stage 1: improve error message if error union is cast to payload (#10770)

Also: Added special error message for for `?T` to `T` casting

src/stage1/ir.cpp

@@ -8242,6 +8242,34 @@ static Stage1AirInst *ir_analyze_cast(IrAnalyze *ira, Scope *scope, AstNode *sou
         return ir_implicit_cast2(ira, scope, source_node, cast1, wanted_type);
     }
 
+    // E!T to T
+    if (actual_type->id == ZigTypeIdErrorUnion) {
+        if (types_match_const_cast_only(ira, actual_type->data.error_union.payload_type, wanted_type,
+            source_node, false).id == ConstCastResultIdOk)
+        {
+            ErrorMsg *parent_msg = ir_add_error_node(ira, source_node,
+                buf_sprintf("cannot convert error union to payload type. consider using `try`, `catch`, or `if`. expected type '%s', found '%s'",
+                    buf_ptr(&wanted_type->name),
+                    buf_ptr(&actual_type->name)));
+            report_recursive_error(ira, source_node, &const_cast_result, parent_msg);
+            return ira->codegen->invalid_inst_gen;
+        }
+    }
+
+    //?T to T
+    if (actual_type->id == ZigTypeIdOptional) {
+        if (types_match_const_cast_only(ira, actual_type->data.maybe.child_type, wanted_type,
+            source_node, false).id == ConstCastResultIdOk)
+        {
+            ErrorMsg *parent_msg = ir_add_error_node(ira, source_node,
+                buf_sprintf("cannot convert optional to payload type. consider using `.?`, `orelse`, or `if`. expected type '%s', found '%s'",
+                    buf_ptr(&wanted_type->name),
+                    buf_ptr(&actual_type->name)));
+            report_recursive_error(ira, source_node, &const_cast_result, parent_msg);
+            return ira->codegen->invalid_inst_gen;
+        }
+    }
+
     ErrorMsg *parent_msg = ir_add_error_node(ira, source_node,
         buf_sprintf("expected type '%s', found '%s'",
             buf_ptr(&wanted_type->name),

test/compile_errors.zig

@@ -790,9 +790,9 @@ pub fn addCases(ctx: *TestContext) !void {
         "tmp.zig:1:17: note: function cannot return an error",
         "tmp.zig:8:5: error: expected type 'void', found '@typeInfo(@typeInfo(@TypeOf(bar)).Fn.return_type.?).ErrorUnion.error_set'",
         "tmp.zig:7:17: note: function cannot return an error",
-        "tmp.zig:11:15: error: expected type 'u32', found '@typeInfo(@typeInfo(@TypeOf(bar)).Fn.return_type.?).ErrorUnion.error_set!u32'",
+        "tmp.zig:11:15: error: cannot convert error union to payload type. consider using `try`, `catch`, or `if`. expected type 'u32', found '@typeInfo(@typeInfo(@TypeOf(bar)).Fn.return_type.?).ErrorUnion.error_set!u32'",
         "tmp.zig:10:17: note: function cannot return an error",
-        "tmp.zig:15:14: error: expected type 'u32', found '@typeInfo(@typeInfo(@TypeOf(bar)).Fn.return_type.?).ErrorUnion.error_set!u32'",
+        "tmp.zig:15:14: error: cannot convert error union to payload type. consider using `try`, `catch`, or `if`. expected type 'u32', found '@typeInfo(@typeInfo(@TypeOf(bar)).Fn.return_type.?).ErrorUnion.error_set!u32'",
         "tmp.zig:14:5: note: cannot store an error in type 'u32'",
     });
 
@@ -1879,7 +1879,7 @@ pub fn addCases(ctx: *TestContext) !void {
         \\    _ = afoo;
         \\}
     , &[_][]const u8{
-        "tmp.zig:12:25: error: expected type 'u32', found '@typeInfo(@typeInfo(@TypeOf(get_uval)).Fn.return_type.?).ErrorUnion.error_set!u32'",
+        "tmp.zig:12:25: error: cannot convert error union to payload type. consider using `try`, `catch`, or `if`. expected type 'u32', found '@typeInfo(@typeInfo(@TypeOf(get_uval)).Fn.return_type.?).ErrorUnion.error_set!u32'",
     });
 
     ctx.objErrStage1("assigning to struct or union fields that are not optionals with a function that returns an optional",
@@ -1899,7 +1899,7 @@ pub fn addCases(ctx: *TestContext) !void {
         \\    _ = s;
         \\}
     , &[_][]const u8{
-        "tmp.zig:11:27: error: expected type 'u8', found '?u8'",
+        "tmp.zig:11:27: error: cannot convert optional to payload type. consider using `.?`, `orelse`, or `if`. expected type 'u8', found '?u8'",
     });
 
     ctx.objErrStage1("missing result type for phi node",
@@ -2308,7 +2308,7 @@ pub fn addCases(ctx: *TestContext) !void {
         \\    not_optional: i32,
         \\};
     , &[_][]const u8{
-        "tmp.zig:3:36: error: expected type 'i32', found '?i32'",
+        "tmp.zig:3:36: error: cannot convert optional to payload type. consider using `.?`, `orelse`, or `if`. expected type 'i32', found '?i32'",
     });
 
     ctx.objErrStage1("result location incompatibility mismatching handle_is_ptr",
@@ -2325,7 +2325,7 @@ pub fn addCases(ctx: *TestContext) !void {
         \\    not_optional: i32,
         \\};
     , &[_][]const u8{
-        "tmp.zig:3:36: error: expected type 'i32', found '?i32'",
+        "tmp.zig:3:36: error: cannot convert optional to payload type. consider using `.?`, `orelse`, or `if`. expected type 'i32', found '?i32'",
     });
 
     ctx.objErrStage1("const frame cast to anyframe",
@@ -8828,7 +8828,7 @@ pub fn addCases(ctx: *TestContext) !void {
         \\    v = u;
         \\}
     , &[_][]const u8{
-        "tmp.zig:4:9: error: expected type '*anyopaque', found '?*anyopaque'",
+        "tmp.zig:4:9: error: cannot convert optional to payload type. consider using `.?`, `orelse`, or `if`. expected type '*anyopaque', found '?*anyopaque'",
     });
 
     ctx.objErrStage1("Issue #6823: don't allow .* to be followed by **",