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compiler: do not propagate result type to
try operand
This commit effectively reverts 9e683f0, and hence un-accepts #19777. While nice in theory, this proposal turned out to have a few problems. Firstly, supplying a result type implicitly coerces the operand to this type -- that's the main point of result types! But for `try`, this is actually a bad idea; we want a redundant `try` to be a compile error, not to silently coerce the non-error value to an error union. In practice, this didn't always happen, because the implementation was buggy anyway; but when it did, it was really quite silly. For instance, `try try ... try .{ ... }` was an accepted expression, with the inner initializer being initially coerced to `E!E!...E!T`. Secondly, the result type inference here didn't play nicely with `return`. If you write `return try`, the operand would actually receive a result type of `E!E!T`, since the `return` gave a result type of `E!T` and the `try` wrapped it in *another* error union. More generally, the problem here is that `try` doesn't know when it should or shouldn't nest error unions. This occasionally broke code which looked like it should work. So, this commit prevents `try` from propagating result types through to its operand. A key motivation for the original proposal here was decl literals; so, as a special case, `try .foo(...)` is still an allowed syntax form, caught by AstGen and specially lowered. This does open the doors to allowing other special cases for decl literals in future, such as `.foo(...) catch ...`, but those proposals are for another time. Resolves: #21991 Resolves: #22633
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,52 @@ | ||
| const S = struct { x: u32 = 0 }; | ||
| const T = struct { []const u8 }; | ||
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| fn test0() !void { | ||
| const x: u8 = try 1; | ||
| _ = x; | ||
| } | ||
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| fn test1() !void { | ||
| const x: S = try .{}; | ||
| _ = x; | ||
| } | ||
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| fn test2() !void { | ||
| const x: S = try .{ .x = 123 }; | ||
| _ = x; | ||
| } | ||
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| fn test3() !void { | ||
| const x: S = try try .{ .x = 123 }; | ||
| _ = x; | ||
| } | ||
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| fn test4() !void { | ||
| const x: T = try .{"hello"}; | ||
| _ = x; | ||
| } | ||
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| fn test5() !void { | ||
| const x: error{Foo}!u32 = 123; | ||
| _ = try try x; | ||
| } | ||
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| comptime { | ||
| _ = &test0; | ||
| _ = &test1; | ||
| _ = &test2; | ||
| _ = &test3; | ||
| _ = &test4; | ||
| _ = &test5; | ||
| } | ||
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| // error | ||
| // | ||
| // :5:23: error: expected error union type, found 'comptime_int' | ||
| // :10:23: error: expected error union type, found '@TypeOf(.{})' | ||
| // :15:23: error: expected error union type, found 'tmp.test2__struct_493' | ||
| // :15:23: note: struct declared here | ||
| // :20:27: error: expected error union type, found 'tmp.test3__struct_495' | ||
| // :20:27: note: struct declared here | ||
| // :25:23: error: expected error union type, found 'struct { comptime *const [5:0]u8 = "hello" }' | ||
| // :31:13: error: expected error union type, found 'u32' |