[Function builders] Use one-way constraints for closure parameters.

Introduce one-way constraints for the parameters of closures to which a
function builder is being applied. This was an intended part of the
model when one-way constraints were introduced, but somehow got
missed. This should further break up large constraints systems for
faster solving, and *most likely* won't break much source code in
practice.

Fixes rdar://problem/64231116.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -7222,6 +7222,24 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
   auto *closure = castToExpr<ClosureExpr>(closureLocator->getAnchor());
   auto *inferredClosureType = getClosureType(closure);
 
+  // Determine whether a function builder will be applied.
+  Type functionBuilderType;
+  ConstraintLocator *calleeLocator = nullptr;
+  if (auto last = locator.last()) {
+    if (auto argToParam = last->getAs<LocatorPathElt::ApplyArgToParam>()) {
+      calleeLocator = getCalleeLocator(getConstraintLocator(locator));
+      functionBuilderType = getFunctionBuilderTypeFor(
+          *this, argToParam->getParamIdx(), calleeLocator);
+    }
+  }
+
+  // Determine whether to introduce one-way constraints between the parameter's
+  // type as seen in the body of the closure and the external parameter
+  // type.
+  bool oneWayConstraints =
+    getASTContext().TypeCheckerOpts.EnableOneWayClosureParameters ||
+    functionBuilderType;
+
   auto *paramList = closure->getParameters();
   SmallVector<AnyFunctionType::Param, 4> parameters;
   for (unsigned i = 0, n = paramList->size(); i != n; ++i) {
@@ -7235,9 +7253,6 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
     }
 
     Type internalType;
-
-    bool oneWayConstraints =
-      getASTContext().TypeCheckerOpts.EnableOneWayClosureParameters;
     if (paramList->get(i)->getTypeRepr()) {
       // Internal type is the type used in the body of the closure,
       // so "external" type translates to it as follows:
@@ -7288,17 +7303,12 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
                         inferredClosureType->getExtInfo());
   assignFixedType(typeVar, closureType, closureLocator);
 
-  if (auto last = locator.last()) {
-    if (auto argToParam = last->getAs<LocatorPathElt::ApplyArgToParam>()) {
-      auto *calleeLocator = getCalleeLocator(getConstraintLocator(locator));
-      if (auto functionBuilderType = getFunctionBuilderTypeFor(
-              *this, argToParam->getParamIdx(), calleeLocator)) {
-        if (auto result = matchFunctionBuilder(
-                closure, functionBuilderType, closureType->getResult(),
-                ConstraintKind::Conversion, calleeLocator, locator)) {
-          return result->isSuccess();
-        }
-      }
+  // If there is a function builder to apply, do so now.
+  if (functionBuilderType) {
+    if (auto result = matchFunctionBuilder(
+            closure, functionBuilderType, closureType->getResult(),
+            ConstraintKind::Conversion, calleeLocator, locator)) {
+      return result->isSuccess();
     }
   }
 

test/Constraints/function_builder_diags.swift

@@ -558,3 +558,14 @@ func rdar61347993() {
   func test_closure(_: () -> Result) {}
   test_closure {} // expected-error {{cannot convert value of type '()' to closure result type 'Result'}}
 }
+
+// One-way constraints through parameters.
+func wrapperifyInfer<T, U>(_ cond: Bool, @WrapperBuilder body: (U) -> T) -> T {
+  fatalError("boom")
+}
+
+let intValue = 17
+wrapperifyInfer(true) { x in // expected-error{{unable to infer type of a closure parameter 'x' in the current context}}
+  intValue + x
+}
+

test/Constraints/function_builder_one_way.swift

@@ -49,16 +49,17 @@ func tuplify<C: Collection, T>(_ collection: C, @TupleBuilder body: (C.Element)
 }
 
 // CHECK: ---Connected components---
-// CHECK-NEXT: 0: $T1 $T2 $T3 $T5 $T6 $T7 $T8 $T10 $T77 $T78 depends on 2
-// CHECK-NEXT: 2: $T12 $T17 $T28 $T42 $T53 $T54 $T55 $T56 $T57 $T58 $T59 $T60 $T61 $T62 $T63 $T64 $T65 $T66 $T68 $T69 $T70 $T71 $T72 $T73 $T74 $T75 $T76 depends on 1, 3, 4, 6, 9
-// CHECK-NEXT: 9: $T48 $T49 $T50 $T51 $T52 depends on 8
-// CHECK-NEXT: 8: $T43 $T44 $T45 $T46 $T47
-// CHECK-NEXT: 6: $T31 $T35 $T36 $T37 $T38 $T39 $T40 $T41 depends on 5, 7
-// CHECK-NEXT: 7: $T32 $T33 $T34
-// CHECK-NEXT: 5: $T30
-// CHECK-NEXT: 4: $T18 $T19 $T20 $T21 $T22 $T23 $T24 $T25 $T26 $T27
-// CHECK-NEXT: 3: $T15 $T16
-// CHECK-NEXT: 1: $T11
+// CHECK-NEXT:   1: $T10 depends on 0
+// CHECK-NEXT:   0: $T1 $T2 $T3 $T5 $T6 $T7 $T8 $T77 $T78 depends on 3
+// CHECK-NEXT:   3: $T12 $T17 $T28 $T42 $T53 $T54 $T55 $T56 $T57 $T58 $T59 $T60 $T61 $T62 $T63 $T64 $T65 $T66 $T68 $T69 $T70 $T71 $T72 $T73 $T74 $T75 $T76 depends on 2, 4, 5, 7, 10
+// CHECK-NEXT:   10: $T48 $T49 $T50 $T51 $T52 depends on 9
+// CHECK-NEXT:   9: $T43 $T44 $T45 $T46 $T47
+// CHECK-NEXT:   7: $T31 $T35 $T36 $T37 $T38 $T39 $T40 $T41 depends on 6, 8
+// CHECK-NEXT:   8: $T32 $T33 $T34
+// CHECK-NEXT:   6: $T30
+// CHECK-NEXT:   5: $T18 $T19 $T20 $T21 $T22 $T23 $T24 $T25 $T26 $T27
+// CHECK-NEXT:   4: $T15 $T16
+// CHECK-NEXT:   2: $T11
 let names = ["Alice", "Bob", "Charlie"]
 let b = true
 var number = 17