under -source:future, don't warn on infix usage of Scala 2 defined methods with default arguments

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

@@ -2678,7 +2678,10 @@ class Typer extends Namer
           wrapDefs(defs, lift(app))
         else app
       }
-    checkValidInfix(tree, result.symbol)
+    // issue 10383: we stripBlock because e.g. default arguments desugar to blocks during typing,
+    // and the block itself doesn't have a symbol (because a Block isn't a ProxyTree),
+    // but the last expression in the block does have the right symbol
+    checkValidInfix(tree, stripBlock(result).symbol)
     result
   }
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -62,6 +62,7 @@ class CompilationTests {
       compileFile("tests/pos-custom-args/i9267.scala", defaultOptions.and("-Ystop-after:erasure")),
       compileFile("tests/pos-special/extend-java-enum.scala", defaultOptions.and("-source", "3.0-migration")),
       compileFile("tests/pos-custom-args/help.scala", defaultOptions.and("-help", "-V", "-W", "-X", "-Y")),
+      compileFile("tests/pos-custom-args/i10383.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
     ).checkCompile()
   }
 

tests/pos-custom-args/i10383.scala

@@ -0,0 +1 @@
+def r = BigInt(1) to BigInt(3) // error