[Sema] Downgrade multi-param default inference diag for methods

We recently started enforcing this rule for methods, downgrade to a
warning until the next language mode to avoid source compatibility
breakage.

Author: hamishknight

lib/Sema/TypeCheckConstraints.cpp

@@ -669,12 +669,19 @@ Type TypeChecker::typeCheckParameterDefault(Expr *&defaultValue,
           affectedParams, [&](const unsigned index) { params << "#" << index; },
           [&] { params << ", "; });
 
-      ctx.Diags.diagnose(
+      auto diag = ctx.Diags.diagnose(
           defaultValue->getLoc(),
           diag::
               cannot_default_generic_parameter_inferrable_from_another_parameter,
           paramInterfaceTy, params.str());
-      return Type();
+
+      // In Swift 6.2 and below we incorrectly missed checking this rule for
+      // methods, downgrade to a warning until the next language mode.
+      auto futureVersion = version::Version::getFutureMajorLanguageVersion();
+      if (!anchor->hasCurriedSelf() || ctx.isSwiftVersionAtLeast(futureVersion))
+        return Type();
+
+      diag.warnUntilFutureSwiftVersion();
     }
   }
 

test/Constraints/type_inference_from_default_exprs.swift

@@ -278,7 +278,7 @@ func testInferenceFromClosureVarInvalid<T>(x: T = { let x = "" as Int; return x
 // https://github.com/swiftlang/swift/issues/72199
 enum S72199_1 {
   func testS72199_1<T>(_: T = 42, _: [T]) {}
-  // expected-error@-1 {{cannot use default expression for inference of 'T' because it is inferrable from parameters #0, #1}}
+  // expected-warning@-1 {{cannot use default expression for inference of 'T' because it is inferrable from parameters #0, #1; this will be an error in a future Swift language mode}}
 }
 
 func testS72199_2<T: P>(x: T.X, y: T = S()) { } // Ok

test/Constraints/type_inference_from_default_exprs_swift7.swift

@@ -0,0 +1,11 @@
+// RUN: %target-typecheck-verify-swift -swift-version 7
+// REQUIRES: swift7
+
+// https://github.com/swiftlang/swift/issues/72199
+enum S72199_1 {
+  func testS72199_1<T>(_: T = 42, _: [T]) {}
+  // expected-error@-1 {{cannot use default expression for inference of 'T' because it is inferrable from parameters #0, #1}}
+}
+
+func testS72199<T>(_: T = 42, _: [T]) {}
+// expected-error@-1 {{cannot use default expression for inference of 'T' because it is inferrable from parameters #0, #1}}