Fix ICE/AV on string mixin of version=/debug= statements

compiler/src/dmd/parse.d

@@ -6649,7 +6649,7 @@ class Parser(AST, Lexer = dmd.lexer.Lexer) : Lexer
                 if (condition && ifbody)
                     s = new AST.IfStatement(loc, param, condition, ifbody, elsebody, token.loc);
                 else
-                    s = null; // don't propagate parsing errors
+                    s = new AST.ErrorStatement; // don't propagate parsing errors as null
                 break;
             }
 
@@ -6695,6 +6695,9 @@ class Parser(AST, Lexer = dmd.lexer.Lexer) : Lexer
                 if (auto ds = parseDebugSpecification())
                     eSink.error(ds.loc, "%s `%s` declaration must be at module level", ds.kind, ds.toPrettyChars);
 
+                // Not valid as a statement; return ErrorStatement instead of null so
+                // callers (string mixin semantic) do not null-deref.
+                s = new AST.ErrorStatement;
                 break;
             }
             cond = parseDebugCondition();
@@ -6707,6 +6710,9 @@ class Parser(AST, Lexer = dmd.lexer.Lexer) : Lexer
                 if (auto vs = parseVersionSpecification())
                     eSink.error(vs.loc, "%s `%s` declaration must be at module level", vs.kind, vs.toPrettyChars);
 
+                // Not valid as a statement; return ErrorStatement instead of null so
+                // callers (string mixin semantic) do not null-deref.
+                s = new AST.ErrorStatement;
                 break;
             }
             cond = parseVersionCondition();

compiler/src/dmd/statementsem.d

@@ -5082,7 +5082,9 @@ private Statements* flatten(Statement statement, Scope* sc)
                 Statement s = p.parseStatement(ParseStatementFlags.curlyScope);
                 if (!s || global.errors != errors)
                 {
-                    errorSupplemental(s.loc, "while parsing string mixin statement");
+                    // Prefer a real source location; ErrorStatement uses Loc.initial.
+                    const loc = (s !is null && s.loc.isValid()) ? s.loc : cs.loc;
+                    errorSupplemental(loc, "while parsing string mixin statement");
                     return errorStatements();
                 }
                 a.push(s);

compiler/test/fail_compilation/mixin_version_debug_assign.d

@@ -0,0 +1,21 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/mixin_version_debug_assign.d-mixin-15(15): Error: version `foo` declaration must be at module level
+fail_compilation/mixin_version_debug_assign.d(15):        while parsing string mixin statement
+fail_compilation/mixin_version_debug_assign.d-mixin-20(20): Error: identifier expected, not `1`
+fail_compilation/mixin_version_debug_assign.d(20):        while parsing string mixin statement
+---
+*/
+
+void main()
+{
+    // Previously crashed dmd with ACCESS_VIOLATION: parseStatement returned null,
+    // then string-mixin semantic did errorSupplemental(s.loc, ...).
+    mixin("version = foo;");
+}
+
+void other()
+{
+    mixin("debug = 1;");
+}