Sema: Fix failure to diagnose throwing expressions inside string interpolations

We need to preserve the DiagnoseErrorOnTry bit stored in the Context when
exiting a ContextScope. Otherwise, we fail to produce a diagnostic if the
'try' expression pushes an intertwining Context, such as a string interpolation
or 'await'.

Fixes <https://bugs.swift.org/browse/SR-13654>, <rdar://problem/69958774>.

Author: slavapestov

lib/Sema/TypeCheckEffects.cpp

@@ -878,6 +878,13 @@ class Context {
       HandlesErrors(handlesErrors), HandlesAsync(handlesAsync) { }
 
 public:
+  bool shouldDiagnoseErrorOnTry() const {
+    return DiagnoseErrorOnTry;
+  }
+  void setDiagnoseErrorOnTry(bool b) {
+    DiagnoseErrorOnTry = b;
+  }
+
   /// Whether this is a function that rethrows.
   bool isRethrows() const {
     if (!HandlesErrors)
@@ -1375,6 +1382,7 @@ class CheckEffectsCoverage : public EffectsHandlingWalker<CheckEffectsCoverage>
     DeclContext *OldRethrowsDC;
     ContextFlags OldFlags;
     ThrowingKind OldMaxThrowingKind;
+
   public:
     ContextScope(CheckEffectsCoverage &self, Optional<Context> newContext)
       : Self(self), OldContext(self.CurContext),
@@ -1468,7 +1476,15 @@ class CheckEffectsCoverage : public EffectsHandlingWalker<CheckEffectsCoverage>
     }
 
     ~ContextScope() {
+      // The "DiagnoseErrorOnTry" flag is a bit of mutable state
+      // in the Context itself, used to postpone diagnostic emission
+      // to a parent "try" expression. If something was diagnosed
+      // during this ContextScope, the flag may have been set, and
+      // we need to preseve its value when restoring the old Context.
+      bool DiagnoseErrorOnTry = Self.CurContext.shouldDiagnoseErrorOnTry();
       Self.CurContext = OldContext;
+      Self.CurContext.setDiagnoseErrorOnTry(DiagnoseErrorOnTry);
+
       Self.RethrowsDC = OldRethrowsDC;
       Self.Flags = OldFlags;
       Self.MaxThrowingKind = OldMaxThrowingKind;

test/expr/unary/async_await.swift

@@ -133,3 +133,12 @@ func testStringInterpolation() async throws {
   _ = "Eventually produces \(await getInt())"
   _ = await "Eventually produces \(getInt())"
 }
+
+// Make sure try await works too
+func invalidAsyncFunction() async {
+  _ = try await throwingAndAsync() // expected-error {{errors thrown from here are not handled}}
+}
+
+func validAsyncFunction() async throws {
+  _ = try await throwingAndAsync()
+}

test/stmt/errors.swift

@@ -251,3 +251,16 @@ func sr_11402_func2(_ x: SR_11402_P) {
     print(y)
   }
 }
+
+// https://bugs.swift.org/browse/SR-13654
+
+func sr_13654_func() throws -> String {}
+
+func sr_13654_invalid_interpolation() {
+  _ = try "\(sr_13654_func())" // expected-error {{errors thrown from here are not handled}}
+  _ = "\(try sr_13654_func())" // expected-error {{errors thrown from here are not handled}}
+}
+func sr_13654_valid_interpolation() throws {
+  _ = try "\(sr_13654_func())"
+  _ = "\(try sr_13654_func())"
+}