[Constraint system] Set the contextual type for condition expressions.

Set the contextual type of conditional expressions (to Bool) with the
"condition" purpose so we get customized diagnostics for mistakes such
as

    if x.property = y { }

Prior to this, we'd get generic "() isn't convertible to Bool"
diagnostic. Now we get

    use of '=' in a boolean context, did you mean '=='?

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -3821,6 +3821,7 @@ bool ConstraintSystem::generateConstraints(StmtCondition condition,
     return true;
   }
 
+  Type boolTy = boolDecl->getDeclaredType();
   for (const auto &condElement : condition) {
     switch (condElement.getKind()) {
     case StmtConditionElement::CK_Availability:
@@ -3829,15 +3830,18 @@ bool ConstraintSystem::generateConstraints(StmtCondition condition,
 
     case StmtConditionElement::CK_Boolean: {
       Expr *condExpr = condElement.getBoolean();
+      setContextualType(condExpr, TypeLoc::withoutLoc(boolTy), CTP_Condition);
+
       condExpr = generateConstraints(condExpr, dc);
       if (!condExpr) {
         return true;
       }
 
       addConstraint(ConstraintKind::Conversion,
                     getType(condExpr),
-                    boolDecl->getDeclaredType(),
-                    getConstraintLocator(condExpr));
+                    boolTy,
+                    getConstraintLocator(condExpr,
+                                         LocatorPathElt::ContextualType()));
       continue;
     }
 

test/Constraints/function_builder_diags.swift

@@ -257,7 +257,7 @@ struct MyTuplifiedStruct {
   }
 }
 
-// Check that we're performing syntactic use diagnostics/
+// Check that we're performing syntactic use diagnostics.
 func acceptMetatype<T>(_: T.Type) -> Bool { true }
 
 func syntacticUses<T>(_: T) {
@@ -269,3 +269,18 @@ func syntacticUses<T>(_: T) {
     }
   }
 }
+
+// Check custom diagnostics within "if" conditions.
+struct HasProperty {
+  var property: Bool = false
+}
+
+func checkConditions(cond: Bool) {
+  var x = HasProperty()
+
+  tuplify(cond) { value in
+    if x.property = value { // expected-error{{use of '=' in a boolean context, did you mean '=='?}}
+      "matched it"
+    }
+  }
+}