Warn on use of inferred quote type variable bounds

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

@@ -164,25 +164,36 @@ trait QuotesAndSplices {
       case pt: TypeBounds => pt
       case _ => TypeBounds.empty
 
-    def warnOnInferredBounds(typeSym: Symbol) =
+    def openQuote = if ctx.mode.is(Mode.QuotedExprPattern) then "'{" else "'["
+    def closeQuote = if ctx.mode.is(Mode.QuotedExprPattern) then "}" else "]"
+
+    def warnOnUnusedBounds(typeSym: Symbol) =
       if !(typeSymInfo =:= TypeBounds.empty) && !(typeSym.info <:< typeSymInfo) then
-        val (openQuote, closeQuote) = if ctx.mode.is(Mode.QuotedExprPattern) then ("'{", "}") else ("'[", "]")
         report.warning(em"Ignored bound$typeSymInfo\n\nConsider defining bounds explicitly:\n  $openQuote $typeSym${typeSym.info & typeSymInfo}; ... $closeQuote", tree.srcPos)
 
+    def warnOnInferredBounds(typeSym: Symbol) =
+      if !(typeSymInfo =:= TypeBounds.empty) then
+        if ctx.mode.is(Mode.QuotedExprPattern) then // TODO drop this guard once SIP-53 is non-experimental
+          // TODO state in the message when this will no longer work.
+          //   - We could stabilize SIP-53 and start deprecation warnings for type quoted patterns in 3.4.0.
+          //   - Then we could drop type variable inference 3.5.0.
+          report.deprecationWarning(em"Type variable `$tree` has partially inferred bounds$typeSymInfo.\n\nConsider defining bounds explicitly:\n  $openQuote $typeSym$typeSymInfo; ... $closeQuote", tree.srcPos)
+
     getQuotedPatternTypeVariable(tree.name.asTypeName) match
       case Some(typeSym) =>
         checkExperimentalFeature(
           "support for multiple references to the same type (without backticks) in quoted type patterns (SIP-53)",
           tree.srcPos,
           "\n\nSIP-53: https://docs.scala-lang.org/sips/quote-pattern-type-variable-syntax.html")
-        warnOnInferredBounds(typeSym)
+        warnOnUnusedBounds(typeSym)
         ref(typeSym)
       case None =>
         val spliceContext = quotePatternSpliceContext
         val name = tree.name.toTypeName
         val nameOfSyntheticGiven = PatMatGivenVarName.fresh(tree.name.toTermName)
         val expr = untpd.cpy.Ident(tree)(nameOfSyntheticGiven)
         val typeSym = newSymbol(spliceContext.owner, name, EmptyFlags, typeSymInfo, NoSymbol, tree.span)
+        warnOnInferredBounds(typeSym)
         typeSym.addAnnotation(Annotation(New(ref(defn.QuotedRuntimePatterns_patternTypeAnnot.typeRef)).withSpan(tree.span)))
         addQuotedPatternTypeVariable(typeSym)
         val pat = typedPattern(expr, defn.QuotedTypeClass.typeRef.appliedTo(typeSym.typeRef))(using spliceContext)
@@ -536,8 +547,12 @@ trait QuotesAndSplices {
 object QuotesAndSplices {
   import tpd._
 
+  private enum QuotePattenKind:
+    case Expr, Type
+
   /** Key for mapping from quoted pattern type variable names into their symbol */
   private val TypeVariableKey = new Property.Key[collection.mutable.Map[TypeName, Symbol]]
+  private val QuotePattenKindKey = new Property.Key[QuotePattenKind]
 
