[CS] Add contextual Bool type to switch-case where clause

Seems this was accidentally omitted, which would
crash in CSApply for any non-Bool-convertible type.

lib/Sema/CSApply.cpp

@@ -9224,20 +9224,13 @@ ExprWalker::rewriteTarget(SolutionApplicationTarget target) {
     }
 
     // If there is a guard expression, coerce that.
-    if (auto guardExpr = info.guardExpr) {
-      guardExpr = guardExpr->walk(*this);
-      if (!guardExpr)
+    if (auto *guardExpr = info.guardExpr) {
+      auto target = *cs.getSolutionApplicationTarget(guardExpr);
+      auto resultTarget = rewriteTarget(target);
+      if (!resultTarget)
         return None;
 
-      // FIXME: Feels like we could leverage existing code more.
-      Type boolType = cs.getASTContext().getBoolType();
-      guardExpr = solution.coerceToType(
-          guardExpr, boolType, cs.getConstraintLocator(info.guardExpr));
-      if (!guardExpr)
-        return None;
-
-      (*caseLabelItem)->setGuardExpr(guardExpr);
-      solution.setExprTypes(guardExpr);
+      (*caseLabelItem)->setGuardExpr(resultTarget->getAsExpr());
     }
 
     return target;

lib/Sema/CSGen.cpp

@@ -4768,9 +4768,15 @@ bool ConstraintSystem::generateConstraints(
     // Generate constraints for the guard expression, if there is one.
     Expr *guardExpr = caseLabelItem.getGuardExpr();
     if (guardExpr) {
-      guardExpr = generateConstraints(guardExpr, dc);
-      if (!guardExpr)
+      auto &ctx = dc->getASTContext();
+      SolutionApplicationTarget guardTarget(
+          guardExpr, dc, CTP_Condition, ctx.getBoolType(), /*discarded*/ false);
+
+      if (generateConstraints(guardTarget))
         return true;
+
+      guardExpr = guardTarget.getAsExpr();
+      setSolutionApplicationTarget(guardExpr, guardTarget);
     }
 
     // Save this info.

lib/Sema/CSSyntacticElement.cpp

@@ -529,6 +529,9 @@ public:
   void visitCaseItem(CaseLabelItem *caseItem, ContextualTypeInfo contextInfo) {
     assert(contextInfo.purpose == CTP_CaseStmt);
 
+    auto *DC = context.getAsDeclContext();
+    auto &ctx = DC->getASTContext();
+
     // Resolve the pattern.
     auto *pattern = caseItem->getPattern();
     if (!caseItem->isPatternResolved()) {
@@ -554,11 +557,15 @@ public:
 
     // Generate constraints for `where` clause (if any).
     if (guardExpr) {
-      guardExpr = cs.generateConstraints(guardExpr, context.getAsDeclContext());
-      if (!guardExpr) {
+      SolutionApplicationTarget guardTarget(
+          guardExpr, DC, CTP_Condition, ctx.getBoolType(), /*discarded*/ false);
+
+      if (cs.generateConstraints(guardTarget)) {
         hadError = true;
         return;
       }
+      guardExpr = guardTarget.getAsExpr();
+      cs.setSolutionApplicationTarget(guardExpr, guardTarget);
     }
 
     // Save information about case item so it could be referenced during

test/Constraints/diagnostics.swift

@@ -1531,6 +1531,22 @@ func testUnwrapFixIts(x: Int?) throws {
   let _: Int = try! .optionalThrowsMember ?? 0
 }
 
+// https://github.com/apple/swift/issues/63746
+func issue63746() {
+  let fn1 = {
+    switch 0 {
+      case 1 where 0: // expected-error {{type 'Int' cannot be used as a boolean; test for '!= 0' instead}}
+        ()
+    }
+  }
+  let fn2 = {
+    switch 0 {
+      case 1 where 0: // expected-error {{type 'Int' cannot be used as a boolean; test for '!= 0' instead}}
+        break
+    }
+  }
+}
+
 func rdar86611718(list: [Int]) {
   String(list.count())
   // expected-error@-1 {{cannot call value of non-function type 'Int'}}