Fix deduction crash for function with missing parameters. (#4461)

This is because `var x:! () = ();` modifies the binding index, which
causes `A` to be generic, which causes the params to be used, which
crashes. There may be another issue to fix here so that the invalid
binding doesn't modify the binding index, but at least
`param_patterns_id` should probably be set consistently with
`params_id`.

Author: jonmeow

toolchain/check/handle_function.cpp

@@ -183,8 +183,10 @@ static auto BuildFunctionDecl(Context& context,
 
   auto name = PopNameComponent(context, return_slot_pattern_id);
   if (!name.params_id.is_valid()) {
+    CARBON_CHECK(!name.param_patterns_id.is_valid());
     context.TODO(node_id, "function with positional parameters");
     name.params_id = SemIR::InstBlockId::Empty;
+    name.param_patterns_id = SemIR::InstBlockId::Empty;
   }
 
   auto name_context = context.decl_name_stack().FinishName(name);

toolchain/check/testdata/function/declaration/no_prelude/fail_todo_no_params.carbon

@@ -31,15 +31,40 @@ fn A {
 
 // TODO: We don't have parsing support for this yet.
 library "[[@TEST_NAME]]";
-// CHECK:STDERR: fail_todo_arrow_body.carbon:[[@LINE+7]]:1: error: semantics TODO: `function with positional parameters` [SemanticsTodo]
+// CHECK:STDERR: fail_todo_arrow_body.carbon:[[@LINE+8]]:1: error: semantics TODO: `function with positional parameters` [SemanticsTodo]
 // CHECK:STDERR: fn A => 0;
 // CHECK:STDERR: ^~~~~~~~~~
 // CHECK:STDERR:
-// CHECK:STDERR: fail_todo_arrow_body.carbon:[[@LINE+3]]:6: error: `fn` declarations must either end with a `;` or have a `{ ... }` block for a definition [ExpectedDeclSemiOrDefinition]
+// CHECK:STDERR: fail_todo_arrow_body.carbon:[[@LINE+4]]:6: error: `fn` declarations must either end with a `;` or have a `{ ... }` block for a definition [ExpectedDeclSemiOrDefinition]
 // CHECK:STDERR: fn A => 0;
 // CHECK:STDERR:      ^~
+// CHECK:STDERR:
 fn A => 0;
 
+// --- fail_invalid_file_generic_regression_test.carbon
+
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: fail_invalid_file_generic_regression_test.carbon:[[@LINE+4]]:9: error: `var` declaration cannot declare a compile-time binding [CompileTimeBindingInVarDecl]
+// CHECK:STDERR: var x:! () = ();
+// CHECK:STDERR:         ^~
+// CHECK:STDERR:
+var x:! () = ();
+
+// CHECK:STDERR: fail_invalid_file_generic_regression_test.carbon:[[@LINE+4]]:1: error: semantics TODO: `function with positional parameters` [SemanticsTodo]
+// CHECK:STDERR: fn A {
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR:
+fn A {
+  // CHECK:STDERR: fail_invalid_file_generic_regression_test.carbon:[[@LINE+6]]:3: error: cannot deduce value for generic parameter `x` [DeductionIncomplete]
+  // CHECK:STDERR:   A();
+  // CHECK:STDERR:   ^~
+  // CHECK:STDERR: fail_invalid_file_generic_regression_test.carbon:[[@LINE-4]]:1: note: while deducing parameters of generic declared here [DeductionGenericHere]
+  // CHECK:STDERR: fn A {
+  // CHECK:STDERR: ^~~~~~
+  A();
+}
+
 // CHECK:STDOUT: --- fail_no_body.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -55,7 +80,7 @@ fn A => 0;
 // CHECK:STDOUT:   %A.decl: %A.type = fn_decl @A [template = constants.%A] {} {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @A;
+// CHECK:STDOUT: fn @A();
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_todo_brace_body.carbon
 // CHECK:STDOUT:
@@ -72,7 +97,7 @@ fn A => 0;
 // CHECK:STDOUT:   %A.decl: %A.type = fn_decl @A [template = constants.%A] {} {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @A {
+// CHECK:STDOUT: fn @A() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
@@ -92,5 +117,48 @@ fn A => 0;
 // CHECK:STDOUT:   %A.decl: %A.type = fn_decl @A [template = constants.%A] {} {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @A;
+// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_invalid_file_generic_regression_test.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %.1: type = tuple_type () [template]
+// CHECK:STDOUT:   %x: %.1 = bind_symbolic_name x, 0 [symbolic]
+// CHECK:STDOUT:   %tuple: %.1 = tuple_value () [template]
+// CHECK:STDOUT:   %A.type: type = fn_type @A [template]
+// CHECK:STDOUT:   %A: %A.type = struct_value () [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .x = %x
+// CHECK:STDOUT:     .A = %A.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %.loc8_10.1: %.1 = tuple_literal ()
+// CHECK:STDOUT:   %.loc8_10.2: type = converted %.loc8_10.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %x.var: ref %.1 = var x
+// CHECK:STDOUT:   %x: %.1 = bind_symbolic_name x, 0, %x.var [symbolic = constants.%x]
+// CHECK:STDOUT:   %A.decl: %A.type = fn_decl @A [template = constants.%A] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @A(file.%x: %.1) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn() {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     %A.ref: %A.type = name_ref A, file.%A.decl [template = constants.%A]
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @__global_init() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %.loc8_15.1: %.1 = tuple_literal ()
+// CHECK:STDOUT:   %.loc8_15.2: init %.1 = tuple_init () to file.%x.var [template = constants.%tuple]
+// CHECK:STDOUT:   %.loc8_16: init %.1 = converted %.loc8_15.1, %.loc8_15.2 [template = constants.%tuple]
+// CHECK:STDOUT:   assign file.%x.var, %.loc8_16
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @A(constants.%x) {}
 // CHECK:STDOUT: