify the signature before erasure.
+        // By-name arguments are necesarry in all inline methods that may want to elide the evaluation of the arguments.
+        // Without this signature modification, the following two inline methods would have the same signature.
+        //   `inline def foo(x: => Int): Unit = ???`
+        //   `inline def foo(x: => Long): Unit = ???`
+        // Note that the decalrataions of `inline` method do not reach passed erasure.
+        //
+        // It would not be possible to make this change only to `inline` methods because then overriding would be affected.
+        // This would make merge error logic is quite complex to hadle as is signature based for performance.
+        // It was deemed to tricky to get right.
+        if (ctx.phase.erasedTypes) sigName(defn.FunctionOf(Nil, rt))
+        else tpnme.ARROWkw ++ sigName(rt)
       case tp: TypeVar =>
         val inst = tp.instanceOpt
         if (inst.exists) sigName(inst) else tpnme.Uninstantiated

tests/neg/i6750.scala

@@ -0,0 +1,4 @@
+object Foo {
+  def bar(i: => Int): Unit  = ???
+  def bar(l: => Long): Unit = ??? // error
+}

tests/neg/i6750b.scala

@@ -0,0 +1,4 @@
+object Foo {
+  def bar(i: => Int): Unit  = ???
+  def bar(l: () => Long): Unit = ??? // error
+}

tests/pos/i6750.scala

@@ -0,0 +1,7 @@
+object Foo {
+  inline def foo(i: => Int): Unit  = ???
+  inline def foo(l: => Long): Unit = ???
+
+  foo(2)
+  foo(3L)
+}

tests/run/i6750.scala

@@ -0,0 +1,16 @@
+
+object Test {
+
+  inline def foo(i: => Int)  = i + i
+  inline def foo(l: => Long) = l * l
+
+  inline def bar(i: () => Int)  = ???
+  inline def bar[T](x: () => T) = ???
+
+  def main(args: Array[String]): Unit = {
+    assert(10 == foo(5))
+    assert(25L == foo(5L))
+
+  }
+
+}