Do not check if we are in a discared context in non-immediate contexts

This fixes in a regression introduced by 6eeda06c1.

When deducing the return type of nested function calls, only the
return type of the outermost expression should be ignored.

Instead of assuming all contextes nested in a discared statements
are themselves discarded, only assume that in immediate contexts.

Similarly, only consider contextes immediately in an immediate or
discarded statement as being themselves immediate.

clang/include/clang/Sema/Sema.h

@@ -1324,12 +1324,15 @@ public:
 
     bool isImmediateFunctionContext() const {
       return Context == ExpressionEvaluationContext::ImmediateFunctionContext ||
-             InImmediateFunctionContext;
+             (Context == ExpressionEvaluationContext::DiscardedStatement &&
+              InImmediateFunctionContext);
     }
 
     bool isDiscardedStatementContext() const {
       return Context == ExpressionEvaluationContext::DiscardedStatement ||
-             InDiscardedStatement;
+             (Context ==
+                  ExpressionEvaluationContext::ImmediateFunctionContext &&
+              InDiscardedStatement);
     }
   };
 

clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp

@@ -159,4 +159,19 @@ a:  if constexpr(sizeof(n) == 4) // expected-error {{redefinition}} expected-not
       surprise: {}
   }
 }
+
+namespace deduced_return_type_in_discareded_statement {
+
+template <typename T>
+auto a(const T &t) {
+  return t;
+}
+
+void f() {
+  if constexpr (false) {
+    a(a(0));
+  }
+}
+} // namespace deduced_return_type_in_discareded_statement
+
 #endif