Bug 18594 – X is not an lvalue should have a better error message

--- void main() { assert(1 = 2); } --- currently errors with "foo.d(2): Error: constant 1 is not an lvalue". The error message should be better: (1) not mention lvalue and (2) suggesting `==`.

(PR incoming)

Hmm, just modifying the error message isn't enough. We should probably look up in the tree to check whether we are in if expression, assert, ... diff --git a/src/dmd/expression.d b/src/dmd/expression.d index d93c1ffda..997423b85 100644 --- a/src/dmd/expression.d +++ b/src/dmd/expression.d @@ -1855,7 +1855,7 @@ extern (C++) abstract class Expression : RootObject } } } - error("cannot modify `%s` expression `%s`. Did you mean `==`?", MODtoChars(type.mod), toChars()); + error("cannot modify `%s` expression `%s`", MODtoChars(type.mod), toChars()); return new ErrorExp(); } else if (!type.isAssignable()) @@ -2604,7 +2604,7 @@ extern (C++) final class IntegerExp : Expression e = this; else if (!loc.isValid()) loc = e.loc; - e.error("constant `%s` is not an lvalue. Did you mean `==`?", e.toChars()); + e.error("constant `%s` is not an lvalue", e.toChars()); return new ErrorExp(); } diff --git a/test/fail_compilation/test18594.d b/test/fail_compilation/test18594.d deleted file mode 100644 index 631df3033..000000000 --- a/test/fail_compilation/test18594.d +++ /dev/null @@ -1,11 +0,0 @@ -/*TEST_OUTPUT: ---- -fail_compilation/test18594.d(8): Error: constant `1` is not an lvalue. Did you mean `==`? -fail_compilation/test18594.d(10): Error: cannot modify `const` expression `a`. Did you mean `==`? ---- -*/ -void main() { - assert(1 = 2); - const a = 1; - assert(a = 2); -}

A start: https://github.com/dlang/dmd/pull/8009

Commits pushed to master at https://github.com/dlang/dmd https://github.com/dlang/dmd/commit/ea309c0967e473db3c19e202ffcaf563d9cfff63 Issue 12663, 18594 - Replace lvalue in error messages with 'cannot modify' https://github.com/dlang/dmd/commit/4574f6891b925c768382ae6dba57b93c4b66bf13 Merge pull request #8009 from wilzbach/fix-18594 Issue 18594 - Replace lvalue in error messages with 'cannot modify' merged-on-behalf-of: Mike Franklin <[email protected]>

--- struct Test(T) { @property ref T get() inout { return member; } private: T member; } int main(string[] args) { Test!int t; t.get = 12; return t.get; } --- This code has an very similar issue: > test.d(3): Error: cast(int)this.member is not an lvalue and cannot be modified The correct definition is: --- struct Test(T) { @property ref inout(T) get() inout { return member; } private: T member; } --- but the error message gives no hint in that direction. It confused me quite a bit, since IMO member is a fine lvalue that happens to be const/inout. https://run.dlang.io/is/IJKZ80

the same error message is given when taking the address of a non-lvalue, but "cannot be modified" doesn't necessarily apply there (and can be confusing) // Error: `a[0..2]` is not an lvalue and cannot be modified ubyte[2] a; const void* p1 = &a[0..2]; // Error: cannot modify constant `1` const void* p2 = &1; for these, the message should rather say something like "cannot have its address taken"

This issue has been fixed by: https://github.com/dlang/dmd/pull/8009