Disallow inline secondary constructors

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

@@ -97,6 +97,8 @@ object Inliner {
     if tree.symbol.isExperimental then
       Feature.checkExperimentalDef(tree.symbol, tree)
 
+    if tree.symbol.isConstructor then return tree // error already reported for the inline constructor definition
+
     /** Set the position of all trees logically contained in the expansion of
      *  inlined call `call` to the position of `call`. This transform is necessary
      *  when lifting bindings from the expansion to the outside of the call.

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

@@ -2164,6 +2164,8 @@ class Typer extends Namer
       PrepareInlineable.registerInlineInfo(sym, rhsToInline)
 
     if sym.isConstructor then
+      if sym.is(Inline) then
+        report.error("constructors cannot be `inline`", ddef)
       if sym.isPrimaryConstructor then
         if sym.owner.is(Case) then
           for

tests/neg/i12986a/Bar.scala

@@ -0,0 +1,2 @@
+class Bar(i: Int):
+  inline def this() = this(0) // error

tests/neg/i12986a/Test.scala

@@ -0,0 +1 @@
+val bar = new Bar()

tests/neg/i12986b.scala

@@ -0,0 +1,4 @@
+class Bar(i: Int):
+  transparent inline def this() = this(0) // error
+
+val bar = Bar()