Bug 10755 – 'has no effect in expression' error for return too with comma operator

With dmd 2.064alpha this code gives: void main() { 1; } test.d(2): Error: long has no effect in expression (1) - - - - - - - - - - - - This gives an error, commas are disallowed here: void main() { int[10] a; auto x = a[1, 2]; } test.d(3): Error: only one index allowed to index int[10u] - - - - - - - - - - - - This too gives errors: void main() { int[10, 20] a; } test.d(2): Error: found ',' when expecting ']' test.d(2): Error: no identifier for declarator int[10] test.d(2): Error: semicolon expected, not '20' test.d(2): Error: found ']' when expecting ';' following statement - - - - - - - - - - - - This is disallowed: void main() { int x = 1, 2; } test.d(2): Error: no identifier for declarator int test.d(2): Error: semicolon expected, not '2' - - - - - - - - - - - - This gives an error: void main() { int x; x = 1, 2; } test.d(3): Error: long has no effect in expression (2) - - - - - - - - - - - - While this code compiles with no errors nor warnings: int foo() { return 1, 2; // line 2 } int bar() { return foo(), 3; // line 5 } void main() { assert(bar() == 3); } Inside bar() there is a call to foo(). foo() is not pure, but inside foo() "1" is a pure expression (a literal), so for this program I suggest to generate an error at line 2, and disallow comma syntax at return points, but probably no warning at line 5: test.d(2): Error: int has no effect in expression (1) - - - - - - - - - - - - This generates an error: int foo() { return 1; } void main() { int x; x = foo(), 2; } test.d(6): Error: long has no effect in expression (2) So maybe it's right to generate an error at line 5 too. - - - - - - - - - - - -

There is some dead code in sideeffect.c that implements this: /* This isn't used yet because the only way an expression has an unused sub-expression * is with the CommaExp, and that currently generates messages from rewrites into comma * expressions. Needs more investigation. */ void Expression::useValue() { #if 0 // Disabled because need to cast(void) the a,b code generation void *p; apply(&lambdaUseValue, &p); #endif } #if 0 int lambdaUseValue(Expression *e, void *param) { switch (e->op) { case TOKcomma: { CommaExp *ce = (CommaExp *)e; discardValue(ce->E1); break; } default: break; } return 0; } #endif

Pull request removing the dead code for now: https://github.com/D-Programming-Language/dmd/pull/3217

This now produces a deprecation warning after https://github.com/dlang/dmd/pull/5737, which should cover most occurrences of this situation.