   /** Get the symbol for the quoted pattern type variable if it exists */
   def getQuotedPatternTypeVariable(name: TypeName)(using Context): Option[Symbol] =

tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.check

@@ -0,0 +1,49 @@
+-- Error: tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala:13:18 ----------------------------------
+13 |    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:14:18 ----------------------------------
+14 |    case '{ $x: D[t] } => // error
+   |                  ^
+   |                  Type variable `t` has partially inferred bounds >: Null <: String.
+   |
+   |                  Consider defining bounds explicitly:
+   |                    '{ type t >: Null <: String; ... }
+-- Error: tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala:17:18 ----------------------------------
+17 |    case '{ $x: E[t, 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:19:18 ----------------------------------
+19 |    case '{ $x: F[t, t] } => // error // 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:19:21 ----------------------------------
+19 |    case '{ $x: F[t, t] } => // error // error
+   |                     ^
+   |                     Ignored bound <: String
+   |
+   |                     Consider defining bounds explicitly:
+   |                       '{ type t <: Int & String; ... }
+-- Error: tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala:20:36 ----------------------------------
+20 |    case '{ type t <: Int; $x: F[t, t] } => // error
+   |                                    ^
+   |                                    Ignored bound <: String
+   |
+   |                                    Consider defining bounds explicitly:
+   |                                      '{ type t <: Int & String; ... }
+-- Error: tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala:26:17 ----------------------------------
+26 |    case '[ F[t, t] ] => // error // will have a second error with SIP-53
+   |                 ^
+   |                 Ignored bound <: String
+   |
+   |                 Consider defining bounds explicitly:
+   |                   '[ type t <: Int & String; ... ]

tests/neg-custom-args/fatal-warnings/quote-type-var-with-bounds.scala

@@ -0,0 +1,26 @@
+// scalac: -deprecation
+
+import scala.quoted.*
+
+class C[T <: Int]
+class D[T >: Null <: String]
+class E[T <: Int, U <: Int]
+class F[T <: Int, U <: String]
+
+def test[T: Type](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] } =>
+
+    case '{ $x: E[t, t] } => // error
+
+    case '{ $x: F[t, t] } => // error // error
+    case '{ type t <: Int; $x: F[t, t] } => // error
+
+  Type.of[T] match
+    case '[ C[t] ] => // will have an error with SIP-53
+    case '[ D[t] ] => // will have an error with SIP-53
+    case '[ E[t, t] ] => // will have an error with SIP-53
+    case '[ F[t, t] ] => // error // will have a second error with SIP-53

tests/neg-macros/quote-type-variable-no-inference-2.check

@@ -1,12 +1,19 @@
--- Warning: tests/neg-macros/quote-type-variable-no-inference-2.scala:5:22 ---------------------------------------------
-5 |    case '{ $_ : F[t, t]; () } => // warn // error
+-- Deprecation Warning: tests/neg-macros/quote-type-variable-no-inference-2.scala:7:19 ---------------------------------
+7 |    case '{ $_ : F[t, t]; () } => // warn // error
+  |                   ^
+  |                   Type variable `t` has partially inferred bounds <: Int.
+  |
+  |                   Consider defining bounds explicitly:
+  |                     '{ type t <: Int; ... }
+-- Warning: tests/neg-macros/quote-type-variable-no-inference-2.scala:7:22 ---------------------------------------------
+7 |    case '{ $_ : F[t, t]; () } => // warn // error
   |                      ^
   |                      Ignored bound <: Double
   |
   |                      Consider defining bounds explicitly:
   |                        '{ type t <: Int & Double; ... }
--- [E057] Type Mismatch Error: tests/neg-macros/quote-type-variable-no-inference-2.scala:5:12 --------------------------
-5 |    case '{ $_ : F[t, t]; () } => // warn // error
+-- [E057] Type Mismatch Error: tests/neg-macros/quote-type-variable-no-inference-2.scala:7:12 --------------------------
+7 |    case '{ $_ : F[t, t]; () } => // warn // error
   |            ^
   |            Type argument t does not conform to upper bound Double in inferred type F[t, t]
   |

tests/neg-macros/quote-type-variable-no-inference-2.scala

@@ -1,3 +1,5 @@
+// scalac: -deprecation
+
 import scala.quoted.*
 
 def test2(x: Expr[Any])(using Quotes) =