[Features] Gate same-element requirements behind an experimental feature flag.

Author: hborla

include/swift/Basic/Features.def

@@ -232,6 +232,7 @@ EXPERIMENTAL_FEATURE(CodeItemMacros, false)
 EXPERIMENTAL_FEATURE(PreambleMacros, false)
 EXPERIMENTAL_FEATURE(TupleConformances, false)
 EXPERIMENTAL_FEATURE(FullTypedThrows, false)
+EXPERIMENTAL_FEATURE(SameElementRequirements, false)
 
 // Whether to enable @_used and @_section attributes
 EXPERIMENTAL_FEATURE(SymbolLinkageMarkers, true)

lib/AST/FeatureSet.cpp

@@ -134,6 +134,7 @@ UNINTERESTING_FEATURE(StaticExclusiveOnly)
 UNINTERESTING_FEATURE(ExtractConstantsFromMembers)
 UNINTERESTING_FEATURE(FixedArrays)
 UNINTERESTING_FEATURE(GroupActorErrors)
+UNINTERESTING_FEATURE(SameElementRequirements)
 
 static bool usesFeatureSendingArgsAndResults(Decl *decl) {
   auto isFunctionTypeWithSending = [](Type type) {

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -220,6 +220,17 @@ static void desugarSameTypeRequirement(
         break;
       }
 
+      auto &ctx = firstType->getASTContext();
+      if (!ctx.LangOpts.hasFeature(Feature::SameElementRequirements)) {
+        // If one side is a parameter pack, this is a same-element requirement, which
+        // is not yet supported.
+        if (firstType->isParameterPack() != secondType->isParameterPack()) {
+          errors.push_back(RequirementError::forSameElement(
+              {kind, sugaredFirstType, secondType}, loc));
+          return true;
+        }
+      }
+
       if (firstType->isTypeParameter() && secondType->isTypeParameter()) {
         result.emplace_back(kind, sugaredFirstType, secondType);
         return true;

test/Constraints/pack-expansion-expressions.swift

@@ -63,12 +63,16 @@ func outerArchetype<each T, U>(t: repeat each T, u: U) where repeat each T: P {
 }
 
 func sameElement<each T, U>(t: repeat each T, u: U) where repeat each T: P, repeat each T == U {
-  let _ = (repeat (each t).f(u))
+  // expected-error@-1{{same-element requirements are not yet supported}}
+  let _: (repeat each T) = (repeat (each t).f(u))
+  // expected-error@-1 {{cannot convert value of type 'U' to expected argument type 'each T'}}
 }
 
 func forEachEach<each C, U>(c: repeat each C, function: (U) -> Void)
     where repeat each C: Collection, repeat (each C).Element == U {
-  repeat (each c).forEach(function)
+  // expected-error@-1{{same-element requirements are not yet supported}}
+  _ = (repeat (each c).forEach(function))
+  // expected-error@-1 {{cannot convert value of type '(U) -> Void' to expected argument type '((each C).Element) throws -> Void'}}
 }
 
 func typeReprPacks<each T: ExpressibleByIntegerLiteral>(_ t: repeat each T) {

test/Generics/parameter-pack-same-element-requirements.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -disable-availability-checking 2>&1 | %FileCheck %s
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -enable-experimental-feature SameElementRequirements -disable-availability-checking 2>&1 | %FileCheck %s
 
 // REQUIRES: asserts