[Diagnostics] Make same-shape requirement failure diagnostics less sensitive

to binding order.

Author: hborla

lib/Sema/CSDiagnostics.cpp

@@ -487,6 +487,14 @@ void RequirementFailure::maybeEmitRequirementNote(const Decl *anchor, Type lhs,
     return;
   }
 
+  if (req.getKind() == RequirementKind::SameShape) {
+    // Same-shape requirements are broken down into two ShapeOf
+    // constraints against the same type variable, so a failure is not
+    // necessarily ordered to match the original requriement. For now,
+    // don't emit a note.
+    return;
+  }
+
   emitDiagnosticAt(anchor, diag::where_requirement_failure_both_subst,
                    req.getFirstType(), lhs, req.getSecondType(), rhs);
 }

lib/Sema/CSSimplify.cpp

@@ -2491,9 +2491,9 @@ ConstraintSystem::matchPackExpansionTypes(PackExpansionType *expansion1,
       locator.withPathElement(ConstraintLocator::PackShape));
   auto *shapeTypeVar = createTypeVariable(shapeLoc, TVO_CanBindToPack);
   addConstraint(ConstraintKind::ShapeOf,
-                expansion1->getCountType(), shapeTypeVar, locator);
+                expansion1->getCountType(), shapeTypeVar, shapeLoc);
   addConstraint(ConstraintKind::ShapeOf,
-                expansion2->getCountType(), shapeTypeVar, locator);
+                expansion2->getCountType(), shapeTypeVar, shapeLoc);
 
   auto pattern1 = expansion1->getPatternType();
   auto pattern2 = expansion2->getPatternType();
@@ -6094,6 +6094,16 @@ bool ConstraintSystem::repairFailures(
 
   case ConstraintLocator::PackShape: {
     auto *shapeLocator = getConstraintLocator(locator);
+
+    // FIXME: If the anchor isn't a pack expansion, this shape requirement
+    // came from a same-shape generic requirement, which will fail separately
+    // with an applied requirement fix. Currently, pack shapes can themselves be
+    // pack types with pack expansions, so matching shape types can recursively
+    // add ShapeOf constraints. For now, skip fixing the nested ones to avoid
+    // cascading diagnostics.
+    if (!isExpr<PackExpansionExpr>(shapeLocator->getAnchor()))
+      return true;
+
     auto *fix = SkipSameShapeRequirement::create(*this, lhs, rhs, shapeLocator);
     conversionsOrFixes.push_back(fix);
     break;

test/Constraints/variadic_generic_constraints.swift

@@ -58,7 +58,6 @@ takesParallelSequences(t: Array<String>(), Set<Int>(), u: Array<Int>(), Set<Stri
 // Same-shape requirements
 
 func zip<T..., U...>(t: repeat each T, u: repeat each U) -> (repeat (each T, each U)) {}
-// expected-note@-1 {{where 'T' = 'T', 'U' = 'U'}}
 
 let _ = zip()  // ok
 let _ = zip(t: 1, u: "hi")  // ok
@@ -74,5 +73,5 @@ func goodCallToZip<T..., U...>(t: repeat each T, u: repeat each U) where (repeat
 
 func badCallToZip<T..., U...>(t: repeat each T, u: repeat each U) {
   _ = zip(t: repeat each t, u: repeat each u)
-  // expected-error@-1 {{global function 'zip(t:u:)' requires the type packs 'T' and 'U' have the same shape}}
+  // expected-error@-1 {{global function 'zip(t:u:)' requires the type packs 'U' and 'T' have the same shape}}
 }