Fix bypassing nothrow in debug statements (#20720)

* Fix bypassing nothrow in debug statements

* Fix debug walking null statements

compiler/src/dmd/expressionsem.d

@@ -1363,11 +1363,6 @@ private Expression resolveUFCSProperties(Scope* sc, Expression e1, Expression e2
         auto arguments = new Expressions(1);
         (*arguments)[0] = eleft;
         e = new CallExp(loc, e, arguments);
-
-        // https://issues.dlang.org/show_bug.cgi?id=24017
-        if (sc.flags & SCOPE.debug_)
-            e.isCallExp().inDebugStatement = true;
-
         e = e.expressionSemantic(sc);
         return e;
     }

compiler/src/dmd/statementsem.d

@@ -134,7 +134,16 @@ private Expression checkAssignmentAsCondition(Expression e, Scope* sc)
     return e;
 }
 
-// Performs semantic analysis in Statement AST nodes
+/**
+ * Performs semantic analysis in Statement AST nodes
+ *
+ * Params:
+ *   s = statement to perform semantic analysis on
+ *   sc = scope in which statement resides
+ *
+ * Returns: statement `s` after semantic analysis.
+ * Can be `null`, for example with `pragma(msg, "")`
+ */
 Statement statementSemantic(Statement s, Scope* sc)
 {
     import dmd.compiler;
@@ -3461,6 +3470,7 @@ Statement statementSemanticVisit(Statement s, Scope* sc)
             sc = sc.push();
             sc.flags |= SCOPE.debug_;
             ds.statement = ds.statement.statementSemantic(sc);
+            debugThrowWalker(ds.statement);
             sc.pop();
         }
         result = ds.statement;
@@ -4736,7 +4746,6 @@ private Statements* flatten(Statement statement, Scope* sc)
                 if (dc)
                 {
                     s = new DebugStatement(cs.loc, cs.ifbody);
-                    debugThrowWalker(cs.ifbody);
                 }
                 else
                     s = cs.ifbody;
@@ -4908,7 +4917,8 @@ Params:
 */
 private void debugThrowWalker(Statement s)
 {
-
+    if (!s)
+        return;
     extern(C++) final class DebugWalker : SemanticTimeTransitiveVisitor
     {
         alias visit = SemanticTimeTransitiveVisitor.visit;

compiler/test/compilable/debug_statement_attributes.d

@@ -85,3 +85,17 @@ void test6() nothrow
         () {throw new Exception("");}();
     }
 }
+
+void writeln() {}
+void writeln(string) {}
+
+void test7() nothrow
+{
+    debug writeln("Hello"); // https://issues.dlang.org/show_bug.cgi?id=24017
+    debug "Hello".writeln;
+    debug writeln = "Hello"; // https://github.com/dlang/dmd/issues/20719
+    debug writeln;
+
+    // https://github.com/dlang/dmd/pull/20720#issuecomment-2596892489
+    debug pragma(msg, ""); // Came up as segfault, pragma statement became null after semantic
+}

compiler/test/compilable/test24017.d

@@ -1,11 +0,0 @@
-// https://issues.dlang.org/show_bug.cgi?id=24017
-
-// REQUIRED_ARGS: -debug
-
-void writeln(string) {}
-
-void main() nothrow
-{
-    debug writeln("Hello");
-    debug "Hello".writeln;
-}