[Diagnostics] Diagnose conflicting pattern variables

Diagnose situations where pattern variables with the same name
have conflicting types:

```swift
enum E {
case a(Int)
case b(String)
}

func test(e: E) {
  switch e {
  case .a(let x), .b(let x): ...
  }
}
```

In this example `x` is bound to `Int` and `String` at the same
time which is incorrect.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -8195,3 +8195,12 @@ void MissingExplicitExistentialCoercion::fixIt(
                               "as " + ErasedResultType->getString(printOpts) +
                                   (requiresParens ? ")" : ""));
 }
+
+bool ConflictingPatternVariables::diagnoseAsError() {
+  for (auto *var : Vars) {
+    emitDiagnosticAt(var->getStartLoc(),
+                     diag::type_mismatch_multiple_pattern_list, getType(var),
+                     ExpectedType);
+  }
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -2764,6 +2764,41 @@ class MissingExplicitExistentialCoercion final : public FailureDiagnostic {
   bool fixItRequiresParens() const;
 };
 
+/// Diagnose situations where pattern variables with the same name
+/// have conflicting types:
+///
+/// \code
+/// enum E {
+/// case a(Int)
+/// case b(String)
+/// }
+///
+/// func test(e: E) {
+///   switch e {
+///    case .a(let x), .b(let x): ...
+///   }
+/// }
+/// \endcode
+///
+/// In this example `x` is bound to `Int` and `String` at the same
+/// time which is incorrect.
+class ConflictingPatternVariables final : public FailureDiagnostic {
+  Type ExpectedType;
+  SmallVector<VarDecl *, 4> Vars;
+
+public:
+  ConflictingPatternVariables(const Solution &solution, Type expectedTy,
+                              ArrayRef<VarDecl *> conflicts,
+                              ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator),
+        ExpectedType(resolveType(expectedTy)),
+        Vars(conflicts.begin(), conflicts.end()) {
+    assert(!Vars.empty());
+  }
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -2422,7 +2422,9 @@ AddExplicitExistentialCoercion::create(ConstraintSystem &cs, Type resultTy,
 
 bool RenameConflictingPatternVariables::diagnose(const Solution &solution,
                                                  bool asNote) const {
-  return false;
+  ConflictingPatternVariables failure(solution, ExpectedType,
+                                      getConflictingVars(), getLocator());
+  return failure.diagnose(asNote);
 }
 
 RenameConflictingPatternVariables *