From 63f88a7a45e6de6ef1c75c28bc4deef959b226cd Mon Sep 17 00:00:00 2001
From: Nicolas Stucki <nicolas.stucki@gmail.com>
Date: Thu, 16 Feb 2023 10:41:19 +0100
Subject: [PATCH] Warn on use of inferred quote type variable bounds

This kind of inference is not reliable in general. We can only consider
the bounds from type constructor where the type variable is defined but
any other constraints are ignored. Therefore it is not possible to
properly infer the type bounds of the type variable.

The solution is simple, write the bounds explicitly and just check that
those bounds conform to the use site of the type variable.
---
 .../dotty/tools/dotc/typer/QuotesAndSplices.scala    |  2 ++
 .../fatal-warnings/quote-type-var-with-bounds.check  | 12 ++++++++++++
 .../fatal-warnings/quote-type-var-with-bounds.scala  | 11 +++++++++++
 3 files changed, 25 insertions(+)
 create mode 100644 tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.check
 create mode 100644 tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/QuotesAndSplices.scala b/compiler/src/dotty/tools/dotc/typer/QuotesAndSplices.scala
index 8473bd168bc5..5a356df16cf8 100644
--- a/compiler/src/dotty/tools/dotc/typer/QuotesAndSplices.scala
+++ b/compiler/src/dotty/tools/dotc/typer/QuotesAndSplices.scala
@@ -160,6 +160,8 @@ trait QuotesAndSplices {
       case _ => TypeBounds.empty
     val typeSym = newSymbol(spliceOwner(ctx), name, EmptyFlags, typeSymInfo, NoSymbol, tree.span)
     typeSym.addAnnotation(Annotation(New(ref(defn.QuotedRuntimePatterns_patternTypeAnnot.typeRef)).withSpan(tree.span)))
+    if !(typeSymInfo =:= TypeBounds.empty) then
+      report.warning(em"Type variable `$tree` has partially inferred bounds$pt.\n\nConsider defining bounds explicitly `'{ $typeSym$pt; ... }`", tree.srcPos)
     val pat = typedPattern(expr, defn.QuotedTypeClass.typeRef.appliedTo(typeSym.typeRef))(
         using spliceContext.retractMode(Mode.QuotedPattern).withOwner(spliceOwner(ctx)))
     pat.select(tpnme.Underlying)
diff --git a/tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.check b/tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.check
new file mode 100644
index 000000000000..a75e5c66d1c9
--- /dev/null
+++ b/tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.check
@@ -0,0 +1,12 @@
+-- Error: tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala:9:18 -----------------------------------
+9 |    case '{ $x: C[t] } => // error
+  |                  ^
+  |                  Type variable `t` has partially inferred bounds <: Int.
+  |
+  |                  Consider defining bounds explicitly `'{ type t <: Int; ... }`
+-- Error: tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala:10:18 ----------------------------------
+10 |    case '{ $x: D[t] } => // error
+   |                  ^
+   |                  Type variable `t` has partially inferred bounds >: Null <: String.
+   |
+   |                  Consider defining bounds explicitly `'{ type t >: Null <: String; ... }`
diff --git a/tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala b/tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala
new file mode 100644
index 000000000000..6b2b70ef8025
--- /dev/null
+++ b/tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala
@@ -0,0 +1,11 @@
+import scala.quoted.*
+
+class C[T <: Int]
+class D[T >: Null <: String]
+
+def test(e: Expr[Any])(using Quotes) =
+  e match
+    case '{ $x: t } =>
+    case '{ $x: C[t] } => // error
+    case '{ $x: D[t] } => // error
+    case '{ type t <: Int; $x: C[`t`] } =>