From 104557127ec82fc9237a0d7949fdcb737552ec18 Mon Sep 17 00:00:00 2001
From: Guillaume Martres <smarter@ubuntu.com>
Date: Thu, 11 Apr 2024 20:41:44 +0200
Subject: [PATCH] Retain default parameters with `export`

Default parameters need to have the `HasDefault` flag set to work properly, it
seems that without this flag they were still selected but only under join
compilation.

While we're at it, we define a full set of flags that might be exported. Note
that Given/Implicit/Erased were already set by `tpd.DefDef` and Inlined doesn't
seem to make a difference in practice since the body of the exported inline def
is manually constructed and doesn't contain proxies either way.
---
 compiler/src/dotty/tools/dotc/core/Flags.scala  |  3 +++
 compiler/src/dotty/tools/dotc/typer/Namer.scala |  4 +++-
 tests/pos/export-param-flags/A_1.scala          |  5 +++++
 tests/pos/export-param-flags/B_2.scala          |  2 ++
 tests/printing/export-param-flags.check         | 13 +++++++++++++
 tests/printing/export-param-flags.scala         |  5 +++++
 6 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/export-param-flags/A_1.scala
 create mode 100644 tests/pos/export-param-flags/B_2.scala
 create mode 100644 tests/printing/export-param-flags.check
 create mode 100644 tests/printing/export-param-flags.scala

diff --git a/compiler/src/dotty/tools/dotc/core/Flags.scala b/compiler/src/dotty/tools/dotc/core/Flags.scala
index 1f38289161cd..8110bc769d4f 100644
--- a/compiler/src/dotty/tools/dotc/core/Flags.scala
+++ b/compiler/src/dotty/tools/dotc/core/Flags.scala
@@ -535,6 +535,9 @@ object Flags {
   /** Flags retained in term export forwarders */
   val RetainedExportTermFlags = Infix | Given | Implicit | Inline | Transparent | Erased | HasDefaultParams | NoDefaultParams | ExtensionMethod
 
+  /** Flags retained in parameters of term export forwarders */
+  val RetainedExportTermParamFlags = Given | Implicit | Erased | HasDefault | Inline
+
   val MandatoryExportTermFlags = Exported | Method | Final
 
   /** Flags retained in type export forwarders */
diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 1016fe467a0a..bad78b6714e8 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -1293,7 +1293,7 @@ class Namer { typer: Typer =>
                 getter => addForwarder(
                   getter.name.asTermName, getter.asSeenFrom(path.tpe), span))
 
-            // adding annotations at the parameter level
+            // adding annotations and flags at the parameter level
             // TODO: This probably needs to be filtered to avoid adding some annotation
             // such as MacroAnnotations
             if sym.is(Method) then
@@ -1301,6 +1301,8 @@ class Namer { typer: Typer =>
                   (origParameter, exportedParameter) <- orig.lazyZip(forwarded)
               do
                 exportedParameter.addAnnotations(origParameter.annotations)
+                if exportedParameter.isTerm then
+                  exportedParameter.setFlag(origParameter.flags & RetainedExportTermParamFlags)
       end addForwarder
 
       def addForwardersNamed(name: TermName, alias: TermName, span: Span): Unit =
diff --git a/tests/pos/export-param-flags/A_1.scala b/tests/pos/export-param-flags/A_1.scala
new file mode 100644
index 000000000000..1ac8d10ba930
--- /dev/null
+++ b/tests/pos/export-param-flags/A_1.scala
@@ -0,0 +1,5 @@
+object A:
+  def defaultParam(x: Int = 1) = x
+
+object Exported:
+  export A.*
diff --git a/tests/pos/export-param-flags/B_2.scala b/tests/pos/export-param-flags/B_2.scala
new file mode 100644
index 000000000000..0387f66d7aa7
--- /dev/null
+++ b/tests/pos/export-param-flags/B_2.scala
@@ -0,0 +1,2 @@
+object B:
+  val x = Exported.defaultParam()
diff --git a/tests/printing/export-param-flags.check b/tests/printing/export-param-flags.check
new file mode 100644
index 000000000000..ffab6f77c93d
--- /dev/null
+++ b/tests/printing/export-param-flags.check
@@ -0,0 +1,13 @@
+[[syntax trees at end of                     typer]] // tests/printing/export-param-flags.scala
+package <empty> {
+  final lazy module val A: A = new A()
+  final module class A() extends Object() { this: A.type =>
+    inline def inlinedParam(inline x: Int): Int = x.+(x):Int
+  }
+  final lazy module val Exported: Exported = new Exported()
+  final module class Exported() extends Object() { this: Exported.type =>
+    export A.*
+    final inline def inlinedParam(inline x: Int): Int = A.inlinedParam(x)
+  }
+}
+
diff --git a/tests/printing/export-param-flags.scala b/tests/printing/export-param-flags.scala
new file mode 100644
index 000000000000..cad2d3c8fee8
--- /dev/null
+++ b/tests/printing/export-param-flags.scala
@@ -0,0 +1,5 @@
+object A:
+  inline def inlinedParam(inline x: Int): Int = x + x
+
+object Exported:
+  export A.*