Sema: do not immediately destroy failed generic instantiation

Closes #12535
Closes #12765
Closes #12927

Author: Vexu

src/Module.zig

@@ -3540,8 +3540,8 @@ pub fn destroyDecl(mod: *Module, decl_index: Decl.Index) void {
             if (decl.getInnerNamespace()) |namespace| {
                 namespace.destroyDecls(mod);
             }
-            decl.clearValues(mod);
         }
+        decl.clearValues(mod);
         decl.dependants.deinit(gpa);
         decl.dependencies.deinit(gpa);
         decl.clearName(gpa);
@@ -4610,9 +4610,18 @@ fn semaDecl(mod: *Module, decl_index: Decl.Index) !bool {
 
     // We need the memory for the Type to go into the arena for the Decl
     var decl_arena = std.heap.ArenaAllocator.init(gpa);
-    errdefer decl_arena.deinit();
     const decl_arena_allocator = decl_arena.allocator();
 
+    const decl_arena_state = blk: {
+        errdefer decl_arena.deinit();
+        const s = try decl_arena_allocator.create(std.heap.ArenaAllocator.State);
+        break :blk s;
+    };
+    defer {
+        decl_arena_state.* = decl_arena.state;
+        decl.value_arena = decl_arena_state;
+    }
+
     var analysis_arena = std.heap.ArenaAllocator.init(gpa);
     defer analysis_arena.deinit();
     const analysis_arena_allocator = analysis_arena.allocator();
@@ -4681,8 +4690,6 @@ fn semaDecl(mod: *Module, decl_index: Decl.Index) !bool {
     // not the struct itself.
     try sema.resolveTypeLayout(decl_tv.ty);
 
-    const decl_arena_state = try decl_arena_allocator.create(std.heap.ArenaAllocator.State);
-
     if (decl.is_usingnamespace) {
         if (!decl_tv.ty.eql(Type.type, mod)) {
             return sema.fail(&block_scope, ty_src, "expected type, found {}", .{
@@ -4701,8 +4708,6 @@ fn semaDecl(mod: *Module, decl_index: Decl.Index) !bool {
         decl.@"linksection" = null;
         decl.has_tv = true;
         decl.owns_tv = false;
-        decl_arena_state.* = decl_arena.state;
-        decl.value_arena = decl_arena_state;
         decl.analysis = .complete;
         decl.generation = mod.generation;
 
@@ -4723,16 +4728,14 @@ fn semaDecl(mod: *Module, decl_index: Decl.Index) !bool {
                 if (decl.getFunction()) |prev_func| {
                     prev_is_inline = prev_func.state == .inline_only;
                 }
-                decl.clearValues(mod);
             }
+            decl.clearValues(mod);
 
             decl.ty = try decl_tv.ty.copy(decl_arena_allocator);
             decl.val = try decl_tv.val.copy(decl_arena_allocator);
             // linksection, align, and addrspace were already set by Sema
             decl.has_tv = true;
             decl.owns_tv = owns_tv;
-            decl_arena_state.* = decl_arena.state;
-            decl.value_arena = decl_arena_state;
             decl.analysis = .complete;
             decl.generation = mod.generation;
 
@@ -4767,8 +4770,8 @@ fn semaDecl(mod: *Module, decl_index: Decl.Index) !bool {
     var type_changed = true;
     if (decl.has_tv) {
         type_changed = !decl.ty.eql(decl_tv.ty, mod);
-        decl.clearValues(mod);
     }
+    decl.clearValues(mod);
 
     decl.owns_tv = false;
     var queue_linker_work = false;
@@ -4841,8 +4844,6 @@ fn semaDecl(mod: *Module, decl_index: Decl.Index) !bool {
         };
     };
     decl.has_tv = true;
-    decl_arena_state.* = decl_arena.state;
-    decl.value_arena = decl_arena_state;
     decl.analysis = .complete;
     decl.generation = mod.generation;
 
@@ -5447,8 +5448,8 @@ pub fn clearDecl(
         if (decl.getInnerNamespace()) |namespace| {
             try namespace.deleteAllDecls(mod, outdated_decls);
         }
-        decl.clearValues(mod);
     }
+    decl.clearValues(mod);
 
     if (decl.deletion_flag) {
         decl.deletion_flag = false